[PING] [PATCH v3] gdb: Do autoload before notifying Python side in new_objfile event

[Michael Weghorn <m.weghorn@posteo.de>]

Ping.

On 26/03/2021 09.29, Michael Weghorn via Gdb-patches wrote:
> Previously, the observers attached to the 'gdb::observers::new_objfile'
> observable were always notified in the order in which they had 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, extend the 'observable' class to allow explicitly
> specifying dependencies when attaching observers, by adding
> the possibility to specify a key for the observer, and the
> keys for other observers it depends on, and make use of that
> mechanism to specify a dependency from 'python_new_objfile'
> on 'auto_load_new_objfile'.
> 
> To make sure dependencies are notified before observers
> depending on them, the vector holding the observers is sorted in
> a way that dependencies come before observers depending on them.
> The current implementation for sorting uses the depth-first search
> algorithm for topological sorting as described at [1].
> 
> 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. [2] and [3] for more background.)
> 
> [1] https://en.wikipedia.org/wiki/Topological_sorting#Depth-first_search
> [2] https://bugreports.qt.io/browse/QTCREATORBUG-25339
> [3] https://codereview.qt-project.org/c/qt-creator/qt-creator/+/333857/1
> 
> Tested on x86_64-linux (Debian testing).
> 
> gdb/ChangeLog:
> 
>         * auto-load.c (_initialize_auto_load): Specify key when
>         attaching the 'auto_load_new_objfile' observer, so
>         other observers can specify it as a dependency.
>         * auto-load.h (AUTO_LOAD_H): Declare
>         'auto_load_new_objfile_observer' as key to be used
>         for the 'auto_load_new_objfile' observer.
>         * 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.
> 
> gdbsupport/ChangeLog:
>         * observable.h (struct observer_key): New struct that can be
>         used as a key when attaching observers, and specyfing
>         dependencies to other observers
>         (class observable): Extend to allow specifying dependencies
>         between observers, keep vector holding observers sorted
>         that dependencies are notified before observers depending
>         on them.
> ---
>  gdb/auto-load.c                               |   5 +-
>  gdb/auto-load.h                               |  11 ++
>  gdb/python/py-inferior.c                      |   6 +-
>  ...tty-printers-in-newobjfile-event.so-gdb.py |  44 ++++++
>  ...pretty-printers-in-newobjfile-event-lib.cc |  26 ++++
>  ...-pretty-printers-in-newobjfile-event-lib.h |  31 +++++
>  ...retty-printers-in-newobjfile-event-main.cc |  23 ++++
>  ...ed-pretty-printers-in-newobjfile-event.exp |  96 +++++++++++++
>  ...ded-pretty-printers-in-newobjfile-event.py |  34 +++++
>  gdbsupport/observable.h                       | 127 ++++++++++++++++--
>  10 files changed, 389 insertions(+), 14 deletions(-)
>  create mode 100644 gdb/testsuite/gdb.python/libpy-autoloaded-pretty-printers-in-newobjfile-event.so-gdb.py
>  create mode 100644 gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-lib.cc
>  create mode 100644 gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-lib.h
>  create mode 100644 gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-main.cc
>  create mode 100644 gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.exp
>  create mode 100644 gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.py
> 
> diff --git a/gdb/auto-load.c b/gdb/auto-load.c
> index 1dfcf21eeeb..4ecfdd28f35 100644
> --- a/gdb/auto-load.c
> +++ b/gdb/auto-load.c
> @@ -1494,6 +1494,8 @@ found and/or loaded."),
>    return &retval;
>  }
>  
> +gdb::observers::observer_key auto_load_new_objfile_observer;
> +
>  void _initialize_auto_load ();
>  void
>  _initialize_auto_load ()
> @@ -1503,7 +1505,8 @@ _initialize_auto_load ()
