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, 80 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.exp b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.exp
new file mode 100644
index 0000000..96ae41b
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.exp
@@ -0,0 +1,80 @@
+# 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 are registered when an event
+# handler for the new_objfile event is called.
+
+load_lib gdb-python.exp
+
+standard_testfile -main.cc
+
+set srcfile_lib "${testfile}-lib.cc"
+set python_event_handler_file "${srcdir}/${subdir}/${testfile}.py"
+set libname "lib${testfile}"
+set python_autoload_file "${srcdir}/${subdir}/${libname}.so-gdb.py"
+set binfile_lib [standard_output_file "${libname}.so"]
+
+# Start GDB first - needed for skip_python_tests.
+clean_restart
+
+# Skip all tests if Python scripting is not enabled.
+if { [skip_python_tests] } { continue }
+
+# Compile library.
+if { [gdb_compile_shlib ${srcdir}/${subdir}/${srcfile_lib} ${binfile_lib} \
+      {debug c++}] != "" } {
+    return -1
+}
+
+# Compile main program.
+if { [gdb_compile ${srcdir}/${subdir}/${srcfile} \
+      ${binfile} \
+      executable \
+      [list debug c++ shlib=$binfile_lib]] != "" } {
+    return -1
+}
+
+# Make the -gdb.py script available to gdb, it is automatically loaded by
+# gdb if it is put in the same directory as the library.
+set remote_python_autoload_file \
+    [gdb_remote_download host $python_autoload_file]
+
+gdb_test_no_output \
+    "set auto-load safe-path ${remote_python_autoload_file}" \
+    "set auto-load safe-path"
+
+# Load the Python file that defines a handler for the new_objfile event.
+set remote_python_event_handler_file\
+    [gdb_remote_download host $python_event_handler_file]
+gdb_test_no_output "source ${remote_python_event_handler_file}" "load python file"
+
+gdb_load ${binfile}
+
+gdb_test_no_output "set print pretty on"
+
+if { ![runto_main] } {
+    fail "failed to run to main"
+    return
+}
+
+# Check that the new_objfile handler saw the pretty-printer.
+gdb_test "print all_good" " = true"
+
+# Check that the pretty-printer actually works.
+gdb_test "info pretty-printer" "my_library.*MyClassTestLib.*"
+gdb_breakpoint [gdb_get_line_number "break to inspect"]
+gdb_test "continue" "Breakpoint $decimal, main .*"
+gdb_test "print test" "MyClassTestLib object, id: 1.*"