Fix PR 19494: hang when killing unfollowed fork children

linux_nat_kill relies on get_last_target_status to determine whether
the current inferior is stopped at a unfollowed fork/vfork event.
This is bad because many things can happen ever since we caught the
fork/vfork event...  This commit rewrites that code to instead walk
the thread list looking for unfollowed fork events, similarly to what
was done for remote.c.

New test included.  The main idea of the test is make sure that when
the program stops for a fork catchpoint, and the user kills the
parent, gdb also kills the unfollowed fork child.  Since the child
hasn't been added as an inferior at that point, we need some other
portable way to detect that the child is gone.  The test uses a pipe
for that.  The program forks twice, so you have grandparent, child and
grandchild.  The grandchild inherits the write side of the pipe.  The
grandparent hangs reading from the pipe, since nothing ever writes to
it.  If, when GDB kills the child, it also kills the grandchild, then
the grandparent's pipe read returns 0/EOF and the test passes.
Otherwise, if GDB doesn't kill the grandchild, then the pipe read
never returns and the test times out, like:

 FAIL: gdb.base/catch-fork-kill.exp: fork-kind=fork: exit-kind=kill: fork: kill parent (timeout)
 FAIL: gdb.base/catch-fork-kill.exp: fork-kind=vfork: exit-kind=kill: vfork: kill parent (timeout)

No regressions on x86_64 Fedora 20.  New test passes with gdbserver as
well.

gdb/ChangeLog:
2016-01-25  Pedro Alves  <palves@redhat.com>

	PR gdb/19494
	* linux-nat.c (kill_one_lwp): New, factored out from ...
	(kill_callback): ... this.
	(kill_wait_callback): New, factored out from ...
	(kill_wait_one_lwp): ... this.
	(kill_unfollowed_fork_children): New function.
	(linux_nat_kill): Use it.

gdb/testsuite/ChangeLog:
2016-01-25  Pedro Alves  <palves@redhat.com>

	PR gdb/19494
	* gdb.base/catch-fork-kill.c: New file.
	* gdb.base/catch-fork-kill.exp: New file.

1 files changed, 69 insertions, 35 deletions

diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index eb609c8..6ded38d 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -3466,44 +3466,44 @@ linux_nat_wait (struct target_ops *ops,
   return event_ptid;
 }
 
-static int
-kill_callback (struct lwp_info *lp, void *data)
+/* Kill one LWP.  */
+
+static void
+kill_one_lwp (pid_t pid)
 {
   /* PTRACE_KILL may resume the inferior.  Send SIGKILL first.  */
 
   errno = 0;
-  kill_lwp (ptid_get_lwp (lp->ptid), SIGKILL);
+  kill_lwp (pid, SIGKILL);
   if (debug_linux_nat)
     {
       int save_errno = errno;
 
       fprintf_unfiltered (gdb_stdlog,
-			  "KC:  kill (SIGKILL) %s, 0, 0 (%s)\n",
-			  target_pid_to_str (lp->ptid),
+			  "KC:  kill (SIGKILL) %ld, 0, 0 (%s)\n", (long) pid,
 			  save_errno ? safe_strerror (save_errno) : "OK");
     }
 
   /* Some kernels ignore even SIGKILL for processes under ptrace.  */
 
   errno = 0;
-  ptrace (PTRACE_KILL, ptid_get_lwp (lp->ptid), 0, 0);
+  ptrace (PTRACE_KILL, pid, 0, 0);
   if (debug_linux_nat)
     {
       int save_errno = errno;
 
       fprintf_unfiltered (gdb_stdlog,
-			  "KC:  PTRACE_KILL %s, 0, 0 (%s)\n",
-			  target_pid_to_str (lp->ptid),
+			  "KC:  PTRACE_KILL %ld, 0, 0 (%s)\n", (long) pid,
 			  save_errno ? safe_strerror (save_errno) : "OK");
     }
-
-  return 0;
 }
 
