gdb: do autoload before notifying Python side in new_objfile event

Without any explicit dependencies specified, the observers attached
to the 'gdb::observers::new_objfile' observable are always notified
in the order in which they have been attached.

The new_objfile observer callback to auto-load scripts is attached in
'_initialize_auto_load'.
The new_objfile observer callback that propagates the new_objfile event
to the Python side is attached in 'gdbpy_initialize_inferior', which is
called via '_initialize_python'.
With '_initialize_python' happening before '_initialize_auto_load',
the consequence was that the new_objfile event was emitted on the Python
side before autoloaded scripts had been executed when a new objfile was
loaded.
As a result, trying to access the objfile's pretty printers (defined in
the autoloaded script) from a handler for the Python-side
'new_objfile' event would fail. Those would only be initialized later on
(when the 'auto_load_new_objfile' callback was called).

To make sure that the objfile passed to the Python event handler
is properly initialized (including its 'pretty_printers' member),
make sure that the 'auto_load_new_objfile' observer is notified
before the 'python_new_objfile' one that propagates the event
to the Python side.

To do this, make use of the mechanism to explicitly specify
dependencies between observers (introduced in a preparatory commit).

Add a corresponding testcase that involves a test library with an autoloaded
Python script and a handler for the Python 'new_objfile' event.

(The real world use case where I came across this issue was in an attempt
to extend handling for GDB pretty printers for dynamically loaded
objfiles in the Qt Creator IDE, s. [1] and [2] for more background.)

[1] https://bugreports.qt.io/browse/QTCREATORBUG-25339
[2] https://codereview.qt-project.org/c/qt-creator/qt-creator/+/333857/1

Tested on x86_64-linux (Debian testing).

gdb/ChangeLog:

	* gdb/auto-load.c (_initialize_auto_load): 'Specify token
	when attaching the 'auto_load_new_objfile' observer, so
	other observers can specify it as a dependency.
	* gdb/auto-load.h (struct token): Declare
	'auto_load_new_objfile_observer_token' as token to be used
	for the 'auto_load_new_objfile' observer.
	* gdb/python/py-inferior.c (gdbpy_initialize_inferior): Make
	'python_new_objfile' observer depend on 'auto_load_new_objfile'
	observer, so it gets notified after the latter.

gdb/testsuite/ChangeLog:

	* gdb.python/libpy-autoloaded-pretty-printers-in-newobjfile-event.so-gdb.py: New test.
	* gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-lib.cc: New test.
	* gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-lib.h: New test.
	* gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-main.cc: New test.
	* gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.exp: New test.
	* gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.py: New test.

Change-Id: I8275b3f4c3bec32e56dd7892f9a59d89544edf89

1 files changed, 50 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.py b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.py
new file mode 100644
index 0000000..1d9c75b
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.py
@@ -0,0 +1,50 @@
+# Copyright (C) 2021 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 that python pretty
+# printers defined in a python script that is autoloaded have been
+# registered when a custom event handler for the new_objfile event
+# is called.
+
+import gdb
+import os
+
+
+def new_objfile_handler(event):
+    assert isinstance(event, gdb.NewObjFileEvent)
+    objfile = event.new_objfile
+
+    # Only observe the custom test library.
+    libname = "libpy-autoloaded-pretty-printers-in-newobjfile-event"
+    if libname in os.path.basename(objfile.filename):
+        # If everything went well and the pretty-printer auto-load happened
+        # before notifying the Python listeners, we expect to see one pretty
+        # printer, and it must be ours.
+        all_good = (
+            len(objfile.pretty_printers) == 1
+            and objfile.pretty_printers[0].name == "my_library"
+        )
+
+        if all_good:
+            gdb.parse_and_eval("all_good = 1")
+        else:
+            print("Oops, not all good:")
+            print("pretty printer count: {}".format(len(objfile.pretty_printers)))
+
+            for pp in objfile.pretty_printers:
+                print("  - {}".format(pp.name))
+
+
+gdb.events.new_objfile.connect(new_objfile_handler)