[PATCH v5 06/06] Remote fork catch

[Don Breazeal <donb at codesourcery dot com>]

This patch implements catchpoints for fork events on extended-remote
Linux targets.

Implementation appeared to be straightforward, requiring four new functions
in remote.c to implement insert/remove of fork/vfork catchpoints.  These
functions are essentially stubs that just return 0 ('success') if the
required features are enabled.  If the fork events are being reported, then
catchpoints are set and hit.

However, there are some extra issues that arise with catchpoints.

1) Thread creation reporting -- fork catchpoints are hit before the
   follow_fork has been completed.  When stopped at a fork catchpoint
   in the native implementation, the new process is not 'reported'
   until after the follow is done.  It doesn't show up in the inferiors
   list or the threads list.  However, in the gdbserver case, an
   'info threads' while stopped at a fork catchpoint will retrieve the
   new thread info from the target and add it to GDB's data structures,
   prior to the follow operations.  Because of this premature report,
   things on the GDB side eventually get very confused.

   So in gdbserver, in server.c:handle_qxfer_threads_worker, we check
   'last_status' and if it shows a FORK event, we know that we are in an
   unfollowed fork and we do not report the new (forked) thread to GDB.

2) Kill process before fork is followed -- on the native side in
   linux-nat.c:linux_nat_kill, there is some code to handle the case where
   a fork has occurred but follow_fork hasn't been called yet.  It does
   this by using the last status to determine if a follow is pending, and
   if it is, to kill the child task.  This patch uses similar code in
   linux-low.c:linux_kill.

3) One of the tests related to fork catchpoints,
   gdb.threads/fork-thread-pending.exp, depended on the threads being
   reported in a specific order.  GDBserver reported the threads in a
   different order, that is, 'info threads' showed the same threads, but in
   a different order.  The test used a hard-coded thread number to find a
   threads that (a) was not the main thread (blocked in pthread_join), and
   (b) was not the forking thread (stopped in fork).

   I implemented a new proc, find_unforked_thread, that uses a pretty
   brute-force method of finding a thread.  I considered just hard-coding
   another number (the native case used thread 2, which was the forking
   thread in the remote case), but that didn't seem future-proof.
   Suggestions on how to do this better would be welcome.
  
Tested on x64 Ubuntu Lucid, native, remote, extended-remote.  Tested the
case of killing the forking process before the fork has been followed
manually.  It wasn't clear to me how to check that a process had actually
been killed from a dejagnu test.

Thanks
--Don

gdb/gdbserver/
2015-02-25  Don Breazeal  <donb@codesourcery.com>

	* linux-low.c (linux_kill): Kill forked child when between
	fork event and follow_fork.
	* server.c (handle_qxfer_threads_worker): Skip forked child thread
	when between fork and follow_fork.
	(get_last_target_status): New function.
	* server.h (get_last_target_status): Declare new function.

gdb/
2015-02-25  Don Breazeal  <donb@codesourcery.com>
	* remote.c (remote_insert_fork_catchpoint): New function.
	(remote_remove_fork_catchpoint): New function.
	(remote_insert_vfork_catchpoint): New function.
	(remote_remove_vfork_catchpoint): New function.
	(init_extended_remote_ops): Initialize target vector with
	new fork catchpoint functions.

gdb/testsuite/
2015-02-25  Don Breazeal  <donb@codesourcery.com>
	* gdb.threads/fork-thread-pending.exp (find_unforked_thread):
	New proc.
	(main program): Call find_unforked_thread.

---
 gdb/NEWS                                          |  7 +--
 gdb/doc/gdb.texinfo                               |  6 ++-
 gdb/gdbserver/linux-low.c                         | 18 ++++++++
 gdb/gdbserver/server.c                            | 18 ++++++++
 gdb/gdbserver/server.h                            |  2 +
 gdb/remote.c                                      | 56 +++++++++++++++++++++++
 gdb/testsuite/gdb.threads/fork-thread-pending.exp | 23 +++++++++-
 7 files changed, 124 insertions(+), 6 deletions(-)

diff --git a/gdb/NEWS b/gdb/NEWS
index db62e35..eba4c73 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -83,9 +83,10 @@ T Stop Reply Packet's reason
   The remote stub now reports support for fork and vfork events to
   GDB's qSupported query.
 
