deps: uv: restore prior signal disposition on Android (DCP-4748)

[philippe-distributive]

Summary

When a Node environment is torn down inside an Android app process
(node::FreeEnvironment() → uv_close() on a UV_SIGNAL handle), libuv
resets the signal's disposition to SIG_DFL. On Android that sigaction
call is intercepted by ART's libsigchain, which logs an ERROR-level stack
trace on every worker stop:

E libsigchain: Setting SIGSEGV to SIG_DFL
E libsigchain:   ... uv_close ... node::Environment::CleanupHandles() ...

ART claims SIGSEGV/SIGBUS/SIGFPE/SIGILL/SIGTRAP/SIGABRT for implicit
null-checks, stack-overflow detection, etc., so resetting one to SIG_DFL
also clobbers ART's handler — semantically wrong, not just noisy.

Root cause

uv__signal_unregister_handler() unconditionally installs SIG_DFL once the
last watcher for a signal is removed. libuv never recorded what was installed
before it added its multiplexer — there's a long-standing
/* XXX save old action so we can restore it later on? */ TODO at the install
site.

Fix (gated to __ANDROID__)

On Android, uv__signal_register_handler() captures the previous
struct sigaction (into a per-signum slot) on the genuine
"no libuv handler → installed" transition — gated by a save flag so the
oneshot↔regular re-registration paths never overwrite it with libuv's own
handler. uv__signal_unregister_handler() then restores that saved
disposition instead of forcing SIG_DFL, falling back to SIG_DFL only when
nothing was captured.

The whole save/restore path is #ifdef __ANDROID__; off Android the code
path and behaviour are byte-for-byte unchanged. On Android the restored
handler is ART's (kept by libsigchain), so no SIG_DFL reset — and no
libsigchain report — occurs. Zero hot-path cost (only the first register /
last unregister per signum); the saved table is touched only under the
existing global signal lock.

Why Android-only

A cross-platform version of this was tried upstream (libuv nodejs#4216) and
reverted (nodejs#4302, refs nodejs#4299) because macOS doesn't propagate
SA_RESETHAND in the old action returned by sigaction() — so the captured
disposition is wrong there. bionic (Android) round-trips the flags correctly,
so gating to __ANDROID__ keeps the fix where it's both needed and correct
and leaves macOS/Linux/Windows on stock upstream behaviour. See the upstream
discussion in libuv#5157 (this change floated cross-platform) and libuv issue
nodejs#4299.

Alternatives rejected

• Apply the restore on all platforms — reintroduces the macOS
  SA_RESETHAND bug that got libuv gripe: deprecating fs.existsSync without a replacement that returns a boolean nodejs/node#4216 reverted (fix: workaround for possible V8 bug fixes #4266 nodejs/node#4299). Hence the
  __ANDROID__ gate.
• Raw rt_sigaction(SIG_DFL) syscall to bypass libsigchain — unsafe: it
  overwrites libsigchain's kernel-level handler (the one ART chains behind),
  silencing the log by breaking ART's signal handling.
• Leave libuv's multiplexer installed (never reset) — breaks the
  documented "stop watching → default behaviour restored" contract.

Testing

• libuv signal suite built for the host: 10/10 pass (incl.
  we_get_signals_mixed — the oneshot↔regular path the save gate guards —
  signal_pending_on_close, signal_close_loop_alive); non-Android compiles
  warning-free with stock SIG_DFL behaviour preserved.
• Cross-compiled libnode.so for all four Android ABIs (arm64-v8a, x86_64,
  armeabi-v7a, x86); the 32-bit pair via the ia32 build-host path.
• On an x86_64 emulator (API 35): worker start→stop, libsigchain "Setting SIGSEGV to SIG_DFL" count 1 → 0, with the cleanup path confirmed
  exercised (FreeEnvironment runs, libuv loop closed cleanly).

Upstreaming

The cross-platform variant is not currently acceptable upstream — it hits
the macOS oldact bug (nodejs#4299) that reverted nodejs#4216. Tracked in libuv#5157,
where an __ANDROID__-gated version is proposed; the door reopens if a
supported macOS (≥ 11) starts round-tripping the flags.

Rollout

Cut a tag (e.g. dcp/3.2.1) → bump node-build's GIT_TAG → rebuild the
Android node_api zips → bump the URL/SHA in
dcp-native/externals/node_api/CMakeLists.txt. (The four android-worker
jnilibs/*.so have already been rebuilt from this branch and validated on the
emulator.)

[philippe-distributive]

fixed upstream in libuv/libuv#5157