Decouple user selection from internal selectionusers/simark/user-selection-rfc

I am sending this as an RFC because it's far from complete and
definitive, but I'd like to gather some comments and opinions before
going further in this direction.

The goal of this patch is to decouple the notion of the user-selected
inferior/thread/frame from GDB's internally selected
inferior/thread/frame.

Currently, for example, the inferior_ptid variable has two jobs:

 - it's the user-selected thread: it's changed by the "thread" command.
   Other commands (continue, backtrace, etc) apply to this thread.
 - it's the internally-selected thread: it defines the thread GDB is
   currently "working" on.  For example, implementations of
   to_xfer_partial will refer to it to know from which thread to
   read/write memory.

Because of this dual usage, if we want to do some operations on a thread
other than the currently selected one, we have to save the current
inferior/thread/frame and restore them when we're done.  Failing to do
so would result in an unexpected selection switch for the user.

To improve this, Pedro suggested in [1] to decouple the two concepts.  This
is essentially what this patch is trying to do.

A new "user_selection" object is introduced, which contains the selected
inferior/thread/frame from the point of view of the user.  Before every
command, we "apply" this selection to the core of GDB to make sure the
internal selection matches the user selection.

There is a single user selection for the whole GDB (named "global
user-selection"), but as was mentioned in the linked thread, it opens
the door to having different selections for different UIs.  This means
that each UI would have its own user-selection object, which would be
applied to the core prior to executing commands from this UI.

The global user-selection object only gets modified when we really
intend to change it.  It can be because of the thread / -thread-select /
up / down / frame / inferior commands, a breakpoint hit in all-stop, an
inferior exit, etc.

The problem that initially prompted this effort is that the "--thread"
flag of MI commands changes the user-selected thread under the user's
feet.  My initial attempt to fix it was to restore the selection after
the MI command execution.  However, some cases are hard to get right.
For example:

  (thread 1 is currently selected)
  -interpreter-exec --thread 2 console "thread 3"

Restoring the selected thread to thread 1 after the MI command execution
wrongfully cancels the switch to thread 3.  So it's hard to determine
when we should or shouldn't restore.   With the current patch, it works
naturally: the --thread flag doesn't touch the user-selected thread,
only the internal one.  The "thread 3" command updates the user
selection.

Another difficulty is to send the right notifications to MI when the
user selection changes.  That means to not miss any, but not send too
many either.  Getting it somewhat right lead to ugly hacks (see the
command_notifies_uscc_observer function) and even then it's not perfect
(see the kfails in user-selected-context-sync.exp test).  With the
proposed method, it's easy to know when the user-selection changes and
send notifications.

With this patch, there are probably a few usage of
make_cleanup_restore_current_thread that are not needed anymore, if they
are only used to restore the user selection.  I kept removing them for a
later time though.

In the current state, there are a few minor regressions in the testsuite
(especially some follow-fork stuff I'm not sure how to handle), but the
vast majority of the previously passing tests still pass.

Comments are welcome!

Thanks,

Simon

[1] https://sourceware.org/ml/gdb-patches/2016-08/msg00031.html

1 files changed, 25 insertions, 24 deletions

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 1e5e9f1..a55acdf 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -64,6 +64,7 @@
 #include "event-loop.h"
 #include "thread-fsm.h"
 #include "common/enum-flags.h"
+#include "user-selection.h"
 
 /* Prototypes for local functions */
 
@@ -152,12 +153,6 @@ show_step_stop_if_no_debug (struct ui_file *file, int from_tty,
   fprintf_filtered (file, _("Mode of the step operation is %s.\n"), value);
 }
 
-/* proceed and normal_stop use this to notify the user when the
-   inferior stopped in a different thread than it had been running
-   in.  */
-
-static ptid_t previous_inferior_ptid;
-
 /* If set (default for legacy reasons), when following a fork, GDB
    will detach from one of the fork branches, child or parent.
    Exactly which branch is detached depends on 'set follow-fork-mode'
@@ -779,6 +774,11 @@ follow_fork (void)
 	      {
 		switch_to_thread (child);
 
