Since 0ae842f8 (v1.76.0) ___libc_sigaction is a Go function that records
dispositions for delivery through os/signal and never touches the kernel;
Xabort's ___libc_sigaction(SIGABRT, SIG_DFL) therefore leaves the Go
runtime's SIGABRT handler installed, and the kill(getpid(), SIGABRT) that
follows prints "SIGABRT: abort" and a goroutine dump to stderr before the
runtime re-raises the signal and dies. In v1.75.7 the transpiled musl
__libc_sigaction issued the rt_sigaction syscall, so the process died
silently, as a C abort() does.

The difference is observable: Tcl's exec reports a child that dies by a
signal as "child killed: SIGABRT" only when its stderr stayed empty, and
sqlite's writecrash.test (test_devsym.c calls abort() on the n-th write)
matches on that, so every linux target of modernc.org/libsqlite3 fails
writecrash-1.1.1 with v1.76.0 and passes with v1.75.7 - verified A/B on
linux/amd64 with everything else equal.

resetSigDfl (abort_linux.go, the musl linux ports) installs SIG_DFL with
rt_sigaction and unblocks the signal with rt_sigprocmask, replacing the
runtime's handler for that one signal; Xabort calls it after the
bookkeeping call to ___libc_sigaction and then raises SIGABRT
thread-directed with tgkill, as the Go runtime's raise() and the netbsd
Xabort do, because a process-directed kill(2) is asynchronous and the
calling thread can run on before the signal lands. The kill and the panic
that follow are unreachable fallbacks. With this writecrash.test passes
again (0 errors out of 997 tests), and libsqlite3's full Tcl permutation
on linux/amd64 is back to its waived failures only. Builds on all eight
linux ports.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Dq5kdSFJ1eyaaH2cjpMAKk