PR python/13598 - add before_prompt event

This adds an event that is emitted just before GDB presents a prompt
to the user.  This provides Python code a way to react to whatever
changes might have been made by the previous command.  For example, in
my GUI I use this to track changes to the selected frame and reflect
them in the UI.

Built and regtested on x86-64 Fedora 23.

gdb/ChangeLog
2017-02-14  Tom Tromey  <tom@tromey.com>

	PR python/13598:
	* python/python.c (gdbpy_before_prompt_hook): Emit before_prompt
	event.
	* python/py-evts.c (gdbpy_initialize_py_events): Add
	before_prompt registry.
	* python/py-events.h (events_object) <before_prompt>: New field.

gdb/doc/ChangeLog
2017-02-14  Tom Tromey  <tom@tromey.com>

	PR python/13598:
	* python.texi (Events In Python): Document events.before_prompt.

gdb/testsuite/ChangeLog
2017-02-14  Tom Tromey  <tom@tromey.com>

	PR python/13598:
	* gdb.python/py-events.exp: Add before_prompt event tests.

gdb/ChangeLog

@@ -1,3 +1,12 @@
+2017-02-14  Tom Tromey  <tom@tromey.com>
+
+	PR python/13598:
+	* python/python.c (gdbpy_before_prompt_hook): Emit before_prompt
+	event.
+	* python/py-evts.c (gdbpy_initialize_py_events): Add
+	before_prompt registry.
+	* python/py-events.h (events_object) <before_prompt>: New field.
+
 2017-02-14  Markus Metzger  <markus.t.metzger@intel.com>
 
 	* btrace.c (ftrace_new_switch): Preserve up link and flags.

gdb/doc/ChangeLog

@@ -1,3 +1,8 @@
+2017-02-14  Tom Tromey  <tom@tromey.com>
+
+	PR python/13598:
+	* python.texi (Events In Python): Document events.before_prompt.
+
 2017-02-13  Luis Machado  <lgustavo@codesourcery.com>
 
 	* gdb.texinfo (Target Commands): Document the optional offset

gdb/doc/python.texi

@@ -2981,6 +2981,10 @@ emitted, the @code{gdb.Breakpoint} object will already be in its
 invalid state; that is, the @code{is_valid} method will return
 @code{False}.
 
+@item events.before_prompt
+This event carries no payload.  It is emitted each time @value{GDBN}
+presents a prompt to the user.
+
 @end table
 
 @node Threads In Python

gdb/python/py-events.h

@@ -53,6 +53,7 @@ typedef struct
   eventregistry_object *breakpoint_created;
   eventregistry_object *breakpoint_deleted;
   eventregistry_object *breakpoint_modified;
+  eventregistry_object *before_prompt;
 
   PyObject *module;
 

gdb/python/py-evts.c

@@ -100,6 +100,9 @@ gdbpy_initialize_py_events (void)
 			"breakpoint_modified") < 0)
     return -1;
 
+  if (add_new_registry (&gdb_py_events.before_prompt, "before_prompt") < 0)
+    return -1;
+
   if (gdb_pymodule_addobject (gdb_module,
 			      "events",
 			      (PyObject *) gdb_py_events.module) < 0)

gdb/python/python.c

@@ -100,6 +100,7 @@ const struct extension_language_defn extension_language_python =
 #include "interps.h"
 #include "event-top.h"
 #include "py-ref.h"
+#include "py-event.h"
 
 /* True if Python has been successfully initialized, false
    otherwise.  */
@@ -969,6 +970,10 @@ gdbpy_before_prompt_hook (const struct extension_language_defn *extlang,
 
   gdbpy_enter enter_py (get_current_arch (), current_language);
 
+  if (!evregpy_no_listeners_p (gdb_py_events.before_prompt)
+      && evpy_emit_event (NULL, gdb_py_events.before_prompt) < 0)
+    return EXT_LANG_RC_ERROR;
+
   if (gdb_python_module
       && PyObject_HasAttrString (gdb_python_module, "prompt_hook"))
     {

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2017-02-14  Tom Tromey  <tom@tromey.com>
+
+	PR python/13598:
+	* gdb.python/py-events.exp: Add before_prompt event tests.
+
 2017-02-14  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
 	* gdb.dwarf2/implptrpiece.exp: Fix check for big-endian targets.

gdb/testsuite/gdb.python/py-events.exp

@@ -210,3 +210,28 @@ gdb_test "continue" ".*event type: continue.*
 .*exit code: 12.*
 .*exit inf: 2.*
 dir ok: True.*" "Inferior 2 terminated."
+
+
+# Test before_prompt event.
+gdb_py_test_multiple "define new user command" \
+    "define xxz" "End with a line saying just .end.." \
+    "set variable \$x = 72" "" \
+    "set variable \$y = 93" "" \
+    "end" ""
+
+gdb_py_test_multiple "add before_prompt listener" \
+    "python" "" \
+    "count = 0" "" \
+    "def listener():" "" \
+    "  global count" "" \
+    "  count = count + 1" "" \
+    "gdb.events.before_prompt.connect(listener)" "" \
+    "end" ""
+
+gdb_test_no_output "set variable \$x = 32" "do something"
+# Result is due to one emitted before "set var" and one emitted before
+# this command.
+gdb_test "python print(count)" 2 "check for before_prompt event"
+
+gdb_test_no_output "xxz" "run a canned sequence"
+gdb_test "python print(count)" 4 "check for before_prompt event"