InnoDB architecture: pages, tablespaces, threads

datadir_tour_session.py

"""Every file a brand-new MySQL 8.4.11 data directory holds, and the InnoDB structure behind each name.

A throwaway server (mysql_session.py) is initialised and started, and then the session lists the data directory and
the folder of session temporary tablespaces inside it, and asks the server for the page size and for the files InnoDB
opened at startup. Nothing has been created by a user yet, so every file here belongs to the server itself.

The #innodb_redo folder is left out of the listing on purpose: the number of spare redo files in it changes while the
server starts, so two runs of this program would not print the same thing.

The FILES query shows mysql.ibd, the data dictionary tablespace, with its extent count as a band of 20 to 40. That
count differs by a few extents between fresh servers (25 on one run of this program, 28 on others), so the query
prints the band instead, and the lesson reads growth off the stable ibdata1 (12 extents) and undo files (16 each).
"""

from mysql_session import Server

with Server() as my:
    my.up()
    my.listing("data")
    my.listing("data/#innodb_temp")
    my.mysql("datadir_tour.sql")

$ ls -p data
#ib_16384_0.dblwr  #ib_16384_1.dblwr  #innodb_redo/  #innodb_temp/  auto.cnf
binlog.000001  binlog.index  ca-key.pem  ca.pem  client-cert.pem
client-key.pem  ib_buffer_pool  ibdata1  ibtmp1  mysql.ibd  mysql.sock
mysql.sock.lock  mysql/  mysql_upgrade_history  performance_schema/
private_key.pem  public_key.pem  server-cert.pem  server-key.pem  sys/
undo_001  undo_002

$ ls -p data/#innodb_temp
temp_1.ibt  temp_10.ibt  temp_2.ibt  temp_3.ibt  temp_4.ibt  temp_5.ibt
temp_6.ibt  temp_7.ibt  temp_8.ibt  temp_9.ibt

$ mysql -t -vvv --skip-comments < datadir_tour.sql
--------------
SELECT @@innodb_page_size AS page_bytes, @@innodb_page_size * 64 / 1024 / 1024 AS extent_mb
--------------

+------------+------------+
| page_bytes | extent_mb  |
+------------+------------+
|      16384 | 1.00000000 |
+------------+------------+
1 row in set

--------------
SELECT FILE_NAME, FILE_TYPE, TABLESPACE_NAME,
       IF(TABLESPACE_NAME = 'mysql' AND TOTAL_EXTENTS BETWEEN 20 AND 40, '20 to 40', TOTAL_EXTENTS) AS TOTAL_EXTENTS,
       EXTENT_SIZE
FROM information_schema.FILES
ORDER BY FILE_NAME
--------------

+----------------------+------------+------------------+---------------+-------------+
| FILE_NAME            | FILE_TYPE  | TABLESPACE_NAME  | TOTAL_EXTENTS | EXTENT_SIZE |
+----------------------+------------+------------------+---------------+-------------+
| ./ibdata1            | TABLESPACE | innodb_system    | 12            |     1048576 |
| ./ibtmp1             | TEMPORARY  | innodb_temporary | 12            |     1048576 |
| ./mysql.ibd          | TABLESPACE | mysql            | 20 to 40      |     1048576 |
| ./sys/sys_config.ibd | TABLESPACE | sys/sys_config   | 0             |     1048576 |
| ./undo_001           | UNDO LOG   | innodb_undo_001  | 16            |     1048576 |
| ./undo_002           | UNDO LOG   | innodb_undo_002  | 16            |     1048576 |
+----------------------+------------+------------------+---------------+-------------+
6 rows in set

Bye

datadir_tour.sql

-- The page size of this instance, and the size of an extent of 64 such pages.
SELECT @@innodb_page_size AS page_bytes, @@innodb_page_size * 64 / 1024 / 1024 AS extent_mb;

-- Every InnoDB file the server opened before any user table existed, with the tablespace it belongs to and the
-- extents it is made of. EXTENT_SIZE is in bytes. The data dictionary's mysql.ibd prints its extents as a band,
-- because that count differs by a few extents from one fresh server to the next (a count outside the band prints
-- as it is).
SELECT FILE_NAME, FILE_TYPE, TABLESPACE_NAME,
       IF(TABLESPACE_NAME = 'mysql' AND TOTAL_EXTENTS BETWEEN 20 AND 40, '20 to 40', TOTAL_EXTENTS) AS TOTAL_EXTENTS,
       EXTENT_SIZE
FROM information_schema.FILES
ORDER BY FILE_NAME;

mysql_session.py

"""Throwaway MySQL 8.4.11 servers for the examples of this lesson.

Every session creates a server of its own: `mysqld --initialize-insecure` makes a new data directory inside a new
folder in the examples folder, the server starts on it, the session runs `mysql` client scripts and MySQL's programs
against it and prints what they printed, and then the server is stopped and the folder deleted, also when something
fails. The server is the pinned MySQL 8.4.11 from the mise installations next to the Python that runs this file;
Server(basedir=...) runs the programs of another unpacked MySQL tarball instead (dump_defaults.py uses it).

Safety: the server listens only on 127.0.0.1, an address no other computer can reach, on a port the operating system
reports as unused; the X Protocol listener is off (--mysqlx=OFF); its Unix socket, temporary files, error log and
import/export folder all stay inside the run folder. Right after the first start the session gives root a password made
up for the run (never printed) and connects only with a client option file that holds it, so no password ever appears
on a command line. Every program starts with --no-defaults or --defaults-file, so no option file of the computer
(/etc/my.cnf, ~/.my.cnf) can change a result, and HOME points at the run folder, so ~/.mylogin.cnf is not read either.
A printed mysqld command shows the options that matter to the lesson; the session also passes, on the command line,
the folders it keeps inside the run folder (--basedir, --tmpdir, --log-error, --pid-file, --secure-file-priv) and
the port, which is why performance_schema.variables_info reports those variables as COMMAND_LINE.
learn:verify's run sandbox cannot read the home folder, where mise keeps MySQL, so these sessions are recorded with
LEARN_VERIFY_NETWORK=allow; they read only this folder and the toolchain and connect to nothing but 127.0.0.1.

The output is exactly what the programs printed, with these substitutions so that a record never shows the computer it
ran on and is the same on every run: the run folder is shown as ".", the folder of MySQL's programs is left out of
their paths and MySQL's own folder is shown as <basedir>, the computer's name is shown as "lab-host" and the operating-system user's name as "learner", the time a
statement took (the client's "(0.01 sec)") is left out, the time stamp SET PERSIST writes into mysqld-auto.cnf is
shown as <when it was set>, error-log lines lose their timestamp and thread number, and a session can name more values
that change between runs with my.mask(pattern, replacement), which then prints in their place. A client table whose cells
change is drawn again so that its borders still line up.
The sessions never print the password, and the run's port prints as <port>. A session stops with status 1 when a command that should have worked
fails, or when a script reports a different number of errors than the session expects, so a run that went wrong is
never recorded as if it had worked.
"""

import os
import pwd
import re
import secrets
import shlex
import shutil
import socket
import subprocess
import sys
import tempfile
import textwrap
import time
from pathlib import Path

MYSQL_VERSION = "8.4.11"
# .../installs/python/3.14.8/bin/python3 -> .../installs
INSTALLS = Path(sys.executable).resolve().parents[3]
PINNED = INSTALLS / "mysql" / MYSQL_VERSION
OS_USER = pwd.getpwuid(os.geteuid()).pw_name
HOST = socket.gethostname().split(".")[0]
# 2026-10-17T16:50:01.123456Z 0 [ERROR] ... -> [ERROR] ...
LOG_PREFIX = re.compile(r"^\d{4}-\d\d-\d\dT\d\d:\d\d:\d\d\.\d+Z \d+ ", re.M)


def free_port():
    """A TCP port on 127.0.0.1 that nothing listens on now."""
    with socket.socket() as probe:
        probe.bind(("127.0.0.1", 0))
        return probe.getsockname()[1]


class Server:
    """One throwaway server: initialise, start; stopped and deleted when the with block ends.

