Reduce Hg packet (select remote general thread) bouncing

A patch I wrote made GDB pull the thread list sooner when debugging
with target remote, and I noticed an intended consequence.  GDB
started bouncing around the currently selected remote/general thread
more frequently.  E.g.:

  Sending packet: $qTMinFTPILen#3b...Packet received: 5
 +Sending packet: $Hgp726d.726d#53...Packet received: OK
  Sending packet: $m400680,40#2f...Packet received: 85c0741455bff00d60004889e5ffd05de97bffffff0f1f00e973ffffff0f1f00554889e5c745fc00000000c745fc01000000e900000000c745fc02000000b800
 +Sending packet: $Hgp726d.7278#28...Packet received: OK
  Sending packet: $m4006b2,1#28...Packet received: e9
  Fast tracepoint 2 at 0x4006b2: file gdb/testsuite/gdb.trace/range-stepping.c, line 53.
  Sending packet: $qTStatus#49...Packet received: T0;tnotrun:0;tframes:0;tcreated:0;tfree:500000;tsize:500000;circular:0;disconn:0;starttime:0;stoptime:0;username:;notes::

This ended up breaking "tstart" when one has fast tracepoints set,
because gdbserver isn't expecting an Hg packet in response to
qRelocInsn:

 (gdb) ftrace *set_point
 Fast tracepoint 3 at 0x4006b2: file gdb/testsuite/gdb.trace/range-stepping.c, line 53.
 (gdb) PASS: gdb.trace/range-stepping.exp: ftrace: ftrace *set_point
 tstart
 gdbserver: Malformed response to qRelocInsn, ignoring: Hgp2783.2783

 Target does not support this command.
 (gdb) FAIL: gdb.trace/range-stepping.exp: ftrace: tstart

remote_trace_start should probably start by making sure the remote
current thread matches inferior_ptid (calling set_general_thread), but
still, reducing unnecessary bouncing is a good idea.  It happens
because the memory/symbol/breakpoint routines use
switch_to_program_space_and_thread to do something in the right
context and then revert back to the previously current thread.

The fix is to simply make any_thread_of_process,
find_inferior_for_program_space, etc. give preference to the current
thread/inferior it if matches.

gdb/
2014-10-02  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (any_thread_of_process, any_live_thread_of_process):
	Adjust comments.
	* inferior.c (find_inferior_for_program_space): Give preference to
	the current inferior.
	* inferior.h (find_inferior_for_program_space): Update comment.
	* progspace.c (switch_to_program_space_and_thread): Prefer the
	current inferior if it's bound to the program space requested.  If
	the inferior found doesn't have a PID yet, don't bother looking up
	a thread.
	* progspace.h (switch_to_program_space_and_thread): Adjust
	comment.
	* thread.c (any_thread_of_process, any_live_thread_of_process):
	Give preference to the current thread.

1 files changed, 2 insertions, 3 deletions

diff --git a/gdb/progspace.c b/gdb/progspace.c
index b111a50..48cb34a 100644
--- a/gdb/progspace.c
+++ b/gdb/progspace.c
@@ -459,8 +459,7 @@ save_current_space_and_thread (void)
   return old_chain;
 }
 
-/* Switches full context to program space PSPACE.  Switches to the
-   first thread found bound to PSPACE.  */
+/* See progspace.h  */
 
 void
 switch_to_program_space_and_thread (struct program_space *pspace)
@@ -468,7 +467,7 @@ switch_to_program_space_and_thread (struct program_space *pspace)
   struct inferior *inf;
 
   inf = find_inferior_for_program_space (pspace);
-  if (inf != NULL)
+  if (inf != NULL && inf->pid != 0)
     {
       struct thread_info *tp;