From 758a76922beca511a4b086e45269896607df8a3c Mon Sep 17 00:00:00 2001
From: Ben Noordhuis <info@bnoordhuis.nl>
Date: Mon, 13 Aug 2012 16:36:29 +0200
Subject: [PATCH] unix: fix SIGCHLD race in process.c

Start the SIGCHLD signal watcher before calling fork(). There was a very subtle
race where a child process could quit (and generate a SIGCHILD) before the
signal handler was installed.

To reproduce, call uv_spawn() repeatedly with the below x86_64 program:

    // compile with `gcc -O2 -nostdlib`
    void _start(void)
    {
      // syscall(SYS_exit, 0)
      __asm__ __volatile__ (
        "xorq %rdi, %rdi;"
        "xorq %rax, %rax;"
        "mov $0x3c, %al;"
        "syscall;"
      );
      for (;;);
    }
---
 src/unix/process.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/unix/process.c b/src/unix/process.c
index 36ec40dd..e21e8ea0 100644
--- a/src/unix/process.c
+++ b/src/unix/process.c
@@ -400,6 +400,8 @@ int uv_spawn(uv_loop_t* loop,
   if (uv__make_pipe(signal_pipe, 0))
     goto error;
 
+  uv_signal_start(&loop->child_watcher, uv__chld, SIGCHLD);
+
   pid = fork();
 
   if (pid == -1) {
@@ -440,7 +442,6 @@ int uv_spawn(uv_loop_t* loop,
 
   q = uv__process_queue(loop, pid);
   ngx_queue_insert_tail(q, &process->queue);
-  uv_signal_start(&loop->child_watcher, uv__chld, SIGCHLD);
 
   process->pid = pid;
   process->exit_cb = options.exit_cb;