Add bun.sys.sigaction with bionic-correct layout for Android

[robobun]

Problem

std.c.Sigaction / std.c.sigset_t for .linux assume the glibc/musl layout: { handler, mask[128B], flags, restorer }. bionic LP64 is { int sa_flags; sa_handler; sigset_t sa_mask; sa_restorer } where sigset_t is a single unsigned long.

When Bun calls std.posix.sigaction() on Android, bionic reads sa_handler from offset 8 — which is Zig's mask[0]:

• Every handler installed with mask = sigemptyset() (SIGPIPE/SIGXFSZ in main, the crash handler, SIGINT in repl/filter_run/multi_run/Coordinator) silently becomes SIG_DFL. Broken-pipe kills the process, no crash trace on SEGV, Ctrl+C isn't caught.
• WaiterThread.reloadHandlers() sets mask[0] = 1<<16 (SIGCHLD), so bionic installs the handler 0x10000. When SIGCHLD fires, the kernel jumps there → SEGV_MAPERR, rip=0x10000, rdi=17. Reproduces 100% on a full Android emulator (not under qemu-user).

Fix

Add bun.sys.{Sigaction, sigset_t, sigemptyset, sigaddset, sigaction} in src/sys/sys.zig:

• Android: an extern struct matching bionic bits/signal_types.h (__LP64__) and an @extern to libc sigaction that takes it. sigset_t = c_ulong.
• Everywhere else: transparent aliases of std.posix.*, so nothing changes.

Replace all nine callsites (main.zig, crash_handler.zig, process.zig, repl.zig, filter_run.zig, multi_run.zig, Coordinator.zig, Global.zig).

A comptime tripwire fires if @sizeOf(std.c.Sigaction) ever shrinks to the bionic 32 bytes, so the workaround can be dropped once the Zig stdlib bug is fixed upstream.

Also adds zig build check-android[-debug], gated on -Dandroid_ndk_sysroot like check-freebsd, so the Android-only struct gets compile-checked.

Verification

$ zig build check-android-debug -Dandroid_ndk_sysroot=…/sysroot
check-android-debug success
  compile obj bun-debug Debug x86_64-linux-android  success
  compile obj bun-debug Debug aarch64-linux-android success

bun run zig:check-all still passes all 16 targets. Host smoke tests (spawn/exit-code, spawn/spawn-signal, spawn/spawn-kill-signal, plus BUN_FEATURE_FLAG_FORCE_WAITER_THREAD=1 to hit the modified SIGCHLD path) pass.

No runtime test is included because the misbehaviour only surfaces on a real Android kernel, which CI does not run.

[robobun]

^{Updated 2:12 AM PT - May 8th, 2026}

❌ @robobun, your commit bd939db has 3 failures in Build #52750 (All Failures):

• test/js/bun/s3/s3-storage-class.test.ts - code 1 on 🍎 14 aarch64
• test/js/bun/test/parallel/test-http-should-emit-close-when-connection-is-aborted.ts - timeout on 🪟 2019 x64-baseline
• test/js/bun/test/parallel/test-http-should-emit-close-when-connection-is-aborted.ts - timeout on 🪟 2019 x64
• test/js/bun/test/parallel/test-http-should-emit-close-when-connection-is-aborted.ts - timeout on 🪟 11 aarch64
• test/js/web/fetch/fetch-http2-client.test.ts - code 1 on 🐧 13 x64-asan

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🧪   To try this PR locally:

bunx bun-pr 30389

That installs a local version of the PR into your bun-30389 executable, so you can run:

bun-30389 --bun

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• test/internal/sigaction-layout.test.ts

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Walkthrough

Refactors POSIX signal handling to use an Android-aware bun.sys abstraction, migrates many sigaction usages to bun.sys, adds a TestingAPI + JS binding and POSIX tests to validate sigaction layout, and guards Android build checks on NDK presence.

Changes

Signal Handling Abstraction Migration

Layer / File(s)	Summary
Signal Type & API Abstraction Layer src/sys/sys.zig	New bun.sys.sigset_t, bun.sys.Sigaction, bun.sys.sigemptyset(), bun.sys.sigaddset(), and bun.sys.sigaction() provide Android-specific signal ABI implementations for bionic libc while delegating to posix on other platforms.
Main & Runtime Signal Setup src/main.zig, src/runtime/api/bun/process.zig	Signal handler registrations for SIG.PIPE, SIG.XFSZ, and SIGCHLD are updated to use bun.sys types and functions instead of std.posix equivalents.
Process & REPL Abort Handlers src/cli/filter_run.zig, src/cli/multi_run.zig, src/cli/repl.zig, src/cli/test/parallel/Coordinator.zig	SIGINT/SIGTERM handler registration and restoration in abort handlers migrate from std.posix.Sigaction/std.posix.sigaction to bun.sys equivalents, keeping handler logic and flags unchanged.
Global Panic Signal Handler src/bun_core/Global.zig	raiseIgnoringPanicHandler POSIX signal setup switches from std.c/std.posix APIs to bun.sys equivalents while preserving handler reset and abort flow.
Crash Handler Segfault Setup src/crash_handler/crash_handler.zig	Segfault/crash signal registration functions (updatePosixSegfaultHandler, resetOnPosix, crash()) migrate to construct and register bun.sys.Sigaction structures via bun.sys.sigaction() instead of std.posix calls.
Testing APIs & Tests src/sys_jsc/error_jsc.zig, src/js/internal-for-testing.ts, test/internal/sigaction-layout.test.ts	Adds TestingAPIs.sigactionLayout, exposes sigactionLayout to JS, and adds a POSIX-gated test verifying libc round-trip readback and struct size/handler pointer properties.
CLI Integration Test test/cli/run/run-propagate-sigkill.test.ts	Adds a POSIX-only integration test that spawns a child which self-terminates with SIGKILL and asserts the parent observes/re-raises SIGKILL.
Build Configuration build.zig	Android semantic-check build steps (check-android, check-android-debug) are now conditionally registered only when android_ndk_sysroot is non-null.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding bun.sys.sigaction with bionic-correct layout for Android to fix signal handling on Android platforms.
Description check	✅ Passed	The description comprehensively covers the problem statement, the fix implemented, and verification steps. It follows the template with clear sections explaining what the PR does and how it was verified.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[dylan-conway]

pretty sure this should be c_int instead of c_uint

[robobun]

bionic does declare int sa_flags, but SA_RESETHAND = 0x80000000 — Zig refuses to coerce that into a c_int literal, so .flags = SA.SIGINFO | SA.RESTART | SA.RESETHAND in crash_handler.zig / filter_run.zig / multi_run.zig would stop compiling on Android. c_uint is ABI-identical (same size/align) and is what std.c.Sigaction uses for every other Linux libc for the same reason. Added a comment at the field explaining this.