Translate WASI signal dispositions through libc sentinels - #9024

Scope

Translate Python's SIG_DFL and SIG_IGN values through the actual libc signal dispositions on WASI, and recognize libc's SIG_ERR before updating Python's handler table.

WASI uses function-table pointers for its signal sentinels. Treating them as the usual integer values lets an ignored synchronous signal trap and lets an uncatchable signal registration appear to succeed. A typed host boundary keeps Python's numeric values distinct from libc's default, ignored, and custom handlers.

This is a narrow Python 3.14 compatibility extraction from #8954. It does not include the separate wakeup-fd error-reporting changes.

Regression coverage

The existing signal snippet adds synchronous custom delivery, replacement with SIG_IGN, ignored delivery, and restoration. On WASI it also checks that SIGKILL/SIGSTOP registration raises OSError without changing the Python handler table. Existing canonical tests and assertions are unchanged.

Verification

Actual RustPython modules were built from current main bd8fdb7 and the isolated commit 59f9e3c, using the same source station, official wasm32-wasip1 target, and build flags:

cargo build --locked --profile wasm-release --target wasm32-wasip1 --no-default-features --features freeze-stdlib,stdlib,stdio,importlib,host_env,doc -j 2

Under Wasmtime 49.0.0:

• Baseline: raising an ignored signal traps with an indirect-call type mismatch; registering an uncatchable signal incorrectly succeeds.
• Fixed: both regressions pass, including handler-table preservation. Custom delivery and restoration pass.
• The unchanged canonical RaiseSignalTest.test_sigint and test_handler pass on both versions.
• The complete updated signal snippet passes when run directly as extra_tests/snippets/stdlib_signal.py by the fixed RustPython WASM module.

The updated snippet also passes the repository pytest runner on CPython 3.14.7 and the Linux RustPython baseline. Normal commit hooks pass. Native Linux Clippy with warnings denied, the workspace compile-only aggregate, and the separate C-API compile-only aggregate all pass on the exact isolated source tree. Broad runtime suites were deliberately not run during this extraction.

Reference implementations: CPython 3.14 signalmodule.c, wasi-libc signal constants, and wasi-libc signal implementation.

AI assistance: OpenAI Codex assisted with source comparison, implementation, regression design, and automated verification under the user's direction. No manual validation is claimed.