[RFC][PATCH 4/5] [PR gdb/33469] mi-interp: on_param_changed: pass async = true to redirect()

["Dennis Lambe Jr." <malsyned@malsyned.net>]

When frame filters are enabled and a stack unwinder does something to
cause an async record to be emitted, -stack-list-frames output is
incorrect. A comma is missing from the first async record, and a
spurious comma is inserted before the first frame= of the frame list.

This happens because async record emitters like
mi_interp::on_param_changed() are written as though calling
mi_out::do_redirect() ensures that a comma will be emitted by the next
following mi_ui_out::field_string() call. It's not guaranteed, though.
It usually works out by coincidence because the emitter methods are
called when no other output record is mid-creation.

Pass async = true to the ui_out_redirect_pop constructor to back up and
reset ui_out state related to list generation.

RFC: This version of this patch only changes
mi_interp::on_param_changed(), because that's the async record my code
was emitting when I found this bug and that I have good test cases for.
I'm interested in revving this patch set to change other async record
emitters in mi-interp.c based on comments on this version.
---
 gdb/mi/mi-interp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gdb/mi/mi-interp.c b/gdb/mi/mi-interp.c
index 8373e1a9ea8..10c1e70ce1e 100644
--- a/gdb/mi/mi-interp.c
+++ b/gdb/mi/mi-interp.c
@@ -800,7 +800,8 @@ mi_interp::on_param_changed (const char *param, const char *value)
 
   gdb_printf (this->event_channel, "cmd-param-changed");
 
-  ui_out_redirect_pop redir (mi_uiout, this->event_channel);
+  ui_out_redirect_pop redir (mi_uiout, this->event_channel,
+			     /* async = */ true);
 
   mi_uiout->field_string ("param", param);
   mi_uiout->field_string ("value", value);
-- 
2.51.0