Remote thread create/exit events

When testing with "maint set target-non-stop on", a few
threading-related tests expose an issue that requires new RSP packets.

Say there are 3 threads running, 1-3.  If GDB tries to stop thread 1,
2 and 3, and then waits for their stops, but meanwhile say, thread 2
exits, GDB hangs forever waiting for a stop for thread 2 that won't
ever happen.

This patch fixes the issue by adding support for thread exit events to
the protocol.  However, we don't want these always enabled, as they're
useless most of the time, and would slow down remote debugging.  So I
made it so that GDB can enable/disable them, and then made gdb do that
around the cases that need it, which currently is only
infrun.c:stop_all_threads.

In turn, if we have thread exit events, then the extra "thread x
exited" traffic slows down attach-many-short-lived-threads.exp enough
that gdb has trouble keeping up with new threads that are spawned
while gdb tries to stop existing ones.  To fix that I added support
for the counterpart thread created events too.  Enabling those when we
try to stop threads ensures that new threads never get a chance to
themselves start new threads, killing the race.

gdb/doc/ChangeLog:
2015-11-30  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Remote Configuration): List "set/show remote
	thread-events" command in configuration table.
	(Stop Reply Packets): Document "T05 create" stop
	reason and 'w' stop reply.
	(General Query Packets): Document QThreadEvents packet.  Document
	QThreadEvents qSupported feature.

gdb/gdbserver/ChangeLog:
2015-11-30  Pedro Alves  <palves@redhat.com>

	* linux-low.c (handle_extended_wait): Assert that the LWP's
	waitstatus is TARGET_WAITKIND_IGNORE.  If GDB wants to hear about
	thread create events, leave the new child's status pending.
	(linux_low_filter_event): If GDB wants to hear about thread exit
	events, leave the LWP marked dead and don't delete it.
	(linux_wait_for_event_filtered): Don't check for thread exit.
	(filter_exit_event): New function.
	(linux_wait_1): Use it, when returning an exit event.
	(linux_resume_one_lwp_throw): Assert that the LWP's
	waitstatus is TARGET_WAITKIND_IGNORE.
	* remote-utils.c (prepare_resume_reply): Handle
	TARGET_WAITKIND_THREAD_CREATED and TARGET_WAITKIND_THREAD_EXITED.
	* server.c (report_thread_events): New global.
	(handle_general_set): Handle QThreadEvents.
	(handle_query) <qSupported>: Handle and report QThreadEvents+;
	(handle_target_event): Handle TARGET_WAITKIND_THREAD_CREATED and
	TARGET_WAITKIND_THREAD_EXITED.
	* server.h (report_thread_events): Declare.

gdb/ChangeLog:
2015-11-30  Pedro Alves  <palves@redhat.com>

	* NEWS (New commands): Mention "set/show remote thread-events"
	commands.
	(New remote packets): Mention thread created/exited stop reasons
	and QThreadEvents packet.
	* infrun.c (disable_thread_events): New function.
	(stop_all_threads): Disable/enable thread create/exit events.
	Handle TARGET_WAITKIND_THREAD_EXITED.
	(handle_inferior_event_1): Handle TARGET_WAITKIND_THREAD_CREATED
	and TARGET_WAITKIND_THREAD_EXITED.
	* remote.c (remove_child_of_pending_fork): Also remove threads of
	threads that have TARGET_WAITKIND_THREAD_EXITED events.
	(remote_parse_stop_reply): Handle "create" magic register.  Handle
	'w' stop reply.
	(initialize_remote): Install remote_thread_events as
	to_thread_events target hook.
	(remote_thread_events): New function.
	* target-delegates.c: Regenerate.
	* target.c (target_thread_events): New function.
	* target.h (struct target_ops) <to_thread_events>: New field.
	(target_thread_events): Declare.
	* target/waitstatus.c (target_waitstatus_to_string): Handle
	TARGET_WAITKIND_THREAD_CREATED and TARGET_WAITKIND_THREAD_EXITED.
	* target/waitstatus.h (enum target_waitkind)
	<TARGET_WAITKIND_THREAD_CREATED, TARGET_WAITKIND_THREAD_EXITED):
	New values.

1 files changed, 18 insertions, 0 deletions

diff --git a/gdb/NEWS b/gdb/NEWS
index a7c12e7..a236161 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -48,6 +48,10 @@ set remote multiprocess-extensions-packet
 show remote multiprocess-extensions-packet
   Set/show the use of the remote protocol multiprocess extensions.
 
+set remote thread-events
+show remote thread-events
+  Set/show the use of thread create/exit events.
+
 * The "disassemble" command accepts a new modifier: /s.
   It prints mixed source+disassembly like /m with two differences:
   - disassembled instructions are now printed in program order, and
@@ -88,6 +92,20 @@ vCtrlC
   Equivalent to interrupting with the ^C character, but works in
   non-stop mode.
 
+thread created stop reason (T05 create:...)
+  Indicates that the thread was just created and is stopped at entry.
+
+thread exit stop reply (w exitcode;tid)
+  Indicates that the thread has terminated.
+
+QThreadEvents
+  Enables/disables thread create and exit event reporting.  For
+  example, this is used in non-stop mode when GDB stops a set of
+  threads and synchronously waits for the their corresponding stop
+  replies.  Without exit events, if one of the threads exits, GDB
+  would hang forever not knowing that it should no longer expect a
+  stop for that same thread.
+
 * Extended-remote exec events
 
   ** GDB now has support for exec events on extended-remote Linux targets.