    options: extra mysqld options for every start (for example "--sql-mode=" or "--character-set-server=latin1").
    basedir: the folder of an unpacked MySQL tarball to run instead of the pinned 8.4.11.
    masks: (pattern, replacement) pairs, as for mask().
    """

    def __init__(self, options=(), redo=None, basedir=None, masks=()):
        self.options = [str(o) for o in options]
        self.redo = redo
        self.basedir = Path(basedir).resolve() if basedir else PINNED
        self.bin = self.basedir / "bin"
        self.masks = []
        for pattern, replacement in masks:
            self.mask(pattern, replacement)
        self.proc = None

    def mask(self, pattern, replacement):
        """From now on every printed line shows text matching the regular expression as the replacement: for a value
        that changes from run to run (an uptime, a time stamp), so that the recorded output stays the same."""
        self.masks.append((re.compile(pattern), replacement))

    # ---- the folder ------------------------------------------------------------------------------------------------
    def __enter__(self):
        if not (self.bin / "mysqld").exists():
            sys.exit(f"no MySQL server program in {self.basedir}" if self.basedir != PINNED
                     else f"MySQL {MYSQL_VERSION} was not found next to this Python")
        self.base = Path(tempfile.mkdtemp(prefix="my", dir=".")).resolve()
        for sub in ("tmp", "files"):
            (self.base / sub).mkdir(mode=0o700)
        self.port = free_port()
        self.password = secrets.token_urlsafe(24)
        self.client_cnf = self.base / "client.cnf"
        self._write_client_cnf(password=None)
        self.env = {key: value for key, value in os.environ.items()
                    if not key.startswith("MYSQL") and key not in ("TZ", "HOME", "LANG", "LC_ALL")}
        self.env.update({"HOME": str(self.base), "TZ": "UTC", "LANG": "en_US.UTF-8",
                         "MYSQL_TEST_LOGIN_FILE": str(self.base / "no-login-paths.cnf")})
        for pattern in ("*.sql", "*.cnf", "*.csv", "*.txt"):
            for f in Path(".").glob(pattern):
                shutil.copy(f, self.base / f.name)
        return self

    def __exit__(self, *exc):
        if self.proc and self.proc.poll() is None:
            self.stop(quiet=True)
        shutil.rmtree(self.base, ignore_errors=True)

    def _write_client_cnf(self, password):
        lines = ["[client]", "user=root", "host=127.0.0.1", f"port={self.port}", "protocol=TCP"]
        if password:
            lines.append(f"password={password}")
        self.client_cnf.write_text("\n".join(lines) + "\n")
        self.client_cnf.chmod(0o600)

    def server_options(self):
        b = self.base
        return ["--no-defaults", f"--basedir={self.basedir}", f"--datadir={b / 'data'}", f"--tmpdir={b / 'tmp'}",
                f"--log-error={b / 'mysqld.err'}", f"--pid-file={b / 'mysqld.pid'}",
                f"--secure-file-priv={b / 'files'}",
                *([f"--innodb-redo-log-capacity={self.redo}"] if self.redo else [])]

    # ---- the server ------------------------------------------------------------------------------------------------
    def initialize(self, show=False):
        """mysqld --initialize-insecure: a new data directory whose root account has no password yet."""
        args = ["mysqld", *self.server_options(), "--initialize-insecure"]
        code, out = self._call(args)
        if show:
            _show(["mysqld", "--no-defaults", "--initialize-insecure", "--datadir=./data"], out)
        if code != 0:
            print(self.log())
            sys.exit(f"mysqld --initialize-insecure ended with status {code}")

    def start(self, extra=(), defaults_file=None, expect_fail=False, show=False):
        """Starts mysqld on 127.0.0.1 and waits until it answers. With expect_fail, prints the error-log lines of a
        start that must fail, and returns them."""
        log = self.base / "mysqld.err"
        before = log.read_text() if log.exists() else ""
        opts = self.server_options()
        if defaults_file:
            opts[0] = f"--defaults-file={self.base / defaults_file}"
        args = [str(self.bin / "mysqld"), *opts, "--bind-address=127.0.0.1", f"--port={self.port}",
                "--socket=mysql.sock", "--mysqlx=OFF", *self.options, *[str(e) for e in extra]]
        if show:
            shown = ["mysqld", opts[0].replace(str(self.base) + "/", ""), "--datadir=./data", "--bind-address=127.0.0.1",
                     "--port=$PORT", "--socket=mysql.sock", "--mysqlx=OFF", *self.options, *extra]
            print("$ " + " ".join(_arg(a) if a != "--port=$PORT" else a for a in shown) + " &")
            print()
        with open(self.base / "mysqld.out", "a") as out:
            self.proc = subprocess.Popen(args, stdout=out, stderr=subprocess.STDOUT, cwd=self.base, env=self.env)
        for _ in range(600):
            if self.proc.poll() is not None:
                break
            if self._ping():
                break
            time.sleep(0.1)
        if self.proc.poll() is not None:
            new = (log.read_text() if log.exists() else "")[len(before):]
            lines = [l for l in self.clean(LOG_PREFIX.sub("", new)).splitlines() if "[ERROR]" in l]
            if expect_fail:
                print("\n".join(lines))
                print(f"(mysqld ended with status {self.proc.returncode})")
                print()
                self.proc = None
                return lines
            print("\n".join(lines))
            sys.exit(f"mysqld ended with status {self.proc.returncode} while starting")
        if expect_fail:
            sys.exit("mysqld started, but the session expected it to fail")
        if not self._ping():
            sys.exit("mysqld did not answer within 60 seconds")
        return []

    def _ping(self):
        code, _ = self._call(["mysqladmin", f"--defaults-file={self.client_cnf}", "--connect-timeout=2", "ping"])
        return code == 0

    def up(self, extra=(), defaults_file=None, show=False):
        """Initialise, start (with start()'s options), and give root the run's password, quietly."""
        self.initialize()
        self.start(extra=extra, defaults_file=defaults_file, show=show)
        return self.login()

    def login(self):
        """Gives root the run's password (never printed) on a server started from a new data directory, and makes
        every later client of the session use it."""
        self.silent(f"ALTER USER 'root'@'localhost' IDENTIFIED BY '{self.password}'")
        self._write_client_cnf(self.password)
        return self

    def stop(self, quiet=False):
        """A clean shutdown (mysqladmin shutdown), then waits until the process has ended."""
        code, out = self._call(["mysqladmin", f"--defaults-file={self.client_cnf}", "shutdown"])
        if code != 0 and not quiet:
            sys.exit(f"mysqladmin shutdown failed: {out}")
        try:
            self.proc.wait(timeout=120)
        except subprocess.TimeoutExpired:
            self.proc.terminate()
            self.proc.wait(timeout=60)
        self.proc = None

    def restart(self, extra=(), defaults_file=None, show=False):
        self.stop()
        self.start(extra=extra, defaults_file=defaults_file, show=show)

