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Re: Patch to propagate GDB's knowledge of the executing state to frontend

On 11/09/2012 05:48 PM, Pedro Alves wrote:
On 11/02/2012 06:46 PM, ali_anwar wrote:
On 11/02/2012 09:15 PM, dje@google.com wrote: 
Yao Qi writes:
   >   On 10/25/2012 07:09 PM, ali_anwar wrote:
   >   >   [...]
   >   >   @@ -1,3 +1,13 @@
   >   >   +2012-10-25  Ali Anwar<ali_anwar@codesourcery.com>
   >   >   +
   >   >   +    * infrun.c (handle_inferior_event_stub, regcache_dup_stub):
   >   >   +    New functions.
   >   >   +    (normal_stop): Change to propagate GDB's knowledge of the
   >   >   +    executing state to frontend when not able to fetch registers.
   >   >   +    (wait_for_inferior): Chnage to propagate GDB's knowledge of
   >                                 ^^^^^^ typo
   >
   >
   >   >   +    the executing state if not able to fetch backtrace once the
   >   >   +    step has already occured.
   >                             ^^^^^^^ typo.
   >
   >   In each changelog entry, we'll put 'what do we change' instead of 'why
   >   do we change in this way'.  So this entry can be simplified.

Hi.

I agree with your first sentence, and would add that if such an
explanation is needed, it belongs in the code not the changelog.
[We don't have enough comments in the code explaining *why* things
are the way they are.]

But I'd say that's not the case here, at least for the changelog entries.
Instead, I would remove the leading "Change to", and just say "Propagate ...".

Also, I would add a comment to the code explaining *why* the calls are wrapped
in catch_error (and I would have the comment live at the call to catch_error,
not in the definition of the two new stubs).

One could also say the two new functions also require comments,
but they're pretty simple and hook_stop_stub doesn't have a comment,
so I'd be ok with leaving them out.

Thanks for the review. Please find attached the modified patch.

-Ali

target_state.patch


Index: gdb/ChangeLog
===================================================================
RCS file: /cvs/src/src/gdb/ChangeLog,v
retrieving revision 1.14760
diff -u -r1.14760 ChangeLog
--- gdb/ChangeLog	24 Oct 2012 19:08:15 -0000	1.14760
+++ gdb/ChangeLog	2 Nov 2012 18:41:48 -0000
@@ -1,3 +1,11 @@
+2012-10-25  Ali Anwar<ali_anwar@codesourcery.com>
+
+	* infrun.c (handle_inferior_event_stub, regcache_dup_stub):
+	New functions.
+	(normal_stop): Propagate GDB's knowledge of the executing
+	state to frontend.
+	(wait_for_inferior): Likewise.
+
  2012-10-24  Tristan Gingold<gingold@adacore.com>

  	* ravenscar-sparc-thread.c (ravenscar_sparc_fetch_registers):
Index: gdb/infrun.c
===================================================================
RCS file: /cvs/src/src/gdb/infrun.c,v
retrieving revision 1.559
diff -u -r1.559 infrun.c
--- gdb/infrun.c	17 Sep 2012 07:26:55 -0000	1.559
+++ gdb/infrun.c	2 Nov 2012 18:41:49 -0000
@@ -73,6 +73,10 @@

static int hook_stop_stub (void *);

+static int regcache_dup_stub (void *);
+
+static int handle_inferior_event_stub (void *);
+
  static int restore_selected_frame (void *);

  static int follow_fork (void);
@@ -2700,8 +2704,11 @@
  	 state.  */
        old_chain = make_cleanup (finish_thread_state_cleanup,&minus_one_ptid);

-      /* Now figure out what to do with the result of the result.  */
-      handle_inferior_event (ecs);
+      /* Now figure out what to do with the result of the result. If an
+         error happens while handling the event, catch it to propagate
+         GDB's knowledge of the executing state.  */
+      catch_errors (handle_inferior_event_stub, ecs,
+                    "Error while handling inferior event:\n", RETURN_MASK_ALL);

        /* No error, don't finish the state yet.  */
        discard_cleanups (old_chain);


I don't understand this.  The point of the finish_thread_state_cleanup
is doing exactly what you say is missing.  If you swallow errors, then the
cleanup doesn't run at all (it's discarded immediately afterwards).

@@ -6080,9 +6087,12 @@
        if (stop_registers)
  	regcache_xfree (stop_registers);

-      /* NB: The copy goes through to the target picking up the value of
-	 all the registers.  */
-      stop_registers = regcache_dup (get_current_regcache ());
+      /* NB: The copy goes through to the target picking up the value
+         of all the registers. Catch error to propagate GDB's knowledge
+         of the executing state to frontend even when not able to fetch
+         registers.  */
+      catch_errors (regcache_dup_stub, NULL,
+                    "Error while running regcache_dup:\n", RETURN_MASK_ALL);


normal_stop has the finish_thread_state_cleanup installed too at the
top, and it has been run already above:

   /* Let the user/frontend see the threads as stopped.  */
   do_cleanups (old_chain);

So I'm afraid I don't understand exactly what this is fixing, and _how_
this is fixing it.  On an error, frontends should just no longer trust
any of their state, and issue a full refresh.

Hey,

Should frontends relying on MI information treat ^error specially and not look for any *stopped records?

Suppose a step command was issued and we failed middleway through that command, at a point where gdb ran the inferior, noticed it stop but could not fetch enough data to produce a backtrace. Say, register access error or memory access error.

At this point, the inferior is already stopped, but all the frontend is left with is the ^error output, which does not propagate the state of the inferior very clearly.

For the frontend, the inferior is still running given the lack of a *stopped record.

The MI specification gives room for slightly different interpretations unfortunately.

Luis

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