unix: process: prevent exit event before data events

[bnoordhuis]

See nodejs/node#3676 - the executive summary is that the order in which the child process's stdio I/O watchers and the signal handler I/O watcher run is indeterminate. This can result in the exit callback running before the data callbacks, which is confusing. It appears to be an easy fix, @gireeshpunathil reported good luck with the patch below.

diff --git a/deps/uv/src/unix/aix.c b/deps/uv/src/unix/aix.c
index c90b7e5..b129845 100644
--- a/deps/uv/src/unix/aix.c
+++ b/deps/uv/src/unix/aix.c
@@ -100,6 +100,7 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
   uv__io_t* w;
   uint64_t base;
   uint64_t diff;
+  int have_signals;
   int nevents;
   int count;
   int nfds;
@@ -207,6 +208,7 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
       goto update_timeout;
     }

+    have_signals = 0;
     nevents = 0;

     assert(loop->watchers != NULL);
@@ -237,10 +239,20 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
         continue;
       }

-      w->cb(loop, w, pe->revents);
+      /* Run signal watchers last.  This also affects child process watchers
+       * because those are implemented in terms of signal watchers.
+       */
+      if (w == &loop->signal_io_watcher)
+        have_signals = 1;
+      else
+        w->cb(loop, w, pe->revents);
+
       nevents++;
     }

+    if (have_signals != 0)
+      loop->signal_io_watcher.cb(loop, &loop->signal_io_watcher, UV__POLLIN);
+
     loop->watchers[loop->nwatchers] = NULL;
     loop->watchers[loop->nwatchers + 1] = NULL;

[bnoordhuis]

Note that the patch is not 100% correct. You can still get 'exit' before 'data' when uv__io_poll() decides to loop and poll again.