    # ---- running things --------------------------------------------------------------------------------------------
    def _call(self, args, stdin=None, merge=True):
        """Runs one of MySQL's programs in the run folder: (status, cleaned output). merge=False keeps only what it
        wrote to standard output (its warnings on standard error are dropped)."""
        args = [str(a) for a in args]
        done = subprocess.run([str(self.bin / args[0]), *args[1:]], stdout=subprocess.PIPE,
                              stderr=subprocess.STDOUT if merge else subprocess.DEVNULL,
                              text=True, cwd=self.base, env=self.env, input=stdin)
        return done.returncode, self.clean(done.stdout)

    def output(self, *args, connect=False, status=0):
        """Runs one of MySQL's programs without printing anything and returns its standard output, for example the
        SQL that mysql_tzinfo_to_sql writes, which the session saves with write_file(..., show=False) and runs next."""
        args = [str(a) for a in args]
        run = [args[0], f"--defaults-file={self.client_cnf}", *args[1:]] if connect else args
        code, out = self._call(run, merge=False)
        if code != status:
            sys.exit(f"{args[0]} ended with status {code}, not {status}")
        return out

    def clean(self, text):
        """The substitutions of the docstring. A table of the client whose cells change is drawn again with the
        new column widths, so its borders still line up."""
        lines, out, i = text.split("\n"), [], 0
        while i < len(lines):
            if lines[i].startswith("+-"):
                j = i
                while j < len(lines) and lines[j][:1] in ("+", "|"):
                    j += 1
                block = lines[i:j]
                done = [self._clean(l) for l in block]
                out.extend(done if done == block else _redraw(block, self._clean))
                i = j
            else:
                out.append(self._clean(lines[i]))
                i += 1
        return "\n".join(out)

    def _clean(self, text):
        text = text.replace(str(self.bin) + "/", "").replace(str(self.basedir), "<basedir>")
        text = text.replace(str(self.base) + "/", "./").replace(str(self.base), ".")
        text = text.replace("\x07", "")   # the mysql client rings the terminal bell on an error; drop it
        text = re.sub(r" \(\d+\.\d\d sec\)", "", text)
        text = re.sub(r'"Timestamp": \d+', '"Timestamp": <when it was set>', text)
        text = re.sub(rf"\b{re.escape(HOST)}\b", "lab-host", text)
        text = re.sub(rf"\b{re.escape(OS_USER)}\b", "learner", text)
        text = re.sub(rf"(?<![\d.]){self.port}(?![\d.])", "<port>", text)
        for pattern, replacement in self.masks:
            text = pattern.sub(replacement, text)
        return text

    def log(self, pattern=None):
        """The error log without timestamps and thread numbers, optionally only the lines matching a pattern."""
        text = self.clean(LOG_PREFIX.sub("", (self.base / "mysqld.err").read_text()))
        if pattern:
            text = "\n".join(l for l in text.splitlines() if re.search(pattern, l))
        return text

    def show_log(self, pattern, note=None):
        """Prints the error-log lines that match a pattern, as grep would."""
        print(f"$ grep -E {_arg(pattern)} mysqld.err" + (f"   # {note}" if note else ""))
        print(self.log(pattern))
        print()

    def query(self, sql, db=None):
        """One value from one query, unprinted (the session uses it to wait or to decide)."""
        args = ["mysql", f"--defaults-file={self.client_cnf}", "-N", "-B", *([db] if db else [])]
        code, out = self._call(args, stdin=sql)
        if code != 0:
            sys.exit(f"the query failed: {out}")
        return out.strip()

    def silent(self, sql):
        """Runs a statement without printing it (used only for the run's password)."""
        self.query(sql)

    def mysql(self, script, errors=0, status=None, db=None, flags=("-t", "-vvv", "--skip-comments"), note=None, force=None):
        """Runs a client script from standard input and prints the command and its output. `errors` is how many
        ERROR lines the script must print; with errors > 0 the client runs on past an error (--force) and then ends
        with status 0, without --force it stops at the first error with status 1. The default flags print each
        statement (-vvv) as a table (-t) without the script's comments (--skip-comments)."""
        force = errors > 0 if force is None else force
        shown = ["mysql", *flags, *(["--force"] if force else []), *([db] if db else [])]
        args = ["mysql", f"--defaults-file={self.client_cnf}", *shown[1:]]
        code, out = self._call(args, stdin=(self.base / script).read_text())
        if note:
            print(f"-- {note}")
        print("$ " + " ".join(_arg(a) for a in shown) + f" < {script}")
        if out.strip():
            print(out.rstrip("\n"))
        print()
        found = sum(1 for line in out.splitlines() if line.startswith("ERROR "))
        expected_status = (1 if errors and not force else 0) if status is None else status
        if found != errors or code != expected_status:
            sys.exit(f"{script}: {found} errors and status {code}; the session expects {errors} and {expected_status}")
        return out

    def shell(self, *args, status=0, shown=None, connect=False):
        """Runs one of MySQL's programs and prints the command and its output. connect=True gives a client program
        (mysqladmin, mysqlshow, mysql) the run's client option file, which the printed command leaves out."""
        args = [str(a) for a in args]
        run = [args[0], f"--defaults-file={self.client_cnf}", *args[1:]] if connect else args
        code, out = self._call(run)
        _show(shown or args, out)
        if code != status:
            sys.exit(f"{args[0]} ended with status {code}, not {status}")
        return out

    def sql(self, statement, flags=("-t",), errors=0, db=None, note=None):
        """Runs one statement with mysql -e and prints the command and its output."""
        shown = ["mysql", *flags, *([db] if db else []), "-e", statement]
        code, out = self._call(["mysql", f"--defaults-file={self.client_cnf}", *shown[1:]])
        if note:
            print(f"-- {note}")
        _show(shown, out)
        found = sum(1 for line in out.splitlines() if line.startswith("ERROR "))
        if found != errors or (code != 0) != (errors > 0):
            sys.exit(f"{statement}: {found} errors and status {code}; the session expects {errors}")
        return out

    def write_file(self, path, text, show=True):
        """Writes a text file into the run folder (an option file, a script) and prints it, as cat would."""
        (self.base / path).write_text(textwrap.dedent(text).lstrip("\n"))
        if show:
            self.show_file(path)

    def show_file(self, path):
        print(f"$ cat {path}")
        print(self.clean((self.base / path).read_text()).rstrip("\n"))
        print()

    def listing(self, folder, prefix=""):
        """Prints the names in a folder of the run (sorted, folders with a slash), as `ls -p` would list them."""
        path = self.base / folder
        names = sorted(n + ("/" if (path / n).is_dir() else "") for n in os.listdir(path) if n.startswith(prefix))
        print(f"$ ls -p {folder}" + (f" | grep '^{prefix}'" if prefix else ""))
        print(textwrap.fill("  ".join(names), width=78, break_on_hyphens=False))
        print()
        return names


def _redraw(block, clean):
    """Draws a client table again after its cells were cleaned. The columns come from the first border line; the row
    after it is the header (left-aligned); a column whose data cells were padded on the left (numbers) stays
    right-aligned (an empty cell says nothing about its column)."""
    cuts = [k for k, ch in enumerate(block[0]) if ch == "+"]
    rows = [None if line.startswith("+") else [line[x + 1:y] for x, y in zip(cuts, cuts[1:])] for line in block]
    header = next(k for k, r in enumerate(rows) if r is not None)
    ncol = len(cuts) - 1
    right = [any(r is not None and k != header and r[c].strip() and r[c].startswith("  ") for k, r in enumerate(rows))
             for c in range(ncol)]
    cells = [None if r is None else [clean(x.strip()) for x in r] for r in rows]
    widths = [max(len(r[c]) for r in cells if r is not None) for c in range(ncol)]
    drawn = []
    for k, r in enumerate(cells):
        if r is None:
            drawn.append("+" + "+".join("-" * (w + 2) for w in widths) + "+")
        else:
            drawn.append("|" + "|".join(" " + (t.rjust(w) if right[c] and k != header else t.ljust(w)) + " "
                                        for c, (t, w) in enumerate(zip(r, widths))) + "|")
    return drawn


def _arg(a):
    """A command-line argument as a reader would type it: quotes only where the shell needs them."""
    a = str(a)
    if re.fullmatch(r"[\w@%+=:,./-]+", a):
        return a
    if not re.search(r"[\"$`\\!]", a):
        return f'"{a}"'
    return shlex.quote(a)


def _show(args, out):
    print("$ " + " ".join(_arg(a) for a in args))
    if out.strip():
        print(out.rstrip("\n"))
    print()


# ====================================================================================================================
# Concurrency for the MVCC and locking lessons (undo-logs-mvcc-and-purge, row-gap-next-key-locks).
# Two or more persistent `mysql` clients keep their own transactions open at the same time, so a session can show what
# one transaction sees of another's uncommitted or committed changes, and the record, gap and next-key locks a
# statement takes. One statement can be started without waiting (it blocks on a lock); an observer connection polls
# performance_schema until the wait is visible; the blocked statement then finishes (with its result or its
# ERROR 1205 / ERROR 1213). Output is masked so two runs are byte-identical.
# ====================================================================================================================
import threading
import pty
import termios
import struct
import fcntl

