posix_spawn: fix child spinning on write to a broken pipe

A child process created by posix_spawn reports errors to its parent via
a pipe, retrying infinitely on any write error to prevent falsely
reporting success. If the (original) parent dies before write is
attempted, there is nobody to report to, but the child will remain
stuck in the write loop forever if SIGPIPE is blocked or ignored.
Fix this by not retrying write if it fails with EPIPE.

src/process/posix_spawn.c

@@ -4,6 +4,7 @@
 #include <unistd.h>
 #include <signal.h>
 #include <fcntl.h>
+#include <errno.h>
 #include <sys/wait.h>
 #include "syscall.h"
 #include "lock.h"
@@ -156,7 +157,11 @@ static int child(void *args_vp)
 fail:
 	/* Since sizeof errno < PIPE_BUF, the write is atomic. */
 	ret = -ret;
-	if (ret) while (__syscall(SYS_write, p, &ret, sizeof ret) < 0);
+	if (ret) {
+		int r;
+		do r = __syscall(SYS_write, p, &ret, sizeof ret);
+		while (r<0 && r!=-EPIPE);
+	}
 	_exit(127);
 }