Refuse following the vfork parent if not letting the child run.

[Pedro Alves <pedro at codesourcery dot com>]

If you try to follow a vfork parent, without letting the
child run, GDB hangs.  E.g., gdb 6.8 or head:

 >gdb-6.8 ./testsuite/gdb.base/foll-vfork
 :
 (gdb) set detach-on-fork off
 (gdb) start
 Breakpoint 1 at 0x4005c0: file ../../../src/gdb/testsuite/gdb.base/foll-vfork.c, line 12.
 Starting program: /home/pedro/gdb/sspaces/build/gdb/testsuite/gdb.base/foll-vfork
 main () at ../../../src/gdb/testsuite/gdb.base/foll-vfork.c:12
 12        pid = vfork ();
 (gdb) c
 Continuing.
 <HANG>

No amount of ctrl-c will let you out of this.  Only going to another
terminal and killing gdb bails you out.

I've been keeping the patch below locally to prevent
getting myself in such situation.  (GNU make uses vfork, and much of
my multi-process testing involves running make.  How convenient :-) )

The manual already mentions that you can't debug the vfork parent
until the child execs in the forks section.

Comments?  (This applies on top the the patch that adds the
option to resume all threads of all processes.)

A patched GDB shows this:

 (gdb) set detach-on-fork off
 (gdb) start
 Temporary breakpoint 1 at 0x4005c0: file ../../../src/gdb/testsuite/gdb.base/foll-vfork.c, line 12.
 Starting program: /home/pedro/gdb/sspaces/build/gdb/testsuite/gdb.base/foll-vfork

 Temporary breakpoint 1, main () at ../../../src/gdb/testsuite/gdb.base/foll-vfork.c:12
 12        pid = vfork ();
 (gdb) n
 warning: Can not debug the parent of a vfork in the foreground if not letting
 the child run until it execs or exits, as it would deadlock the terminal.
 0x00007ffff789aee4 in vfork () from /lib/libc.so.6
 (gdb)      

-- 
Pedro Alves

2009-06-08  Pedro Alves  <pedro@codesourcery.com>

	* linux-nat.c (linux_child_follow_fork): Refuse to follow a vfork
	parent in the foreground if not letting the child run.

---
 gdb/linux-nat.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

Index: src/gdb/linux-nat.c
===================================================================
--- src.orig/gdb/linux-nat.c	2009-06-08 13:22:47.000000000 +0100
+++ src/gdb/linux-nat.c	2009-06-08 14:27:05.000000000 +0100
@@ -596,6 +596,21 @@ linux_child_follow_fork (struct target_o
   if (!detach_fork)
     linux_enable_event_reporting (pid_to_ptid (child_pid));
 
+  if (has_vforked
+      && (!target_is_async_p () || sync_execution)
+      && !(follow_child || detach_fork || sched_multi))
+    {
+      /* The parent stays blocked inside the vfork syscall until the
+	 child execs or exits.  If we don't let the child run, then
+	 the parent stays blocked.  If we're telling the parent to run
+	 in the foreground, the user will not be able to ctrl-c to get
+	 back the terminal, effectively hanging the debug session.  */
+      warning (_("\
+Can not debug the parent of a vfork in the foreground if not letting \
+the child run until it execs or exits, as it would deadlock the terminal."));
+      return 1;
+    }
+
   if (! follow_child)
     {
       /* We're already attached to the parent, by default. */