Add bun.sys.ucontext_t with bionic-correct layout on x86_64 Android

[robobun]

Problem

std.c.ucontext_t on .linux forwards to std.os.linux.<arch>.ucontext_t, which is the glibc/musl layout: {flags, link, stack, mcontext, sigmask[128B], fpregs_mem[512B]}. bionic x86_64's ucontext_t (<sys/ucontext.h>) puts only a bare union { sigset_t; sigset64_t; } — one c_ulong, 8 bytes — between uc_mcontext and __fpregs_mem, so __fpregs_mem starts 120 bytes earlier than the Zig stdlib expects.

aarch64 is accidentally correct: bionic explicitly pads uc_sigmask back to 128 bytes there (char __padding[128 - sizeof(sigset_t)]).

No current breakage — handleSegfaultPosix takes the ucontext arg as ?*const anyopaque and never dereferences it. This is proactive: if the crash handler ever starts reading register state from the saved context (e.g. uc_mcontext.gregs[REG_RIP] / REG_RSP instead of the live siginfo), x86_64 Android would read the wrong offsets.

Fix

Same pattern as #30389 (on which this is now based):

• src/sys/sys.zig: add pub const ucontext_t — on x86_64 Android, an extern struct matching bionic <sys/ucontext.h> (__x86_64__), with comptime offset assertions (uc_mcontext @40, uc_sigmask @296, __fpregs_mem @304, sizeof == 816) and a tripwire that fires when std.c.ucontext_t shrinks to the bionic size so the workaround can be dropped. Everywhere else it's a transparent alias of std.posix.ucontext_t. The sigmask field reuses bun.sys.sigset_t.
• src/crash_handler/crash_handler.zig: handleSegfaultPosix's third parameter is now ?*const bun.sys.ucontext_t (still unused, but correctly typed for future use). @ptrCast at the one assignment site since Sigaction.sigaction_fn types the slot as ?*anyopaque — same C ABI.
• test/internal/ucontext-layout.test.ts: pins @sizeOf / @offsetOf(bun.sys.ucontext_t, …) to the host-libc values on Linux x86_64 (glibc/musl on CI, bionic when run on device).

src/test_runner/harness/recover.zig also references std.c.ucontext_t, but that's a separate problem (bionic has no setcontext/getcontext at all — it would need the setjmp/longjmp path like musl) and that file is currently unreferenced.

Verification

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

$ bun bd test test/internal/ucontext-layout.test.ts
(pass) bun.sys.ucontext_t matches host libc on Linux x86_64
  → {sizeof:936, mcontext:40, sigmask:296, fpregs_mem:424, android:false}

bun run zig:check-all passes all 16 targets. Intentionally breaking an offset assertion (@sizeOf == 817) correctly fails the x86_64-android compile.

No Android runtime test is included — the misbehaviour only surfaces when the kernel fills the structure on a real x86_64 Android device, which CI does not run, and nothing in Bun currently dereferences it.

[robobun]

^{Updated 2:59 AM PT - May 8th, 2026}

❌ @robobun, your commit 72aba59 has 1 failures in Build #52781 (All Failures):

• 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

---

🧪   To try this PR locally:

bunx bun-pr 30393

That installs a local version of the PR into your bun-30393 executable, so you can run:

bun-30393 --bun

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

Walkthrough

This PR adds Android x86_64–specific ucontext_t type definition matching bionic ABI layout, updates the POSIX signal handler to use typed context pointers, and exposes layout metadata via testing APIs with validation tests.

Changes

Android x86_64 ucontext_t Implementation

Layer / File(s)	Summary
Type Definition (Data Contract) src/sys/sys.zig	Defines bun.sys.ucontext_t for Android x86_64 as an extern struct matching bionic layout, with compile-time assertions validating field offsets and total size.
Signal Handler Implementation src/crash_handler/crash_handler.zig	Updates handleSegfaultPosix callback parameter from ?*const anyopaque to ?*const bun.sys.ucontext_t and casts the handler to sigaction_fn during registration.
Testing API (JSC) src/sys_jsc/error_jsc.zig	Adds TestingAPIs.ucontextLayout that extracts and returns bun.sys.ucontext_t size, field offsets (mcontext, sigmask, fpregs_mem), and an android platform flag; returns js_undefined on non–Linux x86_64.
JS Testing Binding src/js/internal-for-testing.ts	Exports ucontextLayout() function that calls native TestingAPIs.ucontextLayout and returns optional layout metadata object with sizes, offsets, and android indicator.
Tests test/internal/ucontext-layout.test.ts	Linux x86_64–only test that invokes ucontextLayout() and asserts returned layout matches expected values, with distinct assertions for Android bionic vs glibc/musl offsets and sizes.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: adding a bionic-correct ucontext_t type for x86_64 Android, which is the primary objective of the PR.
Description check	✅ Passed	The PR description comprehensively addresses both required template sections: 'What does this PR do?' explains the problem, fix, and implementation details; 'How did you verify your code works?' provides build commands and test results.
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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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@test/internal/ucontext-layout.test.ts`:
- Around line 29-35: The test currently derives the expected branch from the
value under test (layout!.android) which can hide regressions; change the
selection of the expected object to use the independent isAndroid flag (e.g.,
use isAndroid ? {...android:true} : {...android:false}) instead of
layout!.android, and add an explicit assertion that layout.android equals
isAndroid so the test fails if the reported platform differs from the external
isAndroid signal; update references to the expected object creation (the const
expected) and ensure you still assert other fields against layout.mcontext,
layout.sigmask, layout.fpregs_mem, and layout.sizeof as before.

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Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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