_PROMPT = "READYmk> "   # a distinctive mysql prompt we use as the boundary between statements


class Client:
    """A persistent interactive mysql client on a pseudo-terminal: one connection whose transaction lives across
    calls. Built by Server.client(name). run() sends one statement, waits for the next prompt and prints the result;
    start() sends a statement expected to block and returns at once; finish() waits for that blocked statement.
    A pseudo-terminal is used because the mysql client block-buffers its output to a plain pipe, which would hide the
    result until the process ends; on a terminal it flushes a prompt after every statement. Terminal echo is off (the
    statement is printed by the session, not by the terminal) and the window is made wide so tables never wrap."""

    def __init__(self, server, name):
        self.server = server
        self.name = name
        self.pty_fd, tty_fd = pty.openpty()
        attrs = termios.tcgetattr(tty_fd)
        attrs[3] = attrs[3] & ~termios.ECHO          # lflags: no echo of what we type
        termios.tcsetattr(tty_fd, termios.TCSANOW, attrs)
        fcntl.ioctl(tty_fd, termios.TIOCSWINSZ, struct.pack("HHHH", 100, 1000, 0, 0))  # 1000 columns, no wrap
        args = [str(server.bin / "mysql"), f"--defaults-file={server.client_cnf}",
                f"--prompt={_PROMPT}", "--skip-auto-rehash", "-t"]
        self.proc = subprocess.Popen(args, stdin=tty_fd, stdout=tty_fd, stderr=tty_fd,
                                     cwd=server.base, env=server.env, close_fds=True)
        os.close(tty_fd)
        self._buf = ""
        self._consumed = 0
        self._lock = threading.Lock()
        self._reader = threading.Thread(target=self._read, daemon=True)
        self._reader.start()
        self._read_until_prompt()          # discard the welcome banner up to the first prompt
        self.conn_id = self._value("SELECT CONNECTION_ID()")

    def _read(self):
        while True:
            try:
                data = os.read(self.pty_fd, 4096)
            except OSError:
                break
            if not data:
                break
            with self._lock:
                self._buf += data.decode("utf-8", "replace").replace("\r\n", "\n").replace("\r", "")

    def _send(self, sql):
        os.write(self.pty_fd, (sql.rstrip().rstrip(";") + ";\n").encode())

    def _read_until_prompt(self, timeout=60):
        deadline = time.time() + timeout
        while time.time() < deadline:
            with self._lock:
                idx = self._buf.find(_PROMPT, self._consumed)
                if idx != -1:
                    block = self._buf[self._consumed:idx]
                    self._consumed = idx + len(_PROMPT)
                    return block
            time.sleep(0.02)
        return None

    def _collect(self, sql, timeout=60):
        self._send(sql)
        block = self._read_until_prompt(timeout=timeout)
        if block is None:
            sys.exit(f"{self.name}: statement did not finish within {timeout}s: {sql}")
        return block

    def _value(self, sql):
        text = self.server.clean(self._collect(sql))
        cells = [l for l in text.splitlines() if l.startswith("|")]
        return cells[-1].split("|")[1].strip() if len(cells) >= 2 else ""

    def run(self, sql, note=None, errors=0):
        """Run a statement to completion and print it as the mysql client showed it."""
        out = self._collect(sql)
        self._print(sql, out, note)
        self._check(sql, out, errors)
        return out

    def start(self, sql, note=None):
        """Send a statement expected to block on a lock; return at once. finish() collects its output."""
        self._pending = (sql, note)
        self._send(sql)

    def finish(self, errors=0, timeout=60):
        """Wait for the statement start() sent and print it (its result, or its ERROR 1205/1213)."""
        sql, note = self._pending
        block = self._read_until_prompt(timeout=timeout)
        if block is None:
            sys.exit(f"{self.name}: blocked statement did not finish within {timeout}s: {sql}")
        self._print(sql, block, note)
        self._check(sql, block, errors)
        return block

    def _check(self, sql, out, errors):
        found = sum(1 for l in out.splitlines() if l.startswith("ERROR "))
        if found != errors:
            sys.exit(f"{self.name}: {found} errors on {sql!r}, expected {errors}")

    def _print(self, sql, out, note):
        print(f"-- {self.name}" + (f": {note}" if note else ""))
        print(f"mysql> {sql.rstrip().rstrip(';')};")
        text = self.server.clean(out).strip("\n")
        if text:
            print(text)
        print()

    def close(self):
        try:
            os.write(self.pty_fd, b"\\q\n")
        except OSError:
            pass
        try:
            self.proc.wait(timeout=10)
        except subprocess.TimeoutExpired:
            self.proc.terminate()
        try:
            os.close(self.pty_fd)
        except OSError:
            pass


# ---- Server helpers for the concurrency lessons --------------------------------------------------------------------
def _server_client(self, name):
    """A new persistent connection whose transaction lives across statements."""
    if not hasattr(self, "_clients"):
        self._clients = []
    c = Client(self, name)
    self._clients.append(c)
    return c


def _server_wait_for(self, sql, want, timeout=20):
    """Poll a single-value query (through a short-lived connection) until it returns `want` (as text)."""
    want = str(want)
    deadline = time.time() + timeout
    while time.time() < deadline:
        if self.query(sql) == want:
            return True
        time.sleep(0.05)
    sys.exit(f"wait_for timed out: {sql} never became {want}")


def _server_locks(self, note=None, where="1=1"):
    """Print performance_schema.data_locks, only the columns that do not change between runs, ordered stably."""
    sql = ("SELECT INDEX_NAME, LOCK_TYPE, LOCK_MODE, LOCK_STATUS, LOCK_DATA "
           "FROM performance_schema.data_locks "
           f"WHERE OBJECT_SCHEMA='test' AND ({where}) "
           "ORDER BY INDEX_NAME, FIELD(LOCK_MODE,'X','X,GAP','X,REC_NOT_GAP','X,GAP,INSERT_INTENTION','S','S,GAP'), "
           "LOCK_DATA")
    self.sql(sql, note=note)


def _server_status_deadlock(self, note=None):
    """The LATEST DETECTED DEADLOCK section of SHOW ENGINE INNODB STATUS, trimmed to the parts that do not change
    between runs: the two transactions, the locks each holds and waits for, and which one was rolled back. The
    per-record hex dumps and the run-varying ids, times and thread handles are left out or masked."""
    out = self.output("mysql", "-E", "-e", "SHOW ENGINE INNODB STATUS", connect=True)
    lines = out.splitlines()
    try:
        a = next(i for i, l in enumerate(lines) if "LATEST DETECTED DEADLOCK" in l)
        b = next(i for i in range(a + 2, len(lines)) if lines[i].startswith("-----------"))
    except StopIteration:
        a, b = 1, len(lines)
    kept, i = [], a - 1
    while i < b:
        l = lines[i]
        if l.startswith("Record lock, heap no"):      # skip the record's hex field dump up to the blank line
            while i < b and lines[i].strip():
                i += 1
            continue
        kept.append(l)
        i += 1
    text = "\n".join(kept)
    text = re.sub(r"^\d{4}-\d\d-\d\d \d\d:\d\d:\d\d .*$", "<date and time> <thread>", text, flags=re.M)
    text = re.sub(r"TRANSACTION \d+,", "TRANSACTION <id>,", text)
    text = re.sub(r"ACTIVE \d+ sec", "ACTIVE <n> sec", text)
    text = re.sub(r"heap size \d+", "heap size <n>", text)
    text = re.sub(r"MySQL thread id \d+, OS thread handle \w+, query id \d+",
                  "MySQL thread id <tid>, OS thread handle <h>, query id <q>", text)
    text = re.sub(r"trx id \d+", "trx id <id>", text)
    text = re.sub(r"\n{3,}", "\n\n", text)
    if note:
        print(f"-- {note}")
    print("mysql> SHOW ENGINE INNODB STATUS\\G   (LATEST DETECTED DEADLOCK section)")
    print(self.clean(text).strip("\n"))
    print()


Server.client = _server_client
Server.wait_for = _server_wait_for
Server.locks = _server_locks
Server.status_deadlock = _server_status_deadlock

sdi_session.py

"""A table's own definition, read out of its .ibd file with ibd2sdi, while the server is shut down.

