gdb: add Python events for program space addition and removal

Initially I just wanted a Python event for when GDB removes a program
space, I'm writing a Python extension that caches information for each
program space, and need to know when I should discard entries for a
particular program space.

But, it seemed easy enough to also add an event for when GDB adds a
new program space, so I went ahead and added both new events.

Of course, we don't currently have an observable for program space
addition or removal, so I first needed to add these.  After that it's
pretty simple to add two new Python events and have these trigger.

The two new event registries are:

  events.new_progspace
  events.free_progspace

These emit NewProgspaceEvent and FreeProgspaceEvent objects
respectively, each of these new event types has a 'progspace'
attribute that contains the relevant gdb.Progspace object.

There's a couple of things to be mindful of.

First, it is not possible to catch the NewProgspaceEvent for the very
first program space, the one that is created when GDB first starts, as
this program space is created before any Python scripts are sourced.

In order to allow this event to be caught we would need to defer
creating the first program space, and as a consequence the first
inferior, until some later time.  But, existing scripts could easily
depend on there being an initial inferior, so I really don't think we
should change that -- and so, we end up with the consequence that we
can't catch the event for the first program space.

The second, I think minor, issue, is that GDB doesn't clean up its
program spaces upon exit -- or at least, they are not cleaned up
before Python is shut down.  As a result, any program spaces in use at
the time GDB exits don't generate a FreeProgspaceEvent.  I'm not
particularly worried about this for my use case, I'm using the event
to ensure that a cache doesn't hold stale entries within a single GDB
session.  It's also easy enough to add a Python at-exit callback which
can do any final cleanup if needed.

Finally, when testing, I did hit a slightly weird issue with some of
the remote boards (e.g. remote-stdio-gdbserver).  As a consequence of
this issue I see some output like this in the gdb.log:

  (gdb) PASS: gdb.python/py-progspace-events.exp: inferior 1
  step
  FreeProgspaceEvent: <gdb.Progspace object at 0x7fb7e1d19c10>
  warning: cannot close "target:/lib64/libm.so.6": Cannot execute this command while the target is running.
  Use the "interrupt" command to stop the target
  and then try again.
  warning: cannot close "target:/lib64/libc.so.6": Cannot execute this command while the target is running.
  Use the "interrupt" command to stop the target
  and then try again.
  warning: cannot close "target:/lib64/ld-linux-x86-64.so.2": Cannot execute this command while the target is running.
  Use the "interrupt" command to stop the target
  and then try again.
  do_parent_stuff () at py-progspace-events.c:41
  41        ++global_var;
  (gdb) PASS: gdb.python/py-progspace-events.exp: step

The 'FreeProgspaceEvent ...' line is expected, that's my test Python
extension logging the event.  What isn't expected are all the blocks
like:

  warning: cannot close "target:/lib64/libm.so.6": Cannot execute this command while the target is running.
  Use the "interrupt" command to stop the target
  and then try again.

It turns out that this has nothing to do with my changes, this is just
a consequence of reading files over the remote protocol.  The test
forks a child process which GDB stays attached too.  When the child
exits, GDB cleans up by calling prune_inferiors, which in turn can
result in GDB trying to close some files that are open because of the
inferior being deleted.

If the prune_inferiors call occurs when the remote target is
running (and in non-async mode) then GDB will try to send a fileio
packet while the remote target is waiting for a stop reply, and the
remote target will throw an error, see remote_target::putpkt_binary in
remote.c for details.

I'm going to look at fixing this, but, as I said, this is nothing to
do with this change, I just mention it because I ended up needing to
account for these warning messages in one of my tests, and it all
looks a bit weird.

Approved-By: Tom Tromey <tom@tromey.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>

gdb/NEWS

@@ -319,6 +319,13 @@ show tui mouse-events
      gdb.Progspace) and 'reload' (a Boolean) attributes.  This event
      is emitted when gdb.Progspace.executable_filename changes.
 