+		/* Switch the user-selected thread as well.  */
+		struct thread_info *child_tp = find_thread_ptid (child);
+		gdb_assert (child_tp != nullptr);
+		global_user_selection ()->select_thread (child_tp, false);
+
 		/* ... and preserve the stepping state, in case the
 		   user was stepping over the fork call.  */
 		if (should_resume)
@@ -809,7 +809,14 @@ follow_fork (void)
 		follow_inferior_reset_breakpoints ();
 	      }
 	    else
-	      switch_to_thread (parent);
+	      {
+		switch_to_thread (parent);
+
+		/* Switch the user-selected thread as well.  */
+		struct thread_info *parent_tp = find_thread_ptid (parent);
+		gdb_assert (parent_tp != nullptr);
+		global_user_selection ()->select_thread (parent_tp, false);
+	      }
 	  }
       }
       break;
@@ -2999,9 +3006,6 @@ proceed (CORE_ADDR addr, enum gdb_signal siggnal)
       return;
     }
 
-  /* We'll update this if & when we switch to a new thread.  */
-  previous_inferior_ptid = inferior_ptid;
-
   regcache = get_current_regcache ();
   gdbarch = get_regcache_arch (regcache);
   aspace = get_regcache_aspace (regcache);
@@ -3252,8 +3256,6 @@ init_wait_for_inferior (void)
 
   target_last_wait_ptid = minus_one_ptid;
 
-  previous_inferior_ptid = inferior_ptid;
-
   /* Discard any skipped inlined frames.  */
   clear_inline_frame_state (minus_one_ptid);
 }
@@ -8157,7 +8159,7 @@ save_stop_context (void)
       /* Take a strong reference so that the thread can't be deleted
 	 yet.  */
       sc->thread = inferior_thread ();
-      sc->thread->refcount++;
+      sc->thread->get ();
     }
   else
     sc->thread = NULL;
@@ -8174,7 +8176,8 @@ release_stop_context_cleanup (void *arg)
   struct stop_context *sc = (struct stop_context *) arg;
 
   if (sc->thread != NULL)
-    sc->thread->refcount--;
+    sc->thread->put ();
+
   xfree (sc);
 }
 
@@ -8262,20 +8265,16 @@ normal_stop (void)
      after this event is handled, so we're not really switching, only
      informing of a stop.  */
   if (!non_stop
-      && !ptid_equal (previous_inferior_ptid, inferior_ptid)
       && target_has_execution
       && last.kind != TARGET_WAITKIND_SIGNALLED
       && last.kind != TARGET_WAITKIND_EXITED
       && last.kind != TARGET_WAITKIND_NO_RESUMED)
     {
-      SWITCH_THRU_ALL_UIS ()
-	{
-	  target_terminal_ours_for_output ();
-	  printf_filtered (_("[Switching to %s]\n"),
-			   target_pid_to_str (inferior_ptid));
-	  annotate_thread_changed ();
-	}
-      previous_inferior_ptid = inferior_ptid;
+      struct thread_info *thread = find_thread_ptid (inferior_ptid);
+
+      gdb_assert (thread != nullptr);
+
+      global_user_selection ()->select_thread (thread, false);
     }
 
   if (last.kind == TARGET_WAITKIND_NO_RESUMED)
@@ -8986,7 +8985,7 @@ struct infcall_control_state
   enum stop_stack_kind stop_stack_dummy;
   int stopped_by_random_signal;
 
-  /* ID if the selected frame when the inferior function call was made.  */
+  /* ID of the selected frame when the inferior function call was made.  */
   struct frame_id selected_frame_id;
 };
 
@@ -9080,6 +9079,8 @@ restore_infcall_control_state (struct infcall_control_state *inf_status)
 	/* Error in restoring the selected frame.  Select the innermost
 	   frame.  */
 	select_frame (get_current_frame ());
+
+      global_user_selection ()->select_frame (get_selected_frame (NULL), false);
     }
 
   xfree (inf_status);