The session creates one small table in a throwaway MySQL 8.4.11 server, shows the file the table lives in, stops the
server and then runs ibd2sdi on that file. ibd2sdi prints the whole serialized dictionary information of the
tablespace as JSON, a few hundred lines, so the session prints its first lines as the tool wrote them and then picks
two objects out of the same JSON: the table's name and the definition of one column.
"""

import json

from mysql_session import Server

KEEP = ("name", "type", "column_type_utf8", "ordinal_position", "char_length", "is_nullable", "column_key")

with Server() as my:
    my.up()
    my.mysql("sdi.sql")
    my.listing("data/shop")
    my.stop()                                   # ibd2sdi reads a file no running server is writing to
    dump = my.output("ibd2sdi", "data/shop/orders.ibd")

    print("$ ibd2sdi data/shop/orders.ibd   # the first lines, as the tool printed them")
    print("\n".join(dump.splitlines()[:12]))
    print(f"... {len(dump.splitlines())} lines in all\n")

    records = [r for r in json.loads(dump)[1:] if isinstance(r, dict)]
    table = next(r["object"]["dd_object"] for r in records if r["object"]["dd_object_type"] == "Table")
    columns = table["columns"]
    print("the records in this file:",
          ", ".join(f"{r['object']['dd_object_type']} (type {r['type']}, id {r['id']})" for r in records))
    print("the table it describes:", f"{table['schema_ref']}.{table['name']}, {len(columns)} columns,",
          f"{len(table['indexes'])} indexes, engine {table['engine']}")
    print("its columns, in the order the record lists them:", ", ".join(c["name"] for c in columns))
    print("\nthe second column, with the keys this program keeps:")
    print(json.dumps({k: v for k, v in columns[1].items() if k in KEEP}, indent=2))

$ mysql -t -vvv --skip-comments < sdi.sql
--------------
CREATE DATABASE shop
--------------

Query OK, 1 row affected

--------------
CREATE TABLE shop.orders (
  id        BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
  reference CHAR(12)        NOT NULL,
  amount    DECIMAL(10, 2)  NOT NULL,
  placed_at DATETIME        NOT NULL,
  UNIQUE KEY by_reference (reference)
)
--------------

Query OK, 0 rows affected

--------------
INSERT INTO shop.orders (reference, amount, placed_at)
VALUES ('MSC-00000001', 1250.00, '2031-03-04 09:15:00')
--------------

Query OK, 1 row affected

--------------
SELECT NAME, SPACE_TYPE, PAGE_SIZE FROM information_schema.INNODB_TABLESPACES WHERE NAME = 'shop/orders'
--------------

+-------------+------------+-----------+
| NAME        | SPACE_TYPE | PAGE_SIZE |
+-------------+------------+-----------+
| shop/orders | Single     |     16384 |
+-------------+------------+-----------+
1 row in set

Bye

$ ls -p data/shop
orders.ibd

$ ibd2sdi data/shop/orders.ibd   # the first lines, as the tool printed them
["ibd2sdi"
,
{
	"type": 1,
	"id": 366,
	"object":
		{
    "mysqld_version_id": 80411,
    "dd_version": 80300,
    "sdi_version": 80019,
    "dd_object_type": "Table",
    "dd_object": {
... 396 lines in all

the records in this file: Table (type 1, id 366), Tablespace (type 2, id 7)
the table it describes: shop.orders, 6 columns, 2 indexes, engine InnoDB
its columns, in the order the record lists them: id, reference, amount, placed_at, DB_TRX_ID, DB_ROLL_PTR

the second column, with the keys this program keeps:
{
  "name": "reference",
  "type": 29,
  "is_nullable": false,
  "ordinal_position": 2,
  "char_length": 48,
  "column_key": 3,
  "column_type_utf8": "char(12)"
}

sdi.sql

CREATE DATABASE shop;
CREATE TABLE shop.orders (
  id        BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
  reference CHAR(12)        NOT NULL,
  amount    DECIMAL(10, 2)  NOT NULL,
  placed_at DATETIME        NOT NULL,
  UNIQUE KEY by_reference (reference)
);
INSERT INTO shop.orders (reference, amount, placed_at)
VALUES ('MSC-00000001', 1250.00, '2031-03-04 09:15:00');

-- The tablespace of the new table, as the server sees it.
SELECT NAME, SPACE_TYPE, PAGE_SIZE FROM information_schema.INNODB_TABLESPACES WHERE NAME = 'shop/orders';

mysql_session.py

"""Throwaway MySQL 8.4.11 servers for the examples of this lesson.

Every session creates a server of its own: `mysqld --initialize-insecure` makes a new data directory inside a new
folder in the examples folder, the server starts on it, the session runs `mysql` client scripts and MySQL's programs
against it and prints what they printed, and then the server is stopped and the folder deleted, also when something
fails. The server is the pinned MySQL 8.4.11 from the mise installations next to the Python that runs this file;
Server(basedir=...) runs the programs of another unpacked MySQL tarball instead (dump_defaults.py uses it).

Safety: the server listens only on 127.0.0.1, an address no other computer can reach, on a port the operating system
reports as unused; the X Protocol listener is off (--mysqlx=OFF); its Unix socket, temporary files, error log and
import/export folder all stay inside the run folder. Right after the first start the session gives root a password made
up for the run (never printed) and connects only with a client option file that holds it, so no password ever appears
on a command line. Every program starts with --no-defaults or --defaults-file, so no option file of the computer
(/etc/my.cnf, ~/.my.cnf) can change a result, and HOME points at the run folder, so ~/.mylogin.cnf is not read either.
A printed mysqld command shows the options that matter to the lesson; the session also passes, on the command line,
the folders it keeps inside the run folder (--basedir, --tmpdir, --log-error, --pid-file, --secure-file-priv) and
the port, which is why performance_schema.variables_info reports those variables as COMMAND_LINE.
learn:verify's run sandbox cannot read the home folder, where mise keeps MySQL, so these sessions are recorded with
LEARN_VERIFY_NETWORK=allow; they read only this folder and the toolchain and connect to nothing but 127.0.0.1.

The output is exactly what the programs printed, with these substitutions so that a record never shows the computer it
ran on and is the same on every run: the run folder is shown as ".", the folder of MySQL's programs is left out of
their paths and MySQL's own folder is shown as <basedir>, the computer's name is shown as "lab-host" and the operating-system user's name as "learner", the time a
statement took (the client's "(0.01 sec)") is left out, the time stamp SET PERSIST writes into mysqld-auto.cnf is
shown as <when it was set>, error-log lines lose their timestamp and thread number, and a session can name more values
that change between runs with my.mask(pattern, replacement), which then prints in their place. A client table whose cells
change is drawn again so that its borders still line up.
The sessions never print the password, and the run's port prints as <port>. A session stops with status 1 when a command that should have worked
fails, or when a script reports a different number of errors than the session expects, so a run that went wrong is
never recorded as if it had worked.
"""

import os
import pwd
import re
import secrets
import shlex
import shutil
import socket
import subprocess
import sys
import tempfile
import textwrap
import time
from pathlib import Path

MYSQL_VERSION = "8.4.11"
# .../installs/python/3.14.8/bin/python3 -> .../installs
INSTALLS = Path(sys.executable).resolve().parents[3]
PINNED = INSTALLS / "mysql" / MYSQL_VERSION
OS_USER = pwd.getpwuid(os.geteuid()).pw_name
HOST = socket.gethostname().split(".")[0]
# 2026-10-17T16:50:01.123456Z 0 [ERROR] ... -> [ERROR] ...
LOG_PREFIX = re.compile(r"^\d{4}-\d\d-\d\dT\d\d:\d\d:\d\d\.\d+Z \d+ ", re.M)


def free_port():
    """A TCP port on 127.0.0.1 that nothing listens on now."""
    with socket.socket() as probe:
        probe.bind(("127.0.0.1", 0))
        return probe.getsockname()[1]


class Server:
    """One throwaway server: initialise, start; stopped and deleted when the with block ends.

