Cygwin: fork: attach child not before success

Do not attach to the child before it was successfully initialized, or we
would need more sophisticated cleanup on child initialization failure,
like suppressing SIGCHILD delivery with multiple threads ("waitproc")
involved.

Improves "Cygwin: fork: Remember child not before success.",
commit f03ea8e1c57bd5cea83f6cd47fa02870bdfeb1c5, which leads to fork
problems if cygserver is running:

https://cygwin.com/ml/cygwin-patches/2019-q2/msg00155.html

1 files changed, 11 insertions, 8 deletions

diff --git a/winsup/cygwin/fork.cc b/winsup/cygwin/fork.cc
index 0119581df..7080144b9 100644
--- a/winsup/cygwin/fork.cc
+++ b/winsup/cygwin/fork.cc
@@ -424,14 +424,6 @@ frok::parent (volatile char * volatile stack_here)
 #endif
       goto cleanup;
     }
-  if (!child.reattach ())
-    {
-      this_errno = EAGAIN;
-#ifdef DEBUGGING0
-      error ("child reattach failed");
-#endif
-      goto cleanup;
-    }
 
   /* CHILD IS STOPPED */
   debug_printf ("child is alive (but stopped)");
@@ -516,6 +508,17 @@ frok::parent (volatile char * volatile stack_here)
 	}
     }
 
+  /* Do not attach to the child before it has successfully initialized.
+     Otherwise we may wait forever, or deliver an orphan SIGCHILD. */
+  if (!child.reattach ())
+    {
+      this_errno = EAGAIN;
+#ifdef DEBUGGING0
+      error ("child reattach failed");
+#endif
+      goto cleanup;
+    }
+
   /* Finally start the child up. */
   resume_child (forker_finished);