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[sqlite3] set threadsafety attribute based on default SQLite threaded mode #89776
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Currently, the sqlite3 DB-API 2.0 attribute 'threadsafety' is hard-coded to 1, meaning "threads may share the module, but not connections". This is not always true, since it depends on the default SQLite threaded mode, selected at compile-time with the SQLITE_THREADSAFE define.
SQLite's default compile-time threaded mode is SQLITE_THREADSAFE=1, also known as "serialized threaded mode", meaning SQLite can be safely used by multiple threads with no restriction. This mode equals DB-API 2.0 threadsafety level 3: threads may share the module, connections and cursors.
On macOS 11.6, the system supplied Python 3 and SQLite 3 uses SQLITE_THREADSAFE=2 (also known as "multi-thread mode"), meaning SQLite can be safely used by multiple threads provided that no single database connection is used simultaneously in two or more threads. This mode equals DB-API 2.0 threadsafety level 1: threads may share the module, but not connections.
With SQLITE_THREADSAFE=0 (also known as "single-thread mode"), meaning all mutexes are disabled and SQLite is unsafe to use in more than a single thread at once. This mode equals DB-API 2.0 threadsafety level 0: threads may not share the module.
Suggesting to set the 'treadsafety' dynamically at sqlite3 module load time, either via Lib/sqlite3/dbapi2.py, or in C during module init (slightly faster).
See also:
+1 on setting the attributes dynamically. Other DB-API modules use a
similar approach.FYI, it is also possible to change the threaded mode using sqlite3_config(), however that API is not exposed to the sqlite3 module, and there is no plans for doing so in the near future :)
BTW, I've verified that the overhead of adding this query is negligible.
$ python -m pyperf compare_to --min-speed 1 main.json patched.json Benchmark hidden because not significant (1): sqlite_synth
git switch -
Switched to branch 'main'
$ ./python.exe -m timeit "import sqlite3; sqlite3.threadsafety"
1000000 loops, best of 5: 225 nsec per loop
$ git switch -
Switched to branch 'sqlite-threadsafety'
$ ./python.exe -m timeit "import sqlite3; sqlite3.threadsafety"
1000000 loops, best of 5: 221 nsec per loop
sqlite3.threadsafetydynamically #29227Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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