    options: extra mysqld options for every start (for example "--sql-mode=" or "--character-set-server=latin1").
    basedir: the folder of an unpacked MySQL tarball to run instead of the pinned 8.4.11.
    masks: (pattern, replacement) pairs, as for mask().
    """

    def __init__(self, options=(), redo=None, basedir=None, masks=()):
        self.options = [str(o) for o in options]
        self.redo = redo
        self.basedir = Path(basedir).resolve() if basedir else PINNED
        self.bin = self.basedir / "bin"
        self.masks = []
        for pattern, replacement in masks:
            self.mask(pattern, replacement)
        self.proc = None

    def mask(self, pattern, replacement):
        """From now on every printed line shows text matching the regular expression as the replacement: for a value
        that changes from run to run (an uptime, a time stamp), so that the recorded output stays the same."""
        self.masks.append((re.compile(pattern), replacement))

    # ---- the folder ------------------------------------------------------------------------------------------------
    def __enter__(self):
        if not (self.bin / "mysqld").exists():
            sys.exit(f"no MySQL server program in {self.basedir}" if self.basedir != PINNED
                     else f"MySQL {MYSQL_VERSION} was not found next to this Python")
        self.base = Path(tempfile.mkdtemp(prefix="my", dir=".")).resolve()
        for sub in ("tmp", "files"):
            (self.base / sub).mkdir(mode=0o700)
        self.port = free_port()
        self.password = secrets.token_urlsafe(24)
        self.client_cnf = self.base / "client.cnf"
        self._write_client_cnf(password=None)
        self.env = {key: value for key, value in os.environ.items()
                    if not key.startswith("MYSQL") and key not in ("TZ", "HOME", "LANG", "LC_ALL")}
        self.env.update({"HOME": str(self.base), "TZ": "UTC", "LANG": "en_US.UTF-8",
                         "MYSQL_TEST_LOGIN_FILE": str(self.base / "no-login-paths.cnf")})
        for pattern in ("*.sql", "*.cnf", "*.csv", "*.txt"):
            for f in Path(".").glob(pattern):
                shutil.copy(f, self.base / f.name)
        return self

    def __exit__(self, *exc):
        if self.proc and self.proc.poll() is None:
            self.stop(quiet=True)
        shutil.rmtree(self.base, ignore_errors=True)

    def _write_client_cnf(self, password):
        lines = ["[client]", "user=root", "host=127.0.0.1", f"port={self.port}", "protocol=TCP"]
        if password:
            lines.append(f"password={password}")
        self.client_cnf.write_text("\n".join(lines) + "\n")
        self.client_cnf.chmod(0o600)

    def server_options(self):
        b = self.base
        return ["--no-defaults", f"--basedir={self.basedir}", f"--datadir={b / 'data'}", f"--tmpdir={b / 'tmp'}",
                f"--log-error={b / 'mysqld.err'}", f"--pid-file={b / 'mysqld.pid'}",
                f"--secure-file-priv={b / 'files'}",
                *([f"--innodb-redo-log-capacity={self.redo}"] if self.redo else [])]

    # ---- the server ------------------------------------------------------------------------------------------------
    def initialize(self, show=False):
        """mysqld --initialize-insecure: a new data directory whose root account has no password yet."""
        args = ["mysqld", *self.server_options(), "--initialize-insecure"]
        code, out = self._call(args)
        if show:
            _show(["mysqld", "--no-defaults", "--initialize-insecure", "--datadir=./data"], out)
        if code != 0:
            print(self.log())
            sys.exit(f"mysqld --initialize-insecure ended with status {code}")

    def start(self, extra=(), defaults_file=None, expect_fail=False, show=False):
        """Starts mysqld on 127.0.0.1 and waits until it answers. With expect_fail, prints the error-log lines of a
        start that must fail, and returns them."""
        log = self.base / "mysqld.err"
        before = log.read_text() if log.exists() else ""
        opts = self.server_options()
        if defaults_file:
            opts[0] = f"--defaults-file={self.base / defaults_file}"
        args = [str(self.bin / "mysqld"), *opts, "--bind-address=127.0.0.1", f"--port={self.port}",
                "--socket=mysql.sock", "--mysqlx=OFF", *self.options, *[str(e) for e in extra]]
        if show:
            shown = ["mysqld", opts[0].replace(str(self.base) + "/", ""), "--datadir=./data", "--bind-address=127.0.0.1",
                     "--port=$PORT", "--socket=mysql.sock", "--mysqlx=OFF", *self.options, *extra]
            print("$ " + " ".join(_arg(a) if a != "--port=$PORT" else a for a in shown) + " &")
            print()
        with open(self.base / "mysqld.out", "a") as out:
            self.proc = subprocess.Popen(args, stdout=out, stderr=subprocess.STDOUT, cwd=self.base, env=self.env)
        for _ in range(600):
            if self.proc.poll() is not None:
                break
            if self._ping():
                break
            time.sleep(0.1)
        if self.proc.poll() is not None:
            new = (log.read_text() if log.exists() else "")[len(before):]
            lines = [l for l in self.clean(LOG_PREFIX.sub("", new)).splitlines() if "[ERROR]" in l]
            if expect_fail:
                print("\n".join(lines))
                print(f"(mysqld ended with status {self.proc.returncode})")
                print()
                self.proc = None
                return lines
            print("\n".join(lines))
            sys.exit(f"mysqld ended with status {self.proc.returncode} while starting")
        if expect_fail:
            sys.exit("mysqld started, but the session expected it to fail")
        if not self._ping():
            sys.exit("mysqld did not answer within 60 seconds")
        return []

    def _ping(self):
        code, _ = self._call(["mysqladmin", f"--defaults-file={self.client_cnf}", "--connect-timeout=2", "ping"])
        return code == 0

    def up(self, extra=(), defaults_file=None, show=False):
        """Initialise, start (with start()'s options), and give root the run's password, quietly."""
        self.initialize()
        self.start(extra=extra, defaults_file=defaults_file, show=show)
        return self.login()

    def login(self):
        """Gives root the run's password (never printed) on a server started from a new data directory, and makes
        every later client of the session use it."""
        self.silent(f"ALTER USER 'root'@'localhost' IDENTIFIED BY '{self.password}'")
        self._write_client_cnf(self.password)
        return self

    def stop(self, quiet=False):
        """A clean shutdown (mysqladmin shutdown), then waits until the process has ended."""
        code, out = self._call(["mysqladmin", f"--defaults-file={self.client_cnf}", "shutdown"])
        if code != 0 and not quiet:
            sys.exit(f"mysqladmin shutdown failed: {out}")
        try:
            self.proc.wait(timeout=120)
        except subprocess.TimeoutExpired:
            self.proc.terminate()
            self.proc.wait(timeout=60)
        self.proc = None

    def restart(self, extra=(), defaults_file=None, show=False):
        self.stop()
        self.start(extra=extra, defaults_file=defaults_file, show=show)