>    char *guile_name_help;
>    const char *suffix;
>  
> -  gdb::observers::new_objfile.attach (auto_load_new_objfile);
> +  gdb::observers::new_objfile.attach (auto_load_new_objfile,
> +                                      &auto_load_new_objfile_observer);
>  
>    add_setshow_boolean_cmd ("gdb-scripts", class_support,
>  			   &auto_load_gdb_scripts, _("\
> diff --git a/gdb/auto-load.h b/gdb/auto-load.h
> index f726126c554..e599b862ff9 100644
> --- a/gdb/auto-load.h
> +++ b/gdb/auto-load.h
> @@ -25,6 +25,13 @@ struct program_space;
>  struct auto_load_pspace_info;
>  struct extension_language_defn;
>  
> +namespace gdb
> +{
> +namespace observers {
> +struct observer_key;
> +}
> +}
> +
>  /* Value of the 'set debug auto-load' configuration variable.  */
>  
>  extern bool debug_auto_load;
> @@ -40,6 +47,10 @@ extern bool auto_load_local_gdbinit;
>  extern char *auto_load_local_gdbinit_pathname;
>  extern bool auto_load_local_gdbinit_loaded;
>  
> +/* Key used for the auto_load_new_objfile observer, so other observers can
> + * specify it as a dependency. */
> +extern gdb::observers::observer_key auto_load_new_objfile_observer;
> +
>  extern struct auto_load_pspace_info *
>    get_auto_load_pspace_data_for_loading (struct program_space *pspace);
>  extern void auto_load_objfile_script (struct objfile *objfile,
> diff --git a/gdb/python/py-inferior.c b/gdb/python/py-inferior.c
> index a3d5952a10b..6ec3f1599ca 100644
> --- a/gdb/python/py-inferior.c
> +++ b/gdb/python/py-inferior.c
> @@ -18,6 +18,7 @@
>     along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
>  
>  #include "defs.h"
> +#include "auto-load.h"
>  #include "gdbcore.h"
>  #include "gdbthread.h"
>  #include "inferior.h"
> @@ -913,7 +914,10 @@ gdbpy_initialize_inferior (void)
>    gdb::observers::memory_changed.attach (python_on_memory_change);
>    gdb::observers::register_changed.attach (python_on_register_change);
>    gdb::observers::inferior_exit.attach (python_inferior_exit);
> -  gdb::observers::new_objfile.attach (python_new_objfile);
> +  // needs to run after auto_load_new_objfile observer, so autoloaded
> +  // pretty-printers are available
> +  gdb::observers::new_objfile.attach (python_new_objfile, nullptr,
> +                                      { &auto_load_new_objfile_observer });
>    gdb::observers::inferior_added.attach (python_new_inferior);
>    gdb::observers::inferior_removed.attach (python_inferior_deleted);
>  
> diff --git a/gdb/testsuite/gdb.python/libpy-autoloaded-pretty-printers-in-newobjfile-event.so-gdb.py b/gdb/testsuite/gdb.python/libpy-autoloaded-pretty-printers-in-newobjfile-event.so-gdb.py
> new file mode 100644
> index 00000000000..2327e4a7384
> --- /dev/null
> +++ b/gdb/testsuite/gdb.python/libpy-autoloaded-pretty-printers-in-newobjfile-event.so-gdb.py
> @@ -0,0 +1,44 @@
> +# 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.printing
> +
> +
> +class MyClassTestLibPrinter(object):
> +    "Print a MyClassTestLib"
> +
> +    def __init__(self, val):
> +        self.val = val
> +
> +    def to_string(self):
> +        return "MyClassTestLib object, id: {}".format(self.val['id'])
> +
> +    def display_hint(self):
> +        return "string"
> +
> +
> +def build_pretty_printer():
> +    pp = gdb.printing.RegexpCollectionPrettyPrinter(
> +        "my_library")
> +    pp.add_printer("MyClassTestLib", "^MyClassTestLib$", MyClassTestLibPrinter)
> +    return pp
> +
> +
> +gdb.printing.register_pretty_printer(gdb.current_objfile(), build_pretty_printer())
> diff --git a/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-lib.cc b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-lib.cc
> new file mode 100644
> index 00000000000..7e06cf3903f
> --- /dev/null
> +++ b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-lib.cc
> @@ -0,0 +1,26 @@
> +/* This testcase is part of GDB, the GNU debugger.
