remote: get rid of all the T packets when syncing the thread list

This commit avoids the prune_threads call in the remote target's
target_update_thread_list's implementation, eliminating all the "thread
alive" RSP traffic (one packet per thread) whenever we fetch the
thread list.

IOW, this:

 Sending packet: $Tp2141.2150#82...Packet received: OK
 Sending packet: $Tp2141.214f#b7...Packet received: OK
 Sending packet: $Tp2141.2141#82...Packet received: OK
 ... more T packets; it's one per previously known live thread ...
 Sending packet: $qXfer:threads:read::0,fff#03...Packet received: l<threads>\n<thread id="p2141.2141" core="2"/>\n<thread id="p2141.214f" core="1"/>\n<thread id="p2141.2150" core="2"/>\n</threads>\n

Becomes:

 Sending packet: $qXfer:threads:read::0,fff#03...Packet received: l<threads>\n<thread id="p2141.2141" core="2"/>\n<thread id="p2141.214f" core="1"/>\n<thread id="p2141.2150" core="2"/>\n</threads>\n

Tested on x86_64 Fedora 20, native gdbserver:
  - tests the qXfer:threads:read method.

Tested on x86_64 Fedora 20, native gdbserver with qXfer:threads:read
force-disabled in gdbserver:
  - So that GDB falls back to the qfThreadInfo/qsThreadInfo method.

And also manually smoked tested force disabling both
qXfer:threads:read and qfThreadInfo/qsThreadInfo in gdbserver.

gdb/
2014-10-15  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (ALL_NON_EXITED_THREADS_SAFE): New macro.
	* remote.c (remote_update_thread_list): Skip calling prune_threads
	if any thread listing method is supported, and instead walk over
	the set of remote threads listed, deleting those that are not
	found in GDB's thread list.

3 files changed, 50 insertions, 4 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 3a6bffd..abad945 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,13 @@
 2014-10-15  Pedro Alves  <palves@redhat.com>
 
+	* gdbthread.h (ALL_NON_EXITED_THREADS_SAFE): New macro.
+	* remote.c (remote_update_thread_list): Skip calling prune_threads
+	if any thread listing method is supported, and instead walk over
+	the set of remote threads listed, deleting those that are not
+	found in GDB's thread list.
+
+2014-10-15  Pedro Alves  <palves@redhat.com>
+
 	* ada-tasks.c (print_ada_task_info, task_command_1): Adjust.
 	* bsd-uthread.c (bsd_uthread_find_new_threads): Rename to ...
 	(bsd_uthread_update_thread_list): ... this.  Call prune_threads.
diff --git a/gdb/gdbthread.h b/gdb/gdbthread.h
index 1ed98a2..937bd55 100644
--- a/gdb/gdbthread.h
+++ b/gdb/gdbthread.h
@@ -354,6 +354,14 @@ extern struct thread_info *iterate_over_threads (thread_callback_func, void *);
   for (T = thread_list; T; T = T->next) \
     if ((T)->state != THREAD_EXITED)
 
+/* Like ALL_NON_EXITED_THREADS, but allows deleting the currently
+   iterated thread.  */
+#define ALL_NON_EXITED_THREADS_SAFE(T, TMP)	\
+  for ((T) = thread_list;			\
+       (T) != NULL ? ((TMP) = (T)->next, 1): 0;	\
+       (T) = (TMP))				\
+    if ((T)->state != THREAD_EXITED)
+
 extern int thread_count (void);
 
 /* Switch from one thread to another.  */
diff --git a/gdb/remote.c b/gdb/remote.c
index cd85fd5..98d3f2a 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -2762,9 +2762,7 @@ remote_update_thread_list (struct target_ops *ops)
   struct remote_state *rs = get_remote_state ();
   struct threads_listing_context context;
   struct cleanup *old_chain;
-
-  /* Delete GDB-side threads no longer found on the target.  */
-  prune_threads ();
+  int got_list = 0;
 
   context.items = NULL;
   old_chain = make_cleanup (clear_threads_listing_context, &context);
@@ -2778,8 +2776,31 @@ remote_update_thread_list (struct target_ops *ops)
     {
       int i;
       struct thread_item *item;
+      struct thread_info *tp, *tmp;
+
+      got_list = 1;
+
+      /* CONTEXT now holds the current thread list on the remote
+	 target end.  Delete GDB-side threads no longer found on the
+	 target.  */
+      ALL_NON_EXITED_THREADS_SAFE (tp, tmp)
+        {
+	  for (i = 0;
+	       VEC_iterate (thread_item_t, context.items, i, item);
+	       ++i)
+	    {
+	      if (ptid_equal (item->ptid, tp->ptid))
+		break;
+	    }
 
-      /* Now add threads we don't know about yet to our list.  */
+	  if (i == VEC_length (thread_item_t, context.items))
+	    {
+	      /* Not found.  */
+	      delete_thread (tp->ptid);
+	    }
+        }
+
+      /* And now add threads we don't know about yet to our list.  */
       for (i = 0;
 	   VEC_iterate (thread_item_t, context.items, i, item);
 	   ++i)
@@ -2803,6 +2824,15 @@ remote_update_thread_list (struct target_ops *ops)
 	}
     }
 
+  if (!got_list)
+    {
+      /* If no thread listing method is supported, then query whether
+	 each known thread is alive, one by one, with the T packet.
+	 If the target doesn't support threads at all, then this is a
+	 no-op.  See remote_thread_alive.  */
+      prune_threads ();
+    }
+
   do_cleanups (old_chain);
 }