    # ---- running things --------------------------------------------------------------------------------------------
    def _call(self, args, stdin=None, merge=True):
        """Runs one of MySQL's programs in the run folder: (status, cleaned output). merge=False keeps only what it
        wrote to standard output (its warnings on standard error are dropped)."""
        args = [str(a) for a in args]
        done = subprocess.run([str(self.bin / args[0]), *args[1:]], stdout=subprocess.PIPE,
                              stderr=subprocess.STDOUT if merge else subprocess.DEVNULL,
                              text=True, cwd=self.base, env=self.env, input=stdin)
        return done.returncode, self.clean(done.stdout)

    def output(self, *args, connect=False, status=0):
        """Runs one of MySQL's programs without printing anything and returns its standard output, for example the
        SQL that mysql_tzinfo_to_sql writes, which the session saves with write_file(..., show=False) and runs next."""
        args = [str(a) for a in args]
        run = [args[0], f"--defaults-file={self.client_cnf}", *args[1:]] if connect else args
        code, out = self._call(run, merge=False)
        if code != status:
            sys.exit(f"{args[0]} ended with status {code}, not {status}")
        return out

    def clean(self, text):
        """The substitutions of the docstring. A table of the client whose cells change is drawn again with the
        new column widths, so its borders still line up."""
        lines, out, i = text.split("\n"), [], 0
        while i < len(lines):
            if lines[i].startswith("+-"):
                j = i
                while j < len(lines) and lines[j][:1] in ("+", "|"):
                    j += 1
                block = lines[i:j]
                done = [self._clean(l) for l in block]
                out.extend(done if done == block else _redraw(block, self._clean))
                i = j
            else:
                out.append(self._clean(lines[i]))
                i += 1
        return "\n".join(out)

    def _clean(self, text):
        text = text.replace(str(self.bin) + "/", "").replace(str(self.basedir), "<basedir>")
        text = text.replace(str(self.base) + "/", "./").replace(str(self.base), ".")
        text = text.replace("\x07", "")   # the mysql client rings the terminal bell on an error; drop it
        text = re.sub(r" \(\d+\.\d\d sec\)", "", text)
        text = re.sub(r'"Timestamp": \d+', '"Timestamp": <when it was set>', text)
        text = re.sub(rf"\b{re.escape(HOST)}\b", "lab-host", text)
        text = re.sub(rf"\b{re.escape(OS_USER)}\b", "learner", text)
        text = re.sub(rf"(?<![\d.]){self.port}(?![\d.])", "<port>", text)
        for pattern, replacement in self.masks:
            text = pattern.sub(replacement, text)
        return text

    def log(self, pattern=None):
        """The error log without timestamps and thread numbers, optionally only the lines matching a pattern."""
        text = self.clean(LOG_PREFIX.sub("", (self.base / "mysqld.err").read_text()))
        if pattern:
            text = "\n".join(l for l in text.splitlines() if re.search(pattern, l))
        return text

    def show_log(self, pattern, note=None):
        """Prints the error-log lines that match a pattern, as grep would."""
        print(f"$ grep -E {_arg(pattern)} mysqld.err" + (f"   # {note}" if note else ""))
        print(self.log(pattern))
        print()

    def query(self, sql, db=None):
        """One value from one query, unprinted (the session uses it to wait or to decide)."""
        args = ["mysql", f"--defaults-file={self.client_cnf}", "-N", "-B", *([db] if db else [])]
        code, out = self._call(args, stdin=sql)
        if code != 0:
            sys.exit(f"the query failed: {out}")
        return out.strip()

    def silent(self, sql):
        """Runs a statement without printing it (used only for the run's password)."""
        self.query(sql)

    def mysql(self, script, errors=0, status=None, db=None, flags=("-t", "-vvv", "--skip-comments"), note=None, force=None):
        """Runs a client script from standard input and prints the command and its output. `errors` is how many
        ERROR lines the script must print; with errors > 0 the client runs on past an error (--force) and then ends
        with status 0, without --force it stops at the first error with status 1. The default flags print each
        statement (-vvv) as a table (-t) without the script's comments (--skip-comments)."""
        force = errors > 0 if force is None else force
        shown = ["mysql", *flags, *(["--force"] if force else []), *([db] if db else [])]
        args = ["mysql", f"--defaults-file={self.client_cnf}", *shown[1:]]
        code, out = self._call(args, stdin=(self.base / script).read_text())
        if note:
            print(f"-- {note}")
        print("$ " + " ".join(_arg(a) for a in shown) + f" < {script}")
        if out.strip():
            print(out.rstrip("\n"))
        print()
        found = sum(1 for line in out.splitlines() if line.startswith("ERROR "))
        expected_status = (1 if errors and not force else 0) if status is None else status
        if found != errors or code != expected_status:
            sys.exit(f"{script}: {found} errors and status {code}; the session expects {errors} and {expected_status}")
        return out

    def shell(self, *args, status=0, shown=None, connect=False):
        """Runs one of MySQL's programs and prints the command and its output. connect=True gives a client program
        (mysqladmin, mysqlshow, mysql) the run's client option file, which the printed command leaves out."""
        args = [str(a) for a in args]
        run = [args[0], f"--defaults-file={self.client_cnf}", *args[1:]] if connect else args
        code, out = self._call(run)
        _show(shown or args, out)
        if code != status:
            sys.exit(f"{args[0]} ended with status {code}, not {status}")
        return out

    def sql(self, statement, flags=("-t",), errors=0, db=None, note=None):
        """Runs one statement with mysql -e and prints the command and its output."""
        shown = ["mysql", *flags, *([db] if db else []), "-e", statement]
        code, out = self._call(["mysql", f"--defaults-file={self.client_cnf}", *shown[1:]])
        if note:
            print(f"-- {note}")
        _show(shown, out)
        found = sum(1 for line in out.splitlines() if line.startswith("ERROR "))
        if found != errors or (code != 0) != (errors > 0):
            sys.exit(f"{statement}: {found} errors and status {code}; the session expects {errors}")
        return out

    def write_file(self, path, text, show=True):
        """Writes a text file into the run folder (an option file, a script) and prints it, as cat would."""
        (self.base / path).write_text(textwrap.dedent(text).lstrip("\n"))
        if show:
            self.show_file(path)

    def show_file(self, path):
        print(f"$ cat {path}")
        print(self.clean((self.base / path).read_text()).rstrip("\n"))
        print()

    def listing(self, folder, prefix=""):
        """Prints the names in a folder of the run (sorted, folders with a slash), as `ls -p` would list them."""
        path = self.base / folder
        names = sorted(n + ("/" if (path / n).is_dir() else "") for n in os.listdir(path) if n.startswith(prefix))
        print(f"$ ls -p {folder}" + (f" | grep '^{prefix}'" if prefix else ""))
        print(textwrap.fill("  ".join(names), width=78, break_on_hyphens=False))
        print()
        return names


def _redraw(block, clean):
    """Draws a client table again after its cells were cleaned. The columns come from the first border line; the row
    after it is the header (left-aligned); a column whose data cells were padded on the left (numbers) stays
    right-aligned (an empty cell says nothing about its column)."""
    cuts = [k for k, ch in enumerate(block[0]) if ch == "+"]
    rows = [None if line.startswith("+") else [line[x + 1:y] for x, y in zip(cuts, cuts[1:])] for line in block]
    header = next(k for k, r in enumerate(rows) if r is not None)
    ncol = len(cuts) - 1
    right = [any(r is not None and k != header and r[c].strip() and r[c].startswith("  ") for k, r in enumerate(rows))
             for c in range(ncol)]
    cells = [None if r is None else [clean(x.strip()) for x in r] for r in rows]
    widths = [max(len(r[c]) for r in cells if r is not None) for c in range(ncol)]
    drawn = []
    for k, r in enumerate(cells):
        if r is None:
            drawn.append("+" + "+".join("-" * (w + 2) for w in widths) + "+")
        else:
            drawn.append("|" + "|".join(" " + (t.rjust(w) if right[c] and k != header else t.ljust(w)) + " "
                                        for c, (t, w) in enumerate(zip(r, widths))) + "|")
    return drawn


def _arg(a):
    """A command-line argument as a reader would type it: quotes only where the shell needs them."""
    a = str(a)
    if re.fullmatch(r"[\w@%+=:,./-]+", a):
        return a
    if not re.search(r"[\"$`\\!]", a):
        return f'"{a}"'
    return shlex.quote(a)


def _show(args, out):
    print("$ " + " ".join(_arg(a) for a in args))
    if out.strip():
        print(out.rstrip("\n"))
    print()


# ====================================================================================================================
# Concurrency for the MVCC and locking lessons (undo-logs-mvcc-and-purge, row-gap-next-key-locks).
# Two or more persistent `mysql` clients keep their own transactions open at the same time, so a session can show what
# one transaction sees of another's uncommitted or committed changes, and the record, gap and next-key locks a
# statement takes. One statement can be started without waiting (it blocks on a lock); an observer connection polls
# performance_schema until the wait is visible; the blocked statement then finishes (with its result or its
# ERROR 1205 / ERROR 1213). Output is masked so two runs are byte-identical.
# ====================================================================================================================
import threading
import pty
import termios
import struct
import fcntl

