[RFC] Fix for gdb crash in "info thread" after exec().

["Paul Pluzhnikov" <ppluzhnikov@google.com>]

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Greetings,

Gdb (CVS Head) crashes when 'info thread' is executed after program exec()s:

$ cat execl.c
#include <unistd.h>
int
main (int argc, char* argv[])
{
  if (argc == 1)
    execl (argv[0], argv[0], "second", NULL);

  return 0;
}

$ gcc -g execl.c -pthread && ./gdb -nx -q a.out
(gdb) b main
Breakpoint 1 at 0x8048388: file execl.c, line 5.
(gdb) r
Starting program: /usr/local/build/gdb/a.out
[Thread debugging using libthread_db enabled]

Breakpoint 1, main (argc=1, argv=0xffffd744) at execl.c:5
5         if (argc == 1)
(gdb) c
Continuing.
warning: Error removing breakpoint -2
warning: Error removing breakpoint -3
[New process 5418]
Executing new program: /usr/local/build/gdb/a.out
warning: Cannot initialize thread debugging library: generic error
[Thread debugging using libthread_db enabled]

Breakpoint 1, main (argc=2, argv=0xffffd744) at execl.c:5
5         if (argc == 1)
(gdb) inf thread
../../src/gdb/linux-nat.c:2960: internal-error:
linux_nat_thread_alive: Assertion `is_lwp (ptid)' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) y


The problem is that gdb maintains thread list which is not properly
re-initialized after exec().

Attached patch (originally by Doug Evans <dje@google.com>) fixes
that, and adds a test case for it.

Thanks,
-- 
Paul Pluzhnikov

ChangeLog:
2008-05-12  Paul Pluzhnikov  <ppluzhnikov@google.com>

	* gdb/linux-thread-dl.c (check_for_thread_db): reinitialize
	thread list after exec().

testsuite/ChangeLog:
2008-05-12  Paul Pluzhnikov  <ppluzhnikov@google.com>

	* gdb.threads/execl.c, gdb.threads/execl.exp: Add test for
	"info threads" after exec.

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ChangeLog:
2008-05-12  Paul Pluzhnikov  <ppluzhnikov@google.com>

	* gdb/linux-thread-dl.c (check_for_thread_db): reinitialize
	thread list after exec().

testsuite/ChangeLog:
2008-05-12  Paul Pluzhnikov  <ppluzhnikov@google.com>

	* gdb.threads/execl.c, gdb.threads/execl.exp: Add test for
	"info threads" after exec.


Index: gdb/linux-thread-db.c
===================================================================
RCS file: /cvs/src/src/gdb/linux-thread-db.c,v
retrieving revision 1.42
diff -u -p -u -r1.42 linux-thread-db.c
--- gdb/linux-thread-db.c	4 May 2008 14:12:34 -0000	1.42
+++ gdb/linux-thread-db.c	13 May 2008 01:28:48 -0000
@@ -613,6 +613,10 @@ check_for_thread_db (void)
       push_target (&thread_db_ops);
       using_thread_db = 1;
 
+      /* Existing entries in thread_list might not have thread_info.private
+	 set, so clear it out.  */
+      init_thread_list ();
+
       enable_thread_event_reporting ();
       thread_db_find_new_threads ();
       break;
Index: gdb/testsuite/gdb.threads/execl.c
===================================================================
RCS file: gdb/testsuite/gdb.threads/execl.c
diff -N gdb/testsuite/gdb.threads/execl.c
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ gdb/testsuite/gdb.threads/execl.c	13 May 2008 01:28:48 -0000
@@ -0,0 +1,12 @@
+/* Test handling thread control across an execl.  */
+
+#include <unistd.h>
+
+int
+main (int argc, char* argv[])
+{
+  if (argc == 1)
+    execl (argv[0], argv[0], "second", NULL);
+
+  return 0;
+}
Index: gdb/testsuite/gdb.threads/execl.exp
===================================================================
RCS file: gdb/testsuite/gdb.threads/execl.exp
diff -N gdb/testsuite/gdb.threads/execl.exp
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ gdb/testsuite/gdb.threads/execl.exp	13 May 2008 01:28:48 -0000
@@ -0,0 +1,48 @@
+# Copyright (C) 2008 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/>.
+
+# Please email any bugs, comments, and/or additions to this file to:
+# bug-gdb@prep.ai.mit.edu
+
+# Test handling of threads across an execl.
+
+if $tracelevel then {
+    strace $tracelevel
+}
+
+set testfile "execl"
+set srcfile ${testfile}.c
+set binfile ${objdir}/${subdir}/${testfile}
+
+if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable [list debug "incdir=${objdir}"]] != "" } {
+    return -1
+}
+
+gdb_exit
+gdb_start
+gdb_reinitialize_dir $srcdir/$subdir
+gdb_load ${binfile}
+
+runto_main
+
+# When continuing from this point we'll hit the breakpoint in main()
+# again, this time in the exec'd process.
+gdb_test "continue" ".*Breakpoint 1, main.*" \
+    "continue across exec"
+
+gdb_test "info threads" ".*" "info threads after exec"
+
+gdb_test "continue" ".*Program exited normally\\." \
+    "continue to end"