> +
> +   Copyright 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/>.  */
> +
> +#include "py-autoloaded-pretty-printers-in-newobjfile-event-lib.h"
> +
> +MyClassTestLib::MyClassTestLib(int theId) {
> +    id = theId;
> +}
> +
> +int MyClassTestLib::getId() {
> +    return id;
> +}
> diff --git a/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-lib.h b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-lib.h
> new file mode 100644
> index 00000000000..fa2501bf6de
> --- /dev/null
> +++ b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-lib.h
> @@ -0,0 +1,31 @@
> +/* This testcase is part of GDB, the GNU debugger.
> +
> +   Copyright 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/>.  */
> +
> +#ifndef TESTLIBRARY_H
> +#define TESTLIBRARY_H
> +
> +class MyClassTestLib {
> +
> +public:
> +    explicit MyClassTestLib(int theId);
> +    int getId();
> +
> +private:
> +    int id;
> +};
> +
> +#endif // TESTLIBRARY_H
> diff --git a/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-main.cc b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-main.cc
> new file mode 100644
> index 00000000000..6e66bbe3d43
> --- /dev/null
> +++ b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event-main.cc
> @@ -0,0 +1,23 @@
> +/* This testcase is part of GDB, the GNU debugger.
> +
> +   Copyright 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/>.  */
> +
> +#include "py-autoloaded-pretty-printers-in-newobjfile-event-lib.h"
> +
> +int main() {
> +    MyClassTestLib test(1);
> +    return 0; /* break to inspect */
> +}
> 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 00000000000..ed1c3230835
> --- /dev/null
> +++ b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.exp
> @@ -0,0 +1,96 @@
> +# 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.
> +
> +if [use_gdb_stub] {
> +    return 0
> +}
> +
> +load_lib gdb-python.exp
> +
> +set test "py-autoloaded-pretty-printers-in-newobjfile-event"
> +set srcfile_main "${test}-main.cc"
> +set executable_main ${test}-test
> +set binfile_main [standard_output_file ${executable_main}]
> +set srcfile_lib "${test}-lib.cc"
> +set libname "lib${test}"
> +set pyscriptfile_lib "${libname}.so-gdb.py"
> +set binfile_lib [standard_output_file ${libname}.so]
> +
> +
> +# Start with a fresh gdb.
> +gdb_exit
> +gdb_start
> +
> +# Skip all tests if Python scripting is not enabled.
> +if { [skip_python_tests] } { continue }
> +
> +if {[gdb_compile_shlib ${srcdir}/${subdir}/${srcfile_lib} ${binfile_lib} \
> +                       [list debug c++ -Wl,-soname,${libname}.so]] != ""} {
> +    return -1
> +}
> +
> +if {[gdb_compile "${srcdir}/${subdir}/${srcfile_main}" "${binfile_main}.o" \
> +                 object [list debug c++]] != ""} {
> +    return -1
> +}
> +
> +set testobjdir [standard_output_file {}]
> +if {[gdb_compile "${binfile_main}.o" "${binfile_main}" executable \
> +                 [list debug "additional_flags=-L$testobjdir -l${test} \
> +                              -Wl,-rpath=$testobjdir"]] != ""} {
> +    return -1
> +}
> +
> +# Start with a fresh gdb.
