If the underlying SQLite library sqlite3_threadsafe() function returns 1 (i.e. SERIALIZED) the sqlite3.threadsafety property returns 3, meaning that cursor can be shared between threads.
The sqlite3 module cannot really share cursors between threads, though.
Sometimes it ends up in check_cursor_locked(), more often it segfaults.
Given the dubious value of sharing cursors between threads, introducing the locks needed to support level 3 seems unnecessary.
Until #118172 is fixed, sqlite3.threadsafety should return 1 (share module only).
Reproducible with a test program similar to the one in #118172
import sqlite3
import threading
numthreads=10
KB = None
threadcursor = None
print(f"sqlite3.threadsafety: {sqlite3.threadsafety}")
def execute_query():
threadcursor.execute("SELECT * FROM test_table")
result = threadcursor.fetchall()
assert result == [(1, 'test1'), (2, 'test2'), (3, 'test3')], str(result)
return result
def run_threads():
global threadcursor
threadcursor = KB.cursor()
threads = []
for i in range(numthreads + 1):
thread = threading.Thread(target=execute_query)
threads.append(thread)
thread.start()
for thread in threads:
thread.join()
def test_multithreading():
global KB
KB = sqlite3.connect(
"example.db", check_same_thread=False
)
cursor = KB.cursor()
cursor.execute(
"""CREATE TABLE IF NOT EXISTS test_table (id INTEGER PRIMARY KEY, value TEXT)"""
)
KB.commit()
cursor.execute("""DELETE FROM test_table""")
KB.commit()
cursor.execute("""INSERT INTO test_table (value) VALUES ('test1')""")
cursor.execute("""INSERT INTO test_table (value) VALUES ('test2')""")
cursor.execute("""INSERT INTO test_table (value) VALUES ('test3')""")
KB.commit()
run_threads()
KB.close()
if __name__ == "__main__":
test_multithreading()
Bug report
Bug description:
If the underlying SQLite library sqlite3_threadsafe() function returns 1 (i.e. SERIALIZED) the sqlite3.threadsafety property returns 3, meaning that cursor can be shared between threads. The sqlite3 module cannot really share cursors between threads, though. Sometimes it ends up in check_cursor_locked(), more often it segfaults. Given the dubious value of sharing cursors between threads, introducing the locks needed to support level 3 seems unnecessary. Until #118172 is fixed, sqlite3.threadsafety should return 1 (share module only).
Reproducible with a test program similar to the one in #118172
CPython versions tested on:
CPython main branch
Operating systems tested on:
Linux