Workaround remote targets that report an empty list to qfThreadInfo

In https://sourceware.org/ml/gdb-patches/2014-10/msg00652.html, Sandra
shows a target that was broken by the recent update_thread_list
optimization:

 (gdb) target remote qa8-centos32-cs:10514
 ...
 (gdb) continue
 Continuing.
 Cannot execute this command without a live selected thread.
 (gdb)

The error means that the current thread is in "exited" state when the
continue command is processed.  The root of the problem was found
here:

 > Sending packet: $Hg0#df...Packet received:
 ...
 > Sending packet: $?#3f...Packet received: S00
 > Sending packet: $qfThreadInfo#bb...Packet received: l
 > Sending packet: $Hc-1#09...Packet received:
 > Sending packet: $qC#b4...Packet received: unset

This target doesn't really support threads (no thread indication in
stop reply packets; no support for qC), but then supports
qfThreadInfo, and returns an empty thread list to GDB.

See https://sourceware.org/ml/gdb-patches/2014-10/msg00665.html for
why the target does that.

As remote_update_thread_list deletes threads from GDB's list that are
not found in the thread list that the target reports, the result is
that GDB deletes the "fake" main thread that GDB added itself.  (As
that thread is currently selected, it is marked "exited" instead of
being deleted straight away.)

This commit avoids deleting the main thread in this scenario.

gdb/
2014-10-27  Pedro Alves  <palves@redhat.com>

	* remote.c (remote_thread_alive): New, factored out from ...
	(remote_thread_alive): ... this.
	(remote_update_thread_list): Bail out before deleting threads if
	the target returned an empty list, and, the current thread has a
	magic/fake ptid.

2 files changed, 40 insertions, 3 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 8a34118..b358dd7 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,13 @@
 2014-10-27  Pedro Alves  <palves@redhat.com>
 
+	* remote.c (remote_thread_alive): New, factored out from ...
+	(remote_thread_alive): ... this.
+	(remote_update_thread_list): Bail out before deleting threads if
+	the target returned an empty list, and, the current thread has a
+	magic/fake ptid.
+
+2014-10-27  Pedro Alves  <palves@redhat.com>
+
 	* infrun.c (handle_signal_stop): Also skip handlers when a random
 	signal arrives while handling a "stepi" or a "nexti".  Set the
 	thread's 'step_after_step_resume_breakpoint' flag.
diff --git a/gdb/remote.c b/gdb/remote.c
index 20f2988..4b9b099 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -1842,11 +1842,11 @@ set_general_process (void)
 }
 
 
-/*  Return nonzero if the thread PTID is still alive on the remote
-    system.  */
+/* Return nonzero if this is the main thread that we made up ourselves
+   to model non-threaded targets as single-threaded.  */
 
 static int
-remote_thread_alive (struct target_ops *ops, ptid_t ptid)
+remote_thread_always_alive (struct target_ops *ops, ptid_t ptid)
 {
   struct remote_state *rs = get_remote_state ();
   char *p, *endp;
@@ -1861,6 +1861,23 @@ remote_thread_alive (struct target_ops *ops, ptid_t ptid)
        multi-threading.  */
     return 1;
 
+  return 0;
+}
+
+/* Return nonzero if the thread PTID is still alive on the remote
+   system.  */
+
+static int
+remote_thread_alive (struct target_ops *ops, ptid_t ptid)
+{
+  struct remote_state *rs = get_remote_state ();
+  char *p, *endp;
+
+  /* Check if this is a thread that we made up ourselves to model
+     non-threaded targets as single-threaded.  */
+  if (remote_thread_always_alive (ops, ptid))
+    return 1;
+
   p = rs->buf;
   endp = rs->buf + get_remote_packet_size ();
 
@@ -2780,6 +2797,18 @@ remote_update_thread_list (struct target_ops *ops)
 
       got_list = 1;
 
+      if (VEC_empty (thread_item_t, context.items)
+	  && remote_thread_always_alive (ops, inferior_ptid))
+	{
+	  /* Some targets don't really support threads, but still
+	     reply an (empty) thread list in response to the thread
+	     listing packets, instead of replying "packet not
+	     supported".  Exit early so we don't delete the main
+	     thread.  */
+	  do_cleanups (old_chain);
+	  return;
+	}
+
       /* CONTEXT now holds the current thread list on the remote
 	 target end.  Delete GDB-side threads no longer found on the
 	 target.  */