> +gdb_exit
> +gdb_start
> +gdb_reinitialize_dir $srcdir/$subdir
> +
> +# 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_file_autoload [gdb_remote_download host \
> +                            ${srcdir}/${subdir}/${pyscriptfile_lib}]
> +
> +gdb_test_no_output "set auto-load safe-path ${remote_python_file_autoload}" \
> +                   "set auto-load safe-path"
> +
> +# load the python file that defines a handler for the new_objfile event,
> +# which will generate the output to check later
> +# (prints information on available pretty-printers for objfile)
> +set remote_python_file [gdb_remote_download host \
> +                            ${srcdir}/${subdir}/${test}.py]
> +gdb_test_no_output "source ${remote_python_file}" "load python file"
> +
> +gdb_load ${binfile_main}
> +
> +gdb_test_no_output "set print pretty on"
> +
> +gdb_breakpoint [gdb_get_line_number "break to inspect" ${srcfile_main}]
> +
> +# check that the handler prints output when test library is loaded
> +# and that the pretty printer from autoloaded python file has been registered
> +gdb_test "run" "new_objfile event for test library.*
> +.*number of pretty printers: 1.*
> +.*name of the first pretty printer: my_library.*"
> +
> +# check that pretty printer actually works
> +gdb_test "print test" " .*MyClassTestLib object, id: 1.*"
> +
> +gdb_continue_to_end
> 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 00000000000..924f304fd33
> --- /dev/null
> +++ b/gdb/testsuite/gdb.python/py-autoloaded-pretty-printers-in-newobjfile-event.py
> @@ -0,0 +1,34 @@
> +# 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))
> +    # only observe the custom test library
> +    if os.path.basename(event.new_objfile.filename).find("libpy-autoloaded-pretty-printers-in-newobjfile-event") == 0:
> +        print("new_objfile event for test library")
> +        assert(len(event.new_objfile.pretty_printers) == 1)
> +        print("number of pretty printers: {}".format(len(event.new_objfile.pretty_printers)))
> +        print("name of the first pretty printer: {}".format(event.new_objfile.pretty_printers[0].name))
> +
> +gdb.events.new_objfile.connect(new_objfile_handler)
> diff --git a/gdbsupport/observable.h b/gdbsupport/observable.h
> index 1a9e767ba18..524f98e5a78 100644
> --- a/gdbsupport/observable.h
> +++ b/gdbsupport/observable.h
> @@ -52,6 +52,13 @@ struct token
>    DISABLE_COPY_AND_ASSIGN (token);
>  };
>  
> +struct observer_key
> +{
> +  observer_key () = default;
> +
> +  DISABLE_COPY_AND_ASSIGN (observer_key);
> +};
> +
>  template<typename... T>
>  class observable
>  {
> @@ -59,6 +66,22 @@ class observable
>  
>    typedef std::function<void (T...)> func_type;
>  
> +private:
> +  struct observer_entry
> +  {
> +    const token *tok;
> +    func_type func;
> +    observer_key *key;
> +    std::vector<observer_key *> deps;
> +
> +    observer_entry (const token *t, func_type f, observer_key *k,
> +                    std::vector<observer_key *> d)
> +        : tok (t), func (f), key (k), deps (d)
> +    {
> +    }
> +  };
> +
> +public:
>    explicit observable (const char *name)
>      : m_name (name)
>    {
> @@ -67,17 +90,29 @@ class observable
>    DISABLE_COPY_AND_ASSIGN (observable);
>  
>    /* Attach F as an observer to this observable.  F cannot be
> -     detached.  */
> -  void attach (const func_type &f)
> +     detached.
> +     Optional:
> +     * key that can be used to specify a dependency on
> +       the attached observer
> +     * keys for other observers this observer depends on
> +     Dependencies are notified before observers depending on them. */
> +  void attach (const func_type &f, observer_key *key = nullptr,
> +               std::vector<observer_key *> dependencies = {})
>    {
> -    m_observers.emplace_back (nullptr, f);
> +    attach (f, nullptr, key, dependencies);
>    }
>  
>    /* Attach F as an observer to this observable.  T is a reference to
> -     a token that can be used to later remove F.  */
> -  void attach (const func_type &f, const token &t)
> +     a token that can be used to later remove F.