-  GDBserver extended-remote Linux targets now provides basic support
-  for fork and vfork events.  This enables follow-fork-mode and
-  detach-on-fork for those targets with Linux kernels 2.5.60 and later.
+  GDBserver extended-remote Linux targets now provides support for
+  fork and vfork events.  This enables follow-fork-mode, detach-on-fork,
+  catch fork, and catch vfork for extended-remote targets with Linux
+  kernels 2.5.60 and later.
 
 *** Changes in GDB 7.9
 
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 4510ca7..f2d503a 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -4426,12 +4426,14 @@ Again, in this case @value{GDBN} would not be able to display syscall's names.
 @item fork
 @kindex catch fork
 A call to @code{fork}.  This is currently only available for HP-UX
-and @sc{gnu}/Linux.
+and @sc{gnu}/Linux, and when connected to @code{gdbserver} running
+on @sc{gnu}/Linux with @kbd{target extended-remote}.
 
 @item vfork
 @kindex catch vfork
 A call to @code{vfork}.  This is currently only available for HP-UX
-and @sc{gnu}/Linux.
+and @sc{gnu}/Linux, and when connected to @code{gdbserver} running
+on @sc{gnu}/Linux with @kbd{target extended-remote}.
 
 @item load @r{[}regexp@r{]}
 @itemx unload @r{[}regexp@r{]}
diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 5f9f9d4..07512aa 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -1033,6 +1033,24 @@ linux_kill (int pid)
 {
   struct process_info *process;
   struct lwp_info *lwp;
+  struct target_waitstatus last;
+  ptid_t last_ptid;
+
+  /* If we're stopped while forking and we haven't followed yet,
+     kill the child task.  We need to do this first because the
+     parent will be sleeping if this is a vfork.  */
+
+  get_last_target_status (&last_ptid, &last);
+
+  if (last.kind == TARGET_WAITKIND_FORKED
+      || last.kind == TARGET_WAITKIND_VFORKED)
+    {
+      lwp = find_lwp_pid (last.value.related_pid);
+      gdb_assert (lwp != NULL);
+      kill_wait_lwp (lwp);
+      process = find_process_pid (ptid_get_pid (last.value.related_pid));
+      the_target->mourn (process);
+    }
 
   process = find_process_pid (pid);
   if (process == NULL)
diff --git a/gdb/gdbserver/server.c b/gdb/gdbserver/server.c
index 19e3ba4..bf5586e 100644
--- a/gdb/gdbserver/server.c
+++ b/gdb/gdbserver/server.c
@@ -1356,6 +1356,15 @@ handle_qxfer_threads_worker (struct inferior_list_entry *inf, void *arg)
   int core = target_core_of_thread (ptid);
   char core_s[21];
 
+  /* Skip new threads created as the result of a fork if we are not done
+     handling that fork event.  We won't know whether to tell GDB about
+     the new thread until we are done following the fork.  */
+  if ((last_status.kind == TARGET_WAITKIND_FORKED
+       || last_status.kind == TARGET_WAITKIND_VFORKED)
+      && (ptid_get_pid (last_status.value.related_pid)
+	  == ptid_get_pid (ptid)))
+    return;
+
   write_ptid (ptid_s, ptid);
 
   if (core != -1)
@@ -4124,3 +4133,12 @@ handle_target_event (int err, gdb_client_data client_data)
 
   return 0;
 }
+
+/* Retrieve the last waitstatus reported to GDB.  */
+
+void
+get_last_target_status (ptid_t *ptid, struct target_waitstatus *last)
+{
+  *ptid = last_ptid;
+  *last = last_status;
+}
diff --git a/gdb/gdbserver/server.h b/gdb/gdbserver/server.h
index 5284dac..86e584e 100644
--- a/gdb/gdbserver/server.h
+++ b/gdb/gdbserver/server.h
@@ -102,6 +102,8 @@ typedef int gdb_fildes_t;
 /* Functions from server.c.  */
 extern int handle_serial_event (int err, gdb_client_data client_data);
 extern int handle_target_event (int err, gdb_client_data client_data);
+extern void get_last_target_status (ptid_t *ptid,
+				    struct target_waitstatus *last);
 
 #include "remote-utils.h"
 
diff --git a/gdb/remote.c b/gdb/remote.c
index 13fd837..4c58699 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -1457,6 +1457,54 @@ remote_fork_event_p (struct remote_state *rs)
   return packet_support (PACKET_fork_event_feature) == PACKET_ENABLE;
 }
 
