Fix PR remote/19840: gdb crashes on reverse-stepi

Reverse debugging against a remote target that does reverse debugging
itself (with the bs/bc packets) always trips on:

 (gdb) target remote localhost:...
 (gdb) reverse-stepi
 ../../gdb/target.c:602: internal-error: default_execution_direction: to_execution_direction must be implemented for reverse async

I missed adding a to_execution_direction method to remote.c in commit
3223143295b5 (Adds target_execution_direction to make record targets
support async mode), GDB 7.4 time.  Later, GDB 7.8 switched to
target-async on by default, making the regression user-visible by
default too.

Fix is simply to add the missing to_execution_direction implementation
to target remote.

Tested by Andi Kleen against Simics.

gdb/ChangeLog:
2016-04-13  Pedro Alves  <palves@redhat.com>

	PR remote/19840
	* remote.c (struct remote_state) <last_resume_exec_dir>: New
	field.
	(new_remote_state): Default last_resume_exec_dir to EXEC_FORWARD.
	(remote_open_1): Reset last_resume_exec_dir to EXEC_FORWARD.
	(remote_resume): Store the last execution direction.
	(remote_execution_direction): New function.
	(init_remote_ops): Install it as to_execution_direction target_ops
	method.

2 files changed, 32 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 63e7caa..d03d5ae 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,15 @@
+2016-04-13  Pedro Alves  <palves@redhat.com>
+
+	PR remote/19840
+	* remote.c (struct remote_state) <last_resume_exec_dir>: New
+	field.
+	(new_remote_state): Default last_resume_exec_dir to EXEC_FORWARD.
+	(remote_open_1): Reset last_resume_exec_dir to EXEC_FORWARD.
+	(remote_resume): Store the last execution direction.
+	(remote_execution_direction): New function.
+	(init_remote_ops): Install it as to_execution_direction target_ops
+	method.
+
 2016-04-12  Pedro Alves  <palves@redhat.com>
 
 	* common/common-exceptions.h (GDB_XCPT_TRY): Update comment.
diff --git a/gdb/remote.c b/gdb/remote.c
index b7ff7bf..4c18b4f 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -393,6 +393,9 @@ struct remote_state
 
   int last_sent_step;
 
+  /* The execution direction of the last resume we got.  */
+  enum exec_direction_kind last_resume_exec_dir;
+
   char *finished_object;
   char *finished_annex;
   ULONGEST finished_offset;
@@ -481,6 +484,7 @@ new_remote_state (void)
   result->buf = (char *) xmalloc (result->buf_size);
   result->remote_traceframe_number = -1;
   result->last_sent_signal = GDB_SIGNAL_0;
+  result->last_resume_exec_dir = EXEC_FORWARD;
   result->fs_pid = -1;
 
   return result;
@@ -4999,6 +5003,8 @@ remote_open_1 (const char *name, int from_tty,
   rs->continue_thread = not_sent_ptid;
   rs->remote_traceframe_number = -1;
 
+  rs->last_resume_exec_dir = EXEC_FORWARD;
+
   /* Probe for ability to use "ThreadInfo" query, as required.  */
   rs->use_threadinfo_query = 1;
   rs->use_threadextra_query = 1;
@@ -5634,6 +5640,8 @@ remote_resume (struct target_ops *ops,
   rs->last_sent_signal = siggnal;
   rs->last_sent_step = step;
 
+  rs->last_resume_exec_dir = execution_direction;
+
   /* The vCont packet doesn't need to specify threads via Hc.  */
   /* No reverse support (yet) for vCont.  */
   if (execution_direction != EXEC_REVERSE)
@@ -12985,6 +12993,17 @@ remote_can_do_single_step (struct target_ops *ops)
     return 0;
 }
 
+/* Implementation of the to_execution_direction method for the remote
+   target.  */
+
+static enum exec_direction_kind
+remote_execution_direction (struct target_ops *self)
+{
+  struct remote_state *rs = get_remote_state ();
+
+  return rs->last_resume_exec_dir;
+}
+
 static void
 init_remote_ops (void)
 {
@@ -13130,6 +13149,7 @@ Specify the serial device it is connected to\n\
   remote_ops.to_remove_vfork_catchpoint = remote_remove_vfork_catchpoint;
   remote_ops.to_insert_exec_catchpoint = remote_insert_exec_catchpoint;
   remote_ops.to_remove_exec_catchpoint = remote_remove_exec_catchpoint;
+  remote_ops.to_execution_direction = remote_execution_direction;
 }
 
 /* Set up the extended remote vector by making a copy of the standard