-static int
-kill_wait_callback (struct lwp_info *lp, void *data)
+/* Wait for an LWP to die.  */
+
+static void
+kill_wait_one_lwp (pid_t pid)
 {
-  pid_t pid;
+  pid_t res;
 
   /* We must make sure that there are no pending events (delayed
      SIGSTOPs, pending SIGTRAPs, etc.) to make sure the current
@@ -3511,49 +3511,83 @@ kill_wait_callback (struct lwp_info *lp, void *data)
 
   do
     {
-      pid = my_waitpid (ptid_get_lwp (lp->ptid), NULL, __WALL);
-      if (pid != (pid_t) -1)
+      res = my_waitpid (pid, NULL, __WALL);
+      if (res != (pid_t) -1)
 	{
 	  if (debug_linux_nat)
 	    fprintf_unfiltered (gdb_stdlog,
-				"KWC: wait %s received unknown.\n",
-				target_pid_to_str (lp->ptid));
+				"KWC: wait %ld received unknown.\n",
+				(long) pid);
 	  /* The Linux kernel sometimes fails to kill a thread
 	     completely after PTRACE_KILL; that goes from the stop
 	     point in do_fork out to the one in get_signal_to_deliver
 	     and waits again.  So kill it again.  */
-	  kill_callback (lp, NULL);
+	  kill_one_lwp (pid);
 	}
     }
-  while (pid == ptid_get_lwp (lp->ptid));
+  while (res == pid);
+
+  gdb_assert (res == -1 && errno == ECHILD);
+}
+
+/* Callback for iterate_over_lwps.  */
 
-  gdb_assert (pid == -1 && errno == ECHILD);
+static int
+kill_callback (struct lwp_info *lp, void *data)
+{
+  kill_one_lwp (ptid_get_lwp (lp->ptid));
   return 0;
 }
 
+/* Callback for iterate_over_lwps.  */
+
+static int
+kill_wait_callback (struct lwp_info *lp, void *data)
+{
+  kill_wait_one_lwp (ptid_get_lwp (lp->ptid));
+  return 0;
+}
+
+/* Kill the fork children of any threads of inferior INF that are
+   stopped at a fork event.  */
+
+static void
+kill_unfollowed_fork_children (struct inferior *inf)
+{
+  struct thread_info *thread;
+
+  ALL_NON_EXITED_THREADS (thread)
+    if (thread->inf == inf)
+      {
+	struct target_waitstatus *ws = &thread->pending_follow;
+
+	if (ws->kind == TARGET_WAITKIND_FORKED
+	    || ws->kind == TARGET_WAITKIND_VFORKED)
+	  {
+	    ptid_t child_ptid = ws->value.related_pid;
+	    int child_pid = ptid_get_pid (child_ptid);
+	    int child_lwp = ptid_get_lwp (child_ptid);
+	    int status;
+
+	    kill_one_lwp (child_lwp);
+	    kill_wait_one_lwp (child_lwp);
+
+	    /* Let the arch-specific native code know this process is
+	       gone.  */
+	    linux_nat_forget_process (child_pid);
+	  }
+      }
+}
+
 static void
 linux_nat_kill (struct target_ops *ops)
 {
   struct target_waitstatus last;
-  ptid_t last_ptid;
-  int status;
 
   /* If we're stopped while forking and we haven't followed yet,
      kill the other task.  We need to do this first because the
      parent will be sleeping if this is a vfork.  */
-
-  get_last_target_status (&last_ptid, &last);
-
-  if (last.kind == TARGET_WAITKIND_FORKED
-      || last.kind == TARGET_WAITKIND_VFORKED)
-    {
-      ptrace (PT_KILL, ptid_get_pid (last.value.related_pid), 0, 0);
-      wait (&status);
-
-      /* Let the arch-specific native code know this process is
-	 gone.  */
-      linux_nat_forget_process (ptid_get_pid (last.value.related_pid));
-    }
+  kill_unfollowed_fork_children (current_inferior ());
 
   if (forks_exist_p ())
     linux_fork_killall ();