> +     Optional:
> +     * key that can be used to specify a dependency on
> +       the attached observer
> +     * keys for other observers this observer depends on
> +     Dependencies are notified before observers depending on them. */
> +  void attach (const func_type &f, const token &t, observer_key *key = nullptr,
> +               std::vector<observer_key *> dependencies = {})
>    {
> -    m_observers.emplace_back (&t, f);
> +    attach (f, &t, key, dependencies);
>    }
>  
>    /* Remove observers associated with T from this observable.  T is
> @@ -87,10 +122,9 @@ class observable
>    {
>      auto iter = std::remove_if (m_observers.begin (),
>  				m_observers.end (),
> -				[&] (const std::pair<const token *,
> -				     func_type> &e)
> +                                [&] (const observer_entry &e)
>  				{
> -				  return e.first == &t;
> +				  return e.tok == &t;
>  				});
>  
>      m_observers.erase (iter, m_observers.end ());
> @@ -103,13 +137,82 @@ class observable
>        fprintf_unfiltered (gdb_stdlog, "observable %s notify() called\n",
>  			  m_name);
>      for (auto &&e : m_observers)
> -      e.second (args...);
> +      e.func (args...);
>    }
>  
>  private:
> -
> -  std::vector<std::pair<const token *, func_type>> m_observers;
> +  std::vector<observer_entry> m_observers;
>    const char *m_name;
> +
> +  /* used for sorting algorithm */
> +  enum class mark
> +  {
> +    NONE,
> +    PERMANENT,
> +    TEMPORARY
> +  };
> +
> +  /* Helper method for topological sort using depth-first search algorithm */
> +  void visit_for_sorting (std::vector<observer_entry> &sorted_elems,
> +         std::vector<mark> &marks, int index)
> +  {
> +    if (marks[index] == mark::PERMANENT)
> +      return;
> +    if (marks[index] == mark::TEMPORARY)
> +        error (_("Cyclic dependencies in observers."));
> +
> +    marks[index] = mark::TEMPORARY;
> +
> +    for (observer_key *dep : m_observers[index].deps)
> +      {
> +        auto it_dep
> +            = std::find_if (m_observers.begin (), m_observers.end (),
> +                            [&] (observer_entry e) { return e.key == dep; });
> +        if (it_dep != m_observers.end ())
> +          {
> +            int i = std::distance (m_observers.begin (), it_dep);
> +            visit_for_sorting (sorted_elems, marks, i);
> +          }
> +      }
> +
> +    marks[index] = mark::PERMANENT;
> +    sorted_elems.push_back (m_observers[index]);
> +  }
> +
> +  /* Sorts the elements, so that dependencies come before observers
> +   * depending on them.
> +   *
> +   * Currently uses depth-first search algorithm for topological sorting,
> +   * s. https://en.wikipedia.org/wiki/Topological_sorting#Depth-first_search . */
> +  void sort_elements ()
> +  {
> +    std::vector<observer_entry> sorted_elems;
> +    std::vector<mark> marks (m_observers.size (), mark::NONE);
> +
> +    auto it = marks.begin();
> +    while (it != marks.end ())
> +      {
> +        int index = std::distance (marks.begin (), it);
> +        visit_for_sorting (sorted_elems, marks, index);
> +
> +        it = std::find_if (marks.begin (), marks.end (),
> +                           [] (mark m) { return m == mark::NONE; });
> +      }
> +    // assign sorted result
> +    m_observers = sorted_elems;
> +  }
> +
> +  void attach (const func_type &f, const token *t, observer_key *key,
> +          std::vector<observer_key *> dependencies)
> +  {
> +    m_observers.emplace_back (t, f, key, dependencies);
> +
> +    if (key != nullptr)
> +      {
> +        // other observers might depend on this one -> sort anew
> +        sort_elements ();
> +      }
> +  };
>  };
>  
>  } /* namespace observers */
>