RFC: fix crash when inferior exits during "continue"

[Tom Tromey <tromey@redhat.com>]

I'd appreciate comments on this patch.

This patch fixes PR 13653.  The bug is that you can make gdb crash with
a certain sequence:

    set detach-on-fork off
    set target-async on
    set non-stop on
    ... run inferior, which forks; then the child stops somewhere
    inferior 2
    continue
    ... inferior 2 exits, gdb crashes

The crash happens because do_restore_current_thread_cleanup tries to
select a deleted inferior, causing the assertion in set_current_inferior
to fail.

This patch fixes the problem by noticing that the saved inferior no
longer exists, and arbitrarily selecting some other inferior instead.

Two questions for the reader:

1. Is this the right approach?  I am not sure.  It seems pretty
   reasonable to me, but I don't know this area very well.

2. Are the conditions in the new .exp file correct?  I mostly copied
   these from elsewhere, not knowing what is really right.

Built and regtested on x86-64 Fedora 15.
New test case included.

Tom

b/gdb/ChangeLog:
2012-02-07  Tom Tromey  <tromey@redhat.com>

	PR c++/13653:
	* thread.c (choose_first_inferior): New function.
	(do_restore_current_thread_cleanup): Handle case where inferior
	died.

diff --git a/gdb/testsuite/gdb.base/inferior-died.c b/gdb/testsuite/gdb.base/inferior-died.c
new file mode 100644
index 0000000..66227cf
--- /dev/null
+++ b/gdb/testsuite/gdb.base/inferior-died.c
@@ -0,0 +1,37 @@
+/* Test for fork-related gdb bug
+
+   Copyright 2012 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+void function(void)
+{
+  exit (0);			/* Break here */
+}
+
+int main()
+{
+  pid_t child = fork ();
+
+  if (child == 0)
+    function ();
+  else
+    waitpid (child, NULL, 0);
+}
diff --git a/gdb/testsuite/gdb.base/inferior-died.exp b/gdb/testsuite/gdb.base/inferior-died.exp
new file mode 100644
index 0000000..458dd61
--- /dev/null
+++ b/gdb/testsuite/gdb.base/inferior-died.exp
@@ -0,0 +1,56 @@
+# Copyright 2012 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+if { [is_remote target] || ![isnative] } then {
+    unsupported "inferior-died.exp"
+    continue
+}
+
+# Until "set follow-fork-mode" and "catch fork" are implemented on
+# other targets...
+#
+if {![istarget "hppa*-hp-hpux*"] && ![istarget "*-*-linux*"]} then {
+    unsupported "inferior-died.exp"
+    continue
+}
+
+if { ![support_displaced_stepping] } {
+    unsupported "inferior-died.exp"
+    return -1
+}
+
+set testfile "inferior-died"
+set srcfile ${testfile}.c
+set binfile ${objdir}/${subdir}/${testfile}
+
+if { [prepare_for_testing ${testfile}.exp ${testfile} ${testfile}.c] } {
+    return -1
+}
+
+gdb_test_no_output "set detach-on-fork off"
+gdb_test_no_output "set target-async on"
+gdb_test_no_output "set non-stop on"
+
+if ![runto_main] {
+    return
+}
+
+set line [gdb_get_line_number "Break here"]
+gdb_breakpoint $srcfile:$line
+
+gdb_continue_to_breakpoint "breakpoint"
+
+gdb_test "inferior 2" "Switching to inferior 2.*"
+gdb_test "continue" "exited normally.*"
diff --git a/gdb/thread.c b/gdb/thread.c
index 9a29383..6a667d6 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -1074,6 +1074,16 @@ struct current_thread_cleanup
   int inf_id;
 };
 
+/* A helper function for do_restore_current_thread_cleanup.  This is
+   passed to iterate_over_inferiors and simply returns the first
+   inferior.  */
+
+static int
+choose_first_inferior (struct inferior *inf, void *ignore)
+{
+  return 1;
+}
+
 static void
 do_restore_current_thread_cleanup (void *arg)
 {
@@ -1091,8 +1101,15 @@ do_restore_current_thread_cleanup (void *arg)
     restore_current_thread (old->inferior_ptid);
   else
     {
+      struct inferior *inf = find_inferior_id (old->inf_id);
+
       restore_current_thread (null_ptid);
-      set_current_inferior (find_inferior_id (old->inf_id));
+
+      /* If the inferior was deleted, choose some other inferior.  */
+      if (inf == NULL)
+	inf = iterate_over_inferiors (choose_first_inferior, NULL);
+
+      set_current_inferior (inf);
     }
 
   /* The running state of the originally selected thread may have