_PROMPT = "READYmk> "   # a distinctive mysql prompt we use as the boundary between statements


class Client:
    """A persistent interactive mysql client on a pseudo-terminal: one connection whose transaction lives across
    calls. Built by Server.client(name). run() sends one statement, waits for the next prompt and prints the result;
    start() sends a statement expected to block and returns at once; finish() waits for that blocked statement.
    A pseudo-terminal is used because the mysql client block-buffers its output to a plain pipe, which would hide the
    result until the process ends; on a terminal it flushes a prompt after every statement. Terminal echo is off (the
    statement is printed by the session, not by the terminal) and the window is made wide so tables never wrap."""

    def __init__(self, server, name):
        self.server = server
        self.name = name
        self.pty_fd, tty_fd = pty.openpty()
        attrs = termios.tcgetattr(tty_fd)
        attrs[3] = attrs[3] & ~termios.ECHO          # lflags: no echo of what we type
        termios.tcsetattr(tty_fd, termios.TCSANOW, attrs)
        fcntl.ioctl(tty_fd, termios.TIOCSWINSZ, struct.pack("HHHH", 100, 1000, 0, 0))  # 1000 columns, no wrap
        args = [str(server.bin / "mysql"), f"--defaults-file={server.client_cnf}",
                f"--prompt={_PROMPT}", "--skip-auto-rehash", "-t"]
        self.proc = subprocess.Popen(args, stdin=tty_fd, stdout=tty_fd, stderr=tty_fd,
                                     cwd=server.base, env=server.env, close_fds=True)
        os.close(tty_fd)
        self._buf = ""
        self._consumed = 0
        self._lock = threading.Lock()
        self._reader = threading.Thread(target=self._read, daemon=True)
        self._reader.start()
        self._read_until_prompt()          # discard the welcome banner up to the first prompt
        self.conn_id = self._value("SELECT CONNECTION_ID()")

    def _read(self):
        while True:
            try:
                data = os.read(self.pty_fd, 4096)
            except OSError:
                break
            if not data:
                break
            with self._lock:
                self._buf += data.decode("utf-8", "replace").replace("\r\n", "\n").replace("\r", "")

    def _send(self, sql):
        os.write(self.pty_fd, (sql.rstrip().rstrip(";") + ";\n").encode())

    def _read_until_prompt(self, timeout=60):
        deadline = time.time() + timeout
        while time.time() < deadline:
            with self._lock:
                idx = self._buf.find(_PROMPT, self._consumed)
                if idx != -1:
                    block = self._buf[self._consumed:idx]
                    self._consumed = idx + len(_PROMPT)
                    return block
            time.sleep(0.02)
        return None

    def _collect(self, sql, timeout=60):
        self._send(sql)
        block = self._read_until_prompt(timeout=timeout)
        if block is None:
            sys.exit(f"{self.name}: statement did not finish within {timeout}s: {sql}")
        return block

    def _value(self, sql):
        text = self.server.clean(self._collect(sql))
        cells = [l for l in text.splitlines() if l.startswith("|")]
        return cells[-1].split("|")[1].strip() if len(cells) >= 2 else ""

    def run(self, sql, note=None, errors=0):
        """Run a statement to completion and print it as the mysql client showed it."""
        out = self._collect(sql)
        self._print(sql, out, note)
        self._check(sql, out, errors)
        return out

    def start(self, sql, note=None):
        """Send a statement expected to block on a lock; return at once. finish() collects its output."""
        self._pending = (sql, note)
        self._send(sql)

    def finish(self, errors=0, timeout=60):
        """Wait for the statement start() sent and print it (its result, or its ERROR 1205/1213)."""
        sql, note = self._pending
        block = self._read_until_prompt(timeout=timeout)
        if block is None:
            sys.exit(f"{self.name}: blocked statement did not finish within {timeout}s: {sql}")
        self._print(sql, block, note)
        self._check(sql, block, errors)
        return block

    def _check(self, sql, out, errors):
        found = sum(1 for l in out.splitlines() if l.startswith("ERROR "))
        if found != errors:
            sys.exit(f"{self.name}: {found} errors on {sql!r}, expected {errors}")

    def _print(self, sql, out, note):
        print(f"-- {self.name}" + (f": {note}" if note else ""))
        print(f"mysql> {sql.rstrip().rstrip(';')};")
        text = self.server.clean(out).strip("\n")
        if text:
            print(text)
        print()

    def close(self):
        try:
            os.write(self.pty_fd, b"\\q\n")
        except OSError:
            pass
        try:
            self.proc.wait(timeout=10)
        except subprocess.TimeoutExpired:
            self.proc.terminate()
        try:
            os.close(self.pty_fd)
        except OSError:
            pass


# ---- Server helpers for the concurrency lessons --------------------------------------------------------------------
def _server_client(self, name):
    """A new persistent connection whose transaction lives across statements."""
    if not hasattr(self, "_clients"):
        self._clients = []
    c = Client(self, name)
    self._clients.append(c)
    return c


def _server_wait_for(self, sql, want, timeout=20):
    """Poll a single-value query (through a short-lived connection) until it returns `want` (as text)."""
    want = str(want)
    deadline = time.time() + timeout
    while time.time() < deadline:
        if self.query(sql) == want:
            return True
        time.sleep(0.05)
    sys.exit(f"wait_for timed out: {sql} never became {want}")


def _server_locks(self, note=None, where="1=1"):
    """Print performance_schema.data_locks, only the columns that do not change between runs, ordered stably."""
    sql = ("SELECT INDEX_NAME, LOCK_TYPE, LOCK_MODE, LOCK_STATUS, LOCK_DATA "
           "FROM performance_schema.data_locks "
           f"WHERE OBJECT_SCHEMA='test' AND ({where}) "
           "ORDER BY INDEX_NAME, FIELD(LOCK_MODE,'X','X,GAP','X,REC_NOT_GAP','X,GAP,INSERT_INTENTION','S','S,GAP'), "
           "LOCK_DATA")
    self.sql(sql, note=note)


def _server_status_deadlock(self, note=None):
    """The LATEST DETECTED DEADLOCK section of SHOW ENGINE INNODB STATUS, trimmed to the parts that do not change
    between runs: the two transactions, the locks each holds and waits for, and which one was rolled back. The
    per-record hex dumps and the run-varying ids, times and thread handles are left out or masked."""
    out = self.output("mysql", "-E", "-e", "SHOW ENGINE INNODB STATUS", connect=True)
    lines = out.splitlines()
    try:
        a = next(i for i, l in enumerate(lines) if "LATEST DETECTED DEADLOCK" in l)
        b = next(i for i in range(a + 2, len(lines)) if lines[i].startswith("-----------"))
    except StopIteration:
        a, b = 1, len(lines)
    kept, i = [], a - 1
    while i < b:
        l = lines[i]
        if l.startswith("Record lock, heap no"):      # skip the record's hex field dump up to the blank line
            while i < b and lines[i].strip():
                i += 1
            continue
        kept.append(l)
        i += 1
    text = "\n".join(kept)
    text = re.sub(r"^\d{4}-\d\d-\d\d \d\d:\d\d:\d\d .*$", "<date and time> <thread>", text, flags=re.M)
    text = re.sub(r"TRANSACTION \d+,", "TRANSACTION <id>,", text)
    text = re.sub(r"ACTIVE \d+ sec", "ACTIVE <n> sec", text)
    text = re.sub(r"heap size \d+", "heap size <n>", text)
    text = re.sub(r"MySQL thread id \d+, OS thread handle \w+, query id \d+",
                  "MySQL thread id <tid>, OS thread handle <h>, query id <q>", text)
    text = re.sub(r"trx id \d+", "trx id <id>", text)
    text = re.sub(r"\n{3,}", "\n\n", text)
    if note:
        print(f"-- {note}")
    print("mysql> SHOW ENGINE INNODB STATUS\\G   (LATEST DETECTED DEADLOCK section)")
    print(self.clean(text).strip("\n"))
    print()


Server.client = _server_client
Server.wait_for = _server_wait_for
Server.locks = _server_locks
Server.status_deadlock = _server_status_deadlock