gdb: add maybe_switch_inferior function

Add the maybe_switch_inferior function, which ensures that the given
inferior is the current one.  Return an instantiated
scoped_restore_current_thread object only we actually needed to switch
inferior.

Returning a scoped_restore_current_thread requires it to be
move-constructible, so give it a move constructor.

Change-Id: I1231037102ed6166f2530399e8257ad937fb0569
Reviewed-By: Pedro Alves <pedro@palves.net>

gdb/gdbthread.h

@@ -851,6 +851,8 @@ public:
   scoped_restore_current_thread ();
   ~scoped_restore_current_thread ();
 
+  scoped_restore_current_thread (scoped_restore_current_thread &&rhs);
+
   DISABLE_COPY_AND_ASSIGN (scoped_restore_current_thread);
 
   /* Cancel restoring on scope exit.  */

gdb/inferior.c

@@ -684,6 +684,21 @@ switch_to_inferior_no_thread (inferior *inf)
   set_current_program_space (inf->pspace);
 }
 
+/* See regcache.h.  */
+
+gdb::optional<scoped_restore_current_thread>
+maybe_switch_inferior (inferior *inf)
+{
+  gdb::optional<scoped_restore_current_thread> maybe_restore_thread;
+  if (inf != current_inferior ())
+    {
+      maybe_restore_thread.emplace ();
+      switch_to_inferior_no_thread (inf);
+    }
+
+  return maybe_restore_thread;
+}
+
 static void
 inferior_command (const char *args, int from_tty)
 {

gdb/inferior.h

@@ -340,6 +340,13 @@ extern void set_current_inferior (inferior *);
    selected.  */
 extern void switch_to_inferior_no_thread (inferior *inf);
 
+/* Ensure INF is the current inferior.
+
+   If the current inferior was changed, return an RAII object that will
+   restore the original current context.  */
+extern gdb::optional<scoped_restore_current_thread> maybe_switch_inferior
+  (inferior *inf);
+
 /* Info about an inferior's target description.  There's one of these
    for each inferior.  */
 

gdb/thread.c

@@ -1378,6 +1378,20 @@ scoped_restore_current_thread::scoped_restore_current_thread ()
     }
 }
 
+scoped_restore_current_thread::scoped_restore_current_thread
+  (scoped_restore_current_thread &&rhs)
+  : m_dont_restore (std::move (rhs.m_dont_restore)),
+    m_thread (std::move (rhs.m_thread)),
+    m_inf (std::move (rhs.m_inf)),
+    m_selected_frame_id (std::move (rhs.m_selected_frame_id)),
+    m_selected_frame_level (std::move (rhs.m_selected_frame_level)),
+    m_was_stopped (std::move (rhs.m_was_stopped)),
+    m_lang (std::move (rhs.m_lang))
+{
+  /* Deactivate the rhs.  */
+  rhs.m_dont_restore = true;
+}
+
 /* See gdbthread.h.  */
 
 int