Invalidate rewritten pyc files by source hash

[RonnyPfannschmidt]

Supersedes and thus closes #11418, rebased onto main with @cdce8p's two commits preserved.

Rewritten pycs are invalidated by mtime+size. A checkout gives every source file
a new mtime, so a restored CI cache is thrown away in full and every test module
is rewritten again. This switches them to PEP 552 checked-hash pycs.

The third commit drops the --invalidation-mode option #11418 added and always
writes checked-hash — measured, hashing does not need a timestamp fast path.
500 assert-dense modules (~9 KB each), --collect-only, CPython 3.12:

	main	this PR
cold, rewrite everything	99.3 s	93.9 s
warm cache	5.85 s	5.96 s
after touching every source	93.8 s	5.96 s

Per file that is ~15 µs to read and hash the source, against ~1.1 ms for a cache
hit that unmarshals the code object anyway and ~110 ms for the rewrite it avoids.

Also fixes #13292: the pyc header records whole seconds only, so an edit made
within one second of the previous one, and of the same size, was invisible.

Notes:

• _imp.check_hash_based_pycs is not consulted. It governs how the builtin
  import system treats hash-based pycs, and it never sees these.
• Existing timestamp pycs read as "unsupported flags" and are rewritten once.
  PYTEST_TAG already contains the pytest version, so no two pytest versions
  share a pyc file.
• Private signatures change: _rewrite_test, _write_pyc and _write_pyc_fp
  no longer take or return an os.stat_result; the source stat is gone from
  both the read and the write path.

[Copilot]

Pull request overview

This PR updates pytest’s assertion-rewrite bytecode cache to use PEP 552 checked-hash .pyc headers, so rewritten modules remain valid across checkouts/cache restores that change mtimes and to avoid stale loads when edits occur within the same timestamp second.

Changes:

• Switch rewritten-pyc writing/reading from mtime+size invalidation to checked-hash invalidation (PEP 552).
• Update internal helpers (_rewrite_test, _write_pyc, _write_pyc_fp, _read_pyc) to operate on a computed source hash instead of os.stat_result.
• Expand tests and add changelog entries, including a regression test for edits within one mtime-second.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/_pytest/assertion/rewrite.py	Write/read checked-hash pycs and compute/verify source hashes for cache validity.
testing/test_assertrewrite.py	Update tests for new helper signatures; add coverage for mtime-only changes and same-second edits.
changelog/13292.bugfix.rst	Document fix for stale rewritten module when edits happen within the same second.
changelog/11418.improvement.rst	Document hash-based invalidation improving cache reuse across CI/checkouts.

Suppressed comments (2)

testing/test_assertrewrite.py:1412

• This test only checks pyc_bytes[4] == 3, which would still pass if the other flag bytes are wrong. It also uses hash as a variable name, which shadows the built-in hash(). Consider asserting the full 4-byte flags field and renaming the variable for clarity.

        hash, co = _rewrite_test(fn, config)
        _write_pyc(state, co, hash, pyc)
        assert _read_pyc(fn, pyc, state.trace) is not None

        pyc_bytes = pyc.read_bytes()

testing/test_assertrewrite.py:1435

• Avoid using hash as a local variable name here because it shadows the built-in hash() and can be confusing when reading the test. A name like src_hash is clearer.

        hash, co = _rewrite_test(fn, config)
        _write_pyc(state, co, hash, pyc)

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[Zac-HD]

lgtm, and nice perf improvement!