[rfc] Fix linux_test_for_tracefork for newer kernels

[Daniel Jacobowitz <drow at false dot org>]

This patch makes the run-time test for fork tracing a little more robust.
In particular, it fixes two things:
  - If the test failed in an unexpected way, then two stopped processes
    could be left around.
  - Waitpid can be interrupted by EINTR.  Kernel 2.6.10-rc1 has some
    changes to wait and signal processing, which cause the SIGCHLD
    to be received before wait returns.  I don't think there's any
    actual bug here, since reissuing the wait works fine.

These changes let my testsuite runs finish without leaving hordes of stopped
processes around.  I won't check it in yet, since I haven't tested it on a
system where this runtime check should fail; in the meantime, I'm just
posting it for comments.  Hopefully I got it right this time!

-- 
Daniel Jacobowitz

2004-11-12  Daniel Jacobowitz  <dan@debian.org>

	* linux-nat.c (my_waitpid): New function.
	(linux_test_for_tracefork): Make more robust and verbose.

Index: gdb-6.3/gdb/linux-nat.c
===================================================================
--- gdb-6.3.orig/gdb/linux-nat.c	2004-10-08 16:29:47.000000000 -0400
+++ gdb-6.3/gdb/linux-nat.c	2004-11-12 17:05:30.000000000 -0500
@@ -150,6 +150,21 @@ linux_tracefork_child (void)
   exit (0);
 }
 
+/* Wrapper function for waitpid which handles EINTR.  */
+
+static int
+my_waitpid (int pid, int *status, int flags)
+{
+  int ret;
+  do
+    {
+      ret = waitpid (pid, status, flags);
+    }
+  while (ret == -1 && errno == EINTR);
+
+  return ret;
+}
+
 /* Determine if PTRACE_O_TRACEFORK can be used to follow fork events.  We
    create a child process, attach to it, use PTRACE_SETOPTIONS to enable
    fork tracing, and let it fork.  If the process exits, we assume that
@@ -169,7 +184,7 @@ linux_test_for_tracefork (void)
   if (child_pid == 0)
     linux_tracefork_child ();
 
-  ret = waitpid (child_pid, &status, 0);
+  ret = my_waitpid (child_pid, &status, 0);
   if (ret == -1)
     perror_with_name ("linux_test_for_tracefork: waitpid");
   else if (ret != child_pid)
@@ -182,8 +197,17 @@ linux_test_for_tracefork (void)
   ret = ptrace (PTRACE_SETOPTIONS, child_pid, 0, PTRACE_O_TRACEFORK);
   if (ret != 0)
     {
-      ptrace (PTRACE_KILL, child_pid, 0, 0);
-      waitpid (child_pid, &status, 0);
+      ret = ptrace (PTRACE_KILL, child_pid, 0, 0);
+      if (ret != 0)
+	{
+	  warning ("linux_test_for_tracefork: failed to kill child");
+	  return;
+	}
+
+      ret = my_waitpid (child_pid, &status, 0);
+      if (ret != 0)
+	warning ("linux_test_for_tracefork: failed to wait for killed child");
+
       return;
     }
 
@@ -192,8 +216,12 @@ linux_test_for_tracefork (void)
 		PTRACE_O_TRACEFORK | PTRACE_O_TRACEVFORKDONE);
   linux_supports_tracevforkdone_flag = (ret == 0);
 
-  ptrace (PTRACE_CONT, child_pid, 0, 0);
-  ret = waitpid (child_pid, &status, 0);
+  ret = ptrace (PTRACE_CONT, child_pid, 0, 0);
+  if (ret != 0)
+    warning ("linux_test_for_tracefork: failed to resume child");
+
+  ret = my_waitpid (child_pid, &status, 0);
+
   if (ret == child_pid && WIFSTOPPED (status)
       && status >> 16 == PTRACE_EVENT_FORK)
     {
@@ -204,16 +232,20 @@ linux_test_for_tracefork (void)
 	  int second_status;
 
 	  linux_supports_tracefork_flag = 1;
-	  waitpid (second_pid, &second_status, 0);
-	  ptrace (PTRACE_DETACH, second_pid, 0, 0);
+	  my_waitpid (second_pid, &second_status, 0);
+	  ret = ptrace (PTRACE_KILL, second_pid, 0, 0);
+	  if (ret != 0)
+	    warning ("linux_test_for_tracefork: failed to kill second child");
 	}
     }
+  else
+    warning ("linux_test_for_tracefork: unexpected result from waitpid "
+	     "(%d, status 0x%x)", ret, status);
 
-  if (WIFSTOPPED (status))
-    {
-      ptrace (PTRACE_DETACH, child_pid, 0, 0);
-      waitpid (child_pid, &status, 0);
-    }
+  ret = ptrace (PTRACE_KILL, child_pid, 0, 0);
+  if (ret != 0)
+    warning ("linux_test_for_tracefork: failed to kill child");
+  my_waitpid (child_pid, &status, 0);
 }
 
 /* Return non-zero iff we have tracefork functionality available.