Fix GDB internal error against targets that return a thread in T stop replies but don't support qC.

Yao writes:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

GDB gets an internal error when it connects to GDBserver started with
'--disable-packet=qC'.

Sending packet: $QNonStop:0#8c...Packet received: OK
Sending packet: $?#3f...Packet received: T0505:00000000;04:00f0ffbf;08:b0c2e44c;thread:p4255.4255;core:1;
Sending packet: $Hc-1#09...Packet received: E01
Sending packet: $qC#b4...Packet received:
Sending packet: $qAttached:a410#bf...Packet received: E01
Packet qAttached (query-attached) is supported
warning: Remote failure reply: E01
Sending packet: $qOffsets#4b...Packet received:
../../../git/gdb/target.c:3248: internal-error: Can't determine the current address space of thread Thread 16981

When start remote, the call chain is as follows,

remote_start_remote
  add_current_inferior_and_thread <--[1]
  ...
  start_remote
    wait_for_inferior
      remote_wait_as
        process_stop_reply
          get_thread_arch_regcache   <--[2]
          remote_notice_new_inferior <--[3]

GDB sends packet "qC" in [1] and adds the thread/inferior if the remote
stubs understands "qC".  In [2], GDB looks for the inferior to build a
regcache, and notices a new inferior in [3].  As we can see, GDB assumes
that the inferior can be found in [2].  Once the remote stub doesn't
support "qC", GDB can't look for the inferior in [2], and emits an
internal error.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Right after the initial connection, we query the target for its state,
with the ? packet.  We store the resulting wait status / stop reply
aside, and query the target for the current thread, using qC, which
fails, so we fake a ptid for the target's thread.  We then later,
after the initial setup, end up consuming that set-aside wait status,
parsing the T stop reply, which contains a "thread" "register" (which
was the thread the target would have replied to qC).  We get into
trouble because the ptid in that stop reply doesn't match our faked up
ptid in the initial setup, although the target threads are the same...

So we had the T stop reply handy all along.  We might as well extract
the thread's ptid from it, and avoid all the resulting issues.

qC is also used after vRun, in order to discover the new process'es
main thread.  But, vRun's reply is also a wait status, just like
'?''s, which is quite convenient.

This means that if we have a "Txx thread: ptid" reply, then we don't
really need qC.  The patch makes GDB look in the T reply first, and if
not found, try with qC.  The packet handling seems to have been added
in gdb-4.18 (1999), and I see that in that same release, "Txx thread:
ptid" didn't exist yet, which probably explains why nobody though of
doing this before.

Regression tested against a gdbserver with qC disabled (and then
enabled), on x86_64 Fedora 17.

2013-01-25  Pedro Alves  <palves@redhat.com>

	* remote.c (stop_reply_extract_thread): New.
	(add_current_inferior_and_thread): New parameter 'wait_status'.
	Handle it.
	(remote_start_remote): Pass wait status to
	add_current_inferior_and_thread.
	(extended_remote_run): Update comment.
	(extended_remote_create_inferior_1): Pass wait status to
	add_current_inferior_and_thread.

1 files changed, 11 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index af0d7e6..2804269 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,14 @@
+2013-01-25  Pedro Alves  <palves@redhat.com>
+
+	* remote.c (stop_reply_extract_thread): New.
+	(add_current_inferior_and_thread): New parameter 'wait_status'.
+	Handle it.
+	(remote_start_remote): Pass wait status to
+	add_current_inferior_and_thread.
+	(extended_remote_run): Update comment.
+	(extended_remote_create_inferior_1): Pass wait status to
+	add_current_inferior_and_thread.
+
 2013-01-25  Andrew Burgess  <aburgess@broadcom.com>
 	    Ulrich Weigand  <uweigand@de.ibm.com>