Fix foll-fork.exp foll-vfork.exp fork-child-threads.exp

[Pedro Alves <pedro@codesourcery.com>]

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Long story short:

After following a child, detaching from the parent,

('set follow-fork-mode child' + 'set detach-on-fork on')

here in this bit,

 infrun.c:resume():
  {
    ....
    follow_fork ();
    ...
    tp->stop_signal = TARGET_SIGNAL_0;
  }

... `tp' is no longer in the thread list (it was pointing at a thread
of the parent process, which we've detached from, hence no longer
in the thread list), so if the assignment above doesn't crash, it ends
up writing to who-knows-where.

With some local changes I was making, sometimes, `tp' happened to be left pointing
at linux_nat.c:lwp_list, and so that assignment above ended up clearing
lp->waitstatus.kind (of the first lwp in the list), which resulted in
GDB considering that the child process had
exited (because TARGET_SIGNAL_0 == TARGET_WAITKIND_EXITED).

This should fix intermittent foll-fork.exp foll-vfork.exp
fork-child-threads.exp failures.

Checked in.

-- 
Pedro Alves

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2008-11-20  Pedro Alves  <pedro@codesourcery.com>

	* infrun.c (resume): If following a fork, reread the current
	thread.  Avoid dereferencing a possibly dangling pointer.

---
 gdb/infrun.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Index: src/gdb/infrun.c
===================================================================
--- src.orig/gdb/infrun.c	2008-11-20 05:37:35.000000000 +0000
+++ src/gdb/infrun.c	2008-11-20 12:30:26.000000000 +0000
@@ -1053,6 +1053,9 @@ a command like `return' or `jump' to con
       pending_follow.kind = TARGET_WAITKIND_SPURIOUS;
       if (follow_fork ())
 	should_resume = 0;
+
+      /* Following a fork may change inferior_ptid.  */
+      tp = inferior_thread ();
       break;
 
     case TARGET_WAITKIND_EXECD:
@@ -1148,11 +1151,11 @@ a command like `return' or `jump' to con
           displaced_step_dump_bytes (gdb_stdlog, buf, sizeof (buf));
         }
 
-      target_resume (resume_ptid, step, sig);
-
       /* Avoid confusing the next resume, if the next stop/resume
 	 happens to apply to another thread.  */
       tp->stop_signal = TARGET_SIGNAL_0;
+
+      target_resume (resume_ptid, step, sig);
     }
 
   discard_cleanups (old_cleanups);