+  ** New event registries gdb.events.new_progspace and
+     gdb.events.free_progspace, these emit NewProgspaceEvent and
+     FreeProgspaceEvent event types respectively.  Both of these event
+     types have a single 'progspace' attribute, which is the
+     gdb.Progspace that is either being added to GDB, or removed from
+     GDB.
+
 *** Changes in GDB 13
 
 * MI version 1 is deprecated, and will be removed in GDB 14.

gdb/doc/python.texi

@@ -3962,6 +3962,35 @@ command, @value{GDBN} updates both of these fields, but the executable
 file is updated first, so when this event is emitted, the executable
 filename will have changed, but the symbol filename might still hold
 its previous value.
+
+@item events.new_progspace
+This is emitted when @value{GDBN} adds a new program space
+(@pxref{Progspaces In Python,,Program Spaces In Python}).  The event
+is of type @code{gdb.NewProgspaceEvent}, and has a single read-only
+attribute:
+
+@defvar NewProgspaceEvent.progspace
+The @code{gdb.Progspace} that was added to @value{GDBN}.
+@end defvar
+
+No @code{NewProgspaceEvent} is emitted for the very first program
+space, which is assigned to the first inferior.  This first program
+space is created within @value{GDBN} before any Python scripts are
+sourced.
+
+@item events.free_progspace
+This is emitted when @value{GDBN} removes a program space
+(@pxref{Progspaces In Python,,Program Spaces In Python}), for example
+as a result of the @kbd{remove-inferiors} command
+(@pxref{remove_inferiors_cli,,@kbd{remove-inferiors}}).  The event is
+of type @code{gdb.FreeProgspaceEvent}, and has a single read-only
+attribute:
+
+@defvar FreeProgspaceEvent.progspace
+The @code{gdb.Progspace} that is about to be removed from
+@value{GDBN}.
+@end defvar
+
 @end table
 
 @node Threads In Python

gdb/observable.c

@@ -72,6 +72,8 @@ DEFINE_OBSERVABLE (gdb_exiting);
 DEFINE_OBSERVABLE (connection_removed);
 DEFINE_OBSERVABLE (target_pre_wait);
 DEFINE_OBSERVABLE (target_post_wait);
+DEFINE_OBSERVABLE (new_program_space);
+DEFINE_OBSERVABLE (free_program_space);
 
 } /* namespace observers */
 } /* namespace gdb */

gdb/observable.h

@@ -242,6 +242,12 @@ extern observable <ptid_t /* ptid */> target_pre_wait;
 /* About to leave target_wait (). */
 extern observable <ptid_t /* event_ptid */> target_post_wait;
 
+/* New program space PSPACE was created.  */
+extern observable <program_space */* pspace */> new_program_space;
+
+/* The program space PSPACE is about to be deleted.  */
+extern observable <program_space */* pspace */> free_program_space;
+
 } /* namespace observers */
 
 } /* namespace gdb */

gdb/progspace.c

@@ -28,6 +28,7 @@
 #include "inferior.h"
 #include <algorithm>
 #include "cli/cli-style.h"
+#include "observable.h"
 
 /* The last program space number assigned.  */
 static int last_program_space_num = 0;
@@ -98,6 +99,7 @@ program_space::program_space (address_space *aspace_)
     aspace (aspace_)
 {
   program_spaces.push_back (this);
+  gdb::observers::new_program_space.notify (this);
 }
 
 /* See progspace.h.  */
@@ -106,6 +108,7 @@ program_space::~program_space ()
 {
   gdb_assert (this != current_program_space);
 
+  gdb::observers::free_program_space.notify (this);
   remove_program_space (this);
 
   scoped_restore_current_program_space restore_pspace;

gdb/python/py-all-events.def

@@ -43,3 +43,5 @@ GDB_PY_DEFINE_EVENT(before_prompt)
 GDB_PY_DEFINE_EVENT(gdb_exiting)
 GDB_PY_DEFINE_EVENT(connection_removed)
 GDB_PY_DEFINE_EVENT(executable_changed)
+GDB_PY_DEFINE_EVENT(new_progspace)
+GDB_PY_DEFINE_EVENT(free_progspace)

gdb/python/py-event-types.def

@@ -130,3 +130,13 @@ GDB_PY_DEFINE_EVENT_TYPE (executable_changed,
 			  "ExecutableChangedEvent",
 			  "GDB executable changed event",
 			  event_object_type);
+
+GDB_PY_DEFINE_EVENT_TYPE (new_progspace,
+			  "NewProgspaceEvent",
+			  "GDB new Progspace event object",
+			  event_object_type);
+
+GDB_PY_DEFINE_EVENT_TYPE (free_progspace,
+			  "FreeProgspaceEvent",
+			  "GDB free Progspace event object",
+			  event_object_type);

gdb/python/py-progspace.c

@@ -643,11 +643,74 @@ gdbpy_executable_changed (struct program_space *pspace, bool reload_p)
       gdbpy_print_stack ();
 }
 
