Report a failed link when SIGCHLD is ignored

An ignored SIGCHLD is inherited across exec, e.g. from `trap '' CHLD`
in a shell. The child that fork_child creates is then reaped
automatically, so the parent's waitpid fails with ECHILD. The parent
ignored that and read the untouched zero status as a successful exit,
so a link that failed exited with status 0. Report a failure instead.

Also read and write the completion byte through File, which removes
two raw libc calls.

Based on https://github.com/rui314/mold/pull/1682.

src/subprocess.rs

@@ -2,7 +2,11 @@
 //! handling for disk-full errors, and the `-run` subcommand.
 
 #[cfg(not(windows))]
-use std::os::unix::io::{AsRawFd, FromRawFd, OwnedFd};
+use std::fs::File;
+#[cfg(not(windows))]
+use std::io::{Read, Write};
+#[cfg(not(windows))]
+use std::os::unix::io::{FromRawFd, OwnedFd};
 #[cfg(not(windows))]
 use std::sync::Mutex;
 
@@ -38,12 +42,17 @@ pub fn fork_child() {
         if pid > 0 {
             // Parent
             drop(writer);
-            let mut buf = [0u8; 1];
-            if libc::read(reader.as_raw_fd(), buf.as_mut_ptr().cast(), 1) == 1 {
+            if File::from(reader).read_exact(&mut [0u8]).is_ok() {
                 libc::_exit(0);
             }
+
+            // If SIGCHLD is ignored, which is inherited across exec, the
+            // child is reaped automatically and waitpid fails. Its exit
+            // status is lost then, so report a failure.
             let mut status = 0;
-            libc::waitpid(pid, &raw mut status, 0);
+            if libc::waitpid(pid, &raw mut status, 0) == -1 {
+                libc::_exit(1);
+            }
             if libc::WIFEXITED(status) {
                 libc::_exit(libc::WEXITSTATUS(status));
             }
@@ -67,11 +76,7 @@ pub fn notify_parent() {
     let Some(writer) = PIPE_WRITER.lock().unwrap().take() else {
         return;
     };
-    let buf = [1u8];
-    // SAFETY: writer owns a valid pipe write end.
-    unsafe {
-        libc::write(writer.as_raw_fd(), buf.as_ptr().cast(), 1);
-    }
+    let _ = File::from(writer).write_all(&[1]);
 }
 
 #[cfg(windows)]

tests/sigchld-ignored.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+. $(dirname $0)/common.inc
+
+cat <<EOF | $CC -o $t/a.o -c -xc -
+int foo();
+int main() { return foo(); }
+EOF
+
+# An ignored SIGCHLD is inherited across exec. The forked child is then
+# reaped automatically, but mold must still report that the link failed.
+(trap '' CHLD; not ./mold -o $t/exe $t/a.o)