[linux] fix stepping over fork in follow-child mode.

[Pedro Alves <pedro@codesourcery.com>]

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Hi,

I noticed that issuing a next over a fork call, with
"follow-fork-mode child", wasn't working.  The problem is that when we
update the breakpoints in the child to attach them to the current thread
if their thread their currently attached to doesn't exist anymore, 
inferior_ptid doesn't hold the tid yet.

static int
linux_child_follow_fork (struct target_ops *ops, int follow_child)

(...)

      inferior_ptid = ptid_build (child_pid, child_pid, 0);

      /* Reinstall ourselves, since we might have been removed in
	 target_detach (which does other necessary cleanup).  */

      push_target (ops);
      linux_nat_switch_fork (inferior_ptid);
      check_for_thread_db ();

      /* Reset breakpoints in the child as appropriate.  */
      follow_inferior_reset_breakpoints (); <- this would fail
    }

The patch attached updates fork-child-threads.exp with a
test that would fail without the fix.

No regressions on x86_64-unknown-linux-gnu.

-- 
Pedro Alves

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2008-03-18  Pedro Alves  <pedro@codesourcery.com>

	gdb/
	* linux-nat.c (linux_child_follow_fork): Update inferior_ptid with
	the thread id.

	gdb/testsuite/
	gdb.threads/fork-child-threads.exp: Add "next" test.

---
 gdb/linux-nat.c                                  |    9 +++++++++
 gdb/testsuite/gdb.threads/fork-child-threads.exp |    1 +
 2 files changed, 10 insertions(+)

Index: src/gdb/testsuite/gdb.threads/fork-child-threads.exp
===================================================================
--- src.orig/gdb/testsuite/gdb.threads/fork-child-threads.exp	2008-03-18 18:31:12.000000000 +0000
+++ src/gdb/testsuite/gdb.threads/fork-child-threads.exp	2008-03-18 18:31:53.000000000 +0000
@@ -38,6 +38,7 @@ if ![runto_main] then {
 
 gdb_test "set follow-fork-mode child"
 gdb_breakpoint "start"
+gdb_test "next" ".*pthread_create \\(&thread, NULL, start, NULL\\);.*" "next over fork"
 gdb_test "continue" "Breakpoint 2, start.*" "get to the spawned thread"
 
 # Wrong:
Index: src/gdb/linux-nat.c
===================================================================
--- src.orig/gdb/linux-nat.c	2008-03-18 20:25:45.000000000 +0000
+++ src/gdb/linux-nat.c	2008-03-18 20:26:04.000000000 +0000
@@ -104,6 +104,8 @@ static LONGEST (*super_xfer_partial) (st
 				      const gdb_byte *,
 				      ULONGEST, LONGEST);
 
+static int find_thread_from_lwp (struct thread_info *thr, void *dummy);
+
 static int debug_linux_nat;
 static void
 show_debug_linux_nat (struct ui_file *file, int from_tty,
@@ -460,6 +462,7 @@ linux_child_follow_fork (struct target_o
   else
     {
       char child_pid_spelling[40];
+      struct thread_info *thr;
 
       /* Needed to keep the breakpoint lists in sync.  */
       if (! has_vforked)
@@ -519,6 +522,12 @@ linux_child_follow_fork (struct target_o
       linux_nat_switch_fork (inferior_ptid);
       check_for_thread_db ();
 
+      /* Update inferior_ptid to include the thread id, so updating
+	 step-resume breakpoints in the child works.  */
+      thr = iterate_over_threads (find_thread_from_lwp, &inferior_ptid);
+      if (thr)
+	inferior_ptid = thr->ptid;
+
       /* Reset breakpoints in the child as appropriate.  */
       follow_inferior_reset_breakpoints ();
     }