+/* Helper function to emit NewProgspaceEvent (when ADDING_P is true) or
+   FreeProgspaceEvent events (when ADDING_P is false).  */
+
+static void
+gdbpy_program_space_event (program_space *pspace, bool adding_p)
+{
+  if (!gdb_python_initialized)
+    return;
+
+  gdbpy_enter enter_py;
+
+  eventregistry_object *registry;
+  PyTypeObject *event_type;
+  if (adding_p)
+    {
+      registry = gdb_py_events.new_progspace;
+      event_type = &new_progspace_event_object_type;
+    }
+  else
+    {
+      registry = gdb_py_events.free_progspace;
+      event_type = &free_progspace_event_object_type;
+    }
+
+  if (evregpy_no_listeners_p (registry))
+    return;
+
+  gdbpy_ref<> pspace_obj = pspace_to_pspace_object (pspace);
+  if (pspace_obj == nullptr)
+    {
+      gdbpy_print_stack ();
+      return;
+    }
+
+  gdbpy_ref<> event = create_event_object (event_type);
+  if (event == nullptr
+      || evpy_add_attribute (event.get (), "progspace",
+			     pspace_obj.get ()) < 0
+      || evpy_emit_event (event.get (), registry) < 0)
+    gdbpy_print_stack ();
+}
+
+/* Emit a NewProgspaceEvent to indicate PSPACE has been created.  */
+
+static void
+gdbpy_new_program_space_event (program_space *pspace)
+{
+  gdbpy_program_space_event (pspace, true);
+}
+
+/* Emit a FreeProgspaceEvent to indicate PSPACE is just about to be removed
+   from GDB.  */
+
+static void
+gdbpy_free_program_space_event (program_space *pspace)
+{
+  gdbpy_program_space_event (pspace, false);
+}
+
 static int CPYCHECKER_NEGATIVE_RESULT_SETS_EXCEPTION
 gdbpy_initialize_pspace (void)
 {
   gdb::observers::executable_changed.attach (gdbpy_executable_changed,
 					     "py-progspace");
+  gdb::observers::new_program_space.attach (gdbpy_new_program_space_event,
+					    "py-progspace");
+  gdb::observers::free_program_space.attach (gdbpy_free_program_space_event,
+					     "py-progspace");
 
   if (PyType_Ready (&pspace_object_type) < 0)
     return -1;

gdb/testsuite/gdb.python/py-progspace-events.c

@@ -0,0 +1,73 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2023 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see  <http://www.gnu.org/licenses/>.  */
+
+#include <unistd.h>
+#include <sys/types.h>
+#include <assert.h>
+#include <sys/wait.h>
+
+/* This gives the inferior something to do.  */
+volatile int global_var = 0;
+
+void
+breakpt ()
+{ /* Nothing.  */ }
+
+void
+do_child_stuff ()
+{
+  breakpt ();
+  ++global_var;
+}
+
+void
+do_parent_stuff ()
+{
+  breakpt ();
+  ++global_var;
+}
+
+void
+create_child ()
+{
+  int stat;
+  pid_t wpid;
+  breakpt ();
+  pid_t pid = fork ();
+  assert (pid != -1);
+
+  if (pid == 0)
+    {
+      /* Child.  */
+      do_child_stuff ();
+      return;
+    }
+
+  /* Parent.  */
+  do_parent_stuff ();
+  wpid = waitpid (pid, &stat, 0);
+  assert (wpid == pid);
+}
+
+
+
+int
+main ()
+{
+  create_child ();
+  return 0;
+}