+/* Returns true if vfork events are supported.  */
+
+static int
+remote_vfork_event_p (struct remote_state *rs)
+{
+  return packet_support (PACKET_vfork_event_feature) == PACKET_ENABLE;
+}
+
+/* Insert fork catchpoint target routine.  If fork events are enabled
+   then return success, nothing more to do.  */
+
+static int
+remote_insert_fork_catchpoint (struct target_ops *ops, int pid)
+{
+  struct remote_state *rs = get_remote_state ();
+
+  return !remote_fork_event_p (rs);
+}
+
+/* Remove fork catchpoint target routine.  Nothing to do, just
+   return success.  */
+
+static int
+remote_remove_fork_catchpoint (struct target_ops *ops, int pid)
+{
+  return 0;
+}
+
+/* Insert vfork catchpoint target routine.  If vfork events are enabled
+   then return success, nothing more to do.  */
+
+static int
+remote_insert_vfork_catchpoint (struct target_ops *ops, int pid)
+{
+  struct remote_state *rs = get_remote_state ();
+
+  return !remote_vfork_event_p (rs);
+}
+
+/* Remove vfork catchpoint target routine.  Nothing to do, just
+   return success.  */
+
+static int
+remote_remove_vfork_catchpoint (struct target_ops *ops, int pid)
+{
+  return 0;
+}
+
 /* Tokens for use by the asynchronous signal handlers for SIGINT.  */
 static struct async_signal_handler *async_sigint_remote_twice_token;
 static struct async_signal_handler *async_sigint_remote_token;
@@ -11856,6 +11904,14 @@ Specify the serial device it is connected to (e.g. /dev/ttya).";
   extended_remote_ops.to_supports_disable_randomization
     = extended_remote_supports_disable_randomization;
   extended_remote_ops.to_follow_fork = remote_follow_fork;
+  extended_remote_ops.to_insert_fork_catchpoint
+    = remote_insert_fork_catchpoint;
+  extended_remote_ops.to_remove_fork_catchpoint
+    = remote_remove_fork_catchpoint;
+  extended_remote_ops.to_insert_vfork_catchpoint
+    = remote_insert_vfork_catchpoint;
+  extended_remote_ops.to_remove_vfork_catchpoint
+    = remote_remove_vfork_catchpoint;
 }
 
 static int
diff --git a/gdb/testsuite/gdb.threads/fork-thread-pending.exp b/gdb/testsuite/gdb.threads/fork-thread-pending.exp
index d229232..594f376 100644
--- a/gdb/testsuite/gdb.threads/fork-thread-pending.exp
+++ b/gdb/testsuite/gdb.threads/fork-thread-pending.exp
@@ -31,6 +31,26 @@ if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}" executab
     return -1
 }
 
+# Find a thread that did not fork and is not the main thread and
+# return its thread number.  We can't just hard-code the thread
+# number since we have no guarantee as to the ordering of the threads
+# in gdb.  We know that the main thread is in pthread_join and the
+# forking thread is in fork, so we use this rather ungainly regexp
+# to capture an entry from 'info threads' that doesn't show one of
+# those routines, then extract the thread number.
+
+proc find_unforked_thread { } {
+    gdb_test_multiple "info threads" "find unforked thread" {
+        -re "(\[^\r]*Thread\[^\r]* in \[^fp]\[^ot]\[^rh]\[^kr]\[^e]\[^a]\[^d]\[^_]\[^j]\[^\r]*\r\n)" {
+    	    regexp "(\[ 	]*)(\[0-9]*)(\[    ]*Thread\[^\r]*\r\n)" $expect_out(0,string) ignore lead_spc threadnum rest
+        }
+        timeout {
+	    set threadnum -1
+        }
+    }
+    return $threadnum
+}
+
 clean_restart ${binfile}
 
 if ![runto_main] then {
@@ -46,7 +66,8 @@ gdb_test "continue" "Catchpoint.*" "1, get to the fork event"
 
 gdb_test "info threads" " Thread .* Thread .* Thread .* Thread .*" "1, multiple threads found"
 
-gdb_test "thread 1" ".*" "1, switched away from event thread"
+set threadnum [find_unforked_thread]
+gdb_test "thread $threadnum" ".*" "1, switched away from event thread to thread $threadnum"
 
 gdb_test "continue" "Not resuming.*" "1, refused to resume"
 
-- 
1.8.1.1