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

@@ -0,0 +1,125 @@
+# Copyright (C) 2023 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of the GDB testsuite.  It tests the program space
+# related events in the Python API.
+
+load_lib gdb-python.exp
+
+require allow_python_tests
+
+standard_testfile
+
+if {[prepare_for_testing "preparing" $testfile $srcfile] == -1} {
+    return -1
+}
+
+set pyfile [gdb_remote_download host ${srcdir}/${subdir}/py-progspace-events.py]
+gdb_test_no_output "source ${pyfile}" "load python file"
+
+if {![runto_main]} {
+    return
+}
+
+gdb_breakpoint breakpt
+
+gdb_continue_to_breakpoint "run to breakpt function"
+
+gdb_test_no_output "set detach-on-fork off"
+
+# Continue until the parent process forks and a new child is added.
+# Done this way so we can count the new progspace events; we expect to
+# see exactly one.
+set new_progspace_event_count 0
+gdb_test_multiple "continue" "continue until child process appears" {
+    -re "^NewProgspaceEvent: <gdb.Progspace object at $hex>\r\n" {
+	# This is a correctly formed event line.
+	incr new_progspace_event_count
+	exp_continue
+    }
+
+    -re "^NewProgspaceEvent:\[^\r\n\]+\r\n" {
+	# This is an incorrectly formed event line.
+	fail $gdb_test_name
+    }
+
+    -re "^$gdb_prompt $" {
+	pass $gdb_test_name
+    }
+
+    -re "^\[^\r\n\]*\r\n" {
+	exp_continue
+    }
+}
+
+gdb_assert { $new_progspace_event_count == 1 } \
+    "only a single new progspace event seen"
+
+# Switch to inferior 2 and continue until we hit breakpt.
+gdb_test "inferior 2" "\\\[Switching to inferior 2 .*"
+gdb_continue_to_breakpoint "run to breakpt in inferior 2"
+
+# Let inferior 2 exit.  The new program space is not removed at this
+# point.
+gdb_test "continue" \
+    [multi_line \
+	 "^Continuing\\." \
+	 "\\\[Inferior $decimal \[^\r\n\]+ exited normally\\\]"] \
+    "continue until inferior 2 exits"
+
+gdb_test "inferior 1" "\\\[Switching to inferior 1 .*"
+
+# Step the inferior.  During this process GDB will prune the now
+# defunct inferior, which deletes its program space, which should
+# trigger the FreeProgspaceEvent.
+#
+# However, there is a slight problem.  When the target is remote, and
+# GDB is accessing files using remote fileio, then GDB will attempt to
+# prune the inferior at a point in time when the remote target is
+# waiting for a stop reply.  Pruning an inferior causes GDB to close
+# files associated with that inferior.
+#
+# In non-async mode we can't send fileio packets while waiting for a
+# stop reply, so the attempts to close files fails, and this shows up
+# as an error.
+#
+# As this error has nothing to do with the feature being tested here,
+# we just accept the error message, the important part is the
+# 'FreeProgspaceEvent' string, so long as that appears (just once)
+# then the test is a success.
+set warning_msg \
+    [multi_line \
+	 "warning: cannot close \"\[^\r\n\]+\": Cannot execute this command while the target is running\\." \
+	 "Use the \"interrupt\" command to stop the target" \
+	 "and then try again\\."]
+
+gdb_test "step" \
+    [multi_line \
+	 "^FreeProgspaceEvent: <gdb.Progspace object at $hex>(?:\r\n$warning_msg)*" \
+	 "do_parent_stuff \\(\\) at \[^\r\n\]+" \
+	 "$decimal\\s+\[^\r\n\]+"]
+
+# Let this inferior run to completion.
+gdb_continue_to_end
+
+# Check the program space events trigger when a new inferior is
+# manually added and removed.
+gdb_test "add-inferior" \
+    [multi_line \
+	 "^NewProgspaceEvent: <gdb.Progspace object at $hex>" \
+	 "\\\[New inferior 3\\\]" \
+	 "Added inferior 3\[^\r\n\]*"]
+gdb_test "remove-inferior 3" \
+    "^FreeProgspaceEvent: <gdb.Progspace object at $hex>"