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Author SHA1 Message Date
Simon Marchi
6f3dfea03a gdb/python: remove gdb._mi_commands dict 
The motivation for this patch is the fact that py-micmd.c doesn't build
with Python 2, due to PyDict_GetItemWithError being a Python 3-only
function:

      CXX    python/py-micmd.o
    /home/smarchi/src/binutils-gdb/gdb/python/py-micmd.c: In function ‘int micmdpy_uninstall_command(micmdpy_object*)’:
    /home/smarchi/src/binutils-gdb/gdb/python/py-micmd.c:430:20: error: ‘PyDict_GetItemWithError’ was not declared in this scope; did you mean ‘PyDict_GetItemString’?
      430 |   PyObject *curr = PyDict_GetItemWithError (mi_cmd_dict.get (),
          |                    ^~~~~~~~~~~~~~~~~~~~~~~
          |                    PyDict_GetItemString

A first solution to fix this would be to try to replace
PyDict_GetItemWithError equivalent Python 2 code.  But I looked at why
we are doing this in the first place: it is to maintain the
`gdb._mi_commands` Python dictionary that we use as a `name ->
gdb.MICommand object` map.  Since the `gdb._mi_commands` dictionary is
never actually used in Python, it seems like a lot of trouble to use a
Python object for this.

My first idea was to replace it with a C++ map
(std::unordered_map<std::string, gdbpy_ref<micmdpy_object>>).  While
implementing this, I realized we don't really need this map at all.  The
mi_command_py objects registered in the main MI command table can own
their backing micmdpy_object (that's a gdb.MICommand, but seen from the
C++ code).  To know whether an mi_command is an mi_command_py, we can
use a dynamic cast.  Since there's one less data structure to maintain,
there are less chances of messing things up.

 - Change mi_command_py::m_pyobj to a gdbpy_ref, the mi_command_py is
   now what keeps the MICommand alive.
 - Set micmdpy_object::mi_command in the constructor of mi_command_py.
   If mi_command_py manages setting/clearing that field in
   swap_python_object, I think it makes sense that it also takes care of
   setting it initially.
 - Move a bunch of checks from micmdpy_install_command to
   swap_python_object, and make them gdb_asserts.
 - In micmdpy_install_command, start by doing an mi_cmd_lookup.  This is
   needed to know whether there's a Python MI command already registered
   with that name.  But we can already tell if there's a non-Python
   command registered with that name.  Return an error if that happens,
   rather than waiting for insert_mi_cmd_entry to fail.  Change the
   error message to "name is already in use" rather than "may already be
   in use", since it's more precise.

I asked Andrew about the original intent of using a Python dictionary
object to hold the command objects.  The reason was to make sure the
objects get destroyed when the Python runtime gets finalized, not later.
Holding the objects in global C++ data structures and not doing anything
more means that the held Python objects will be decref'd after the
Python interpreter has been finalized.  That's not desirable.  I tried
it and it indeed segfaults.

Handle this by adding a gdbpy_finalize_micommands function called in
finalize_python.  This is the mirror of gdbpy_initialize_micommands
called in do_start_initialization.  In there, delete all Python MI
commands.  I think it makes sense to do it this way: if it was somehow
possible to unload Python support from GDB in the middle of a session
we'd want to unregister any Python MI command.  Otherwise, these MI
commands would be backed with a stale PyObject or simply nothing.

Delete tests that were related to `gdb._mi_commands`.

Co-Authored-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: I060d5ebc7a096c67487998a8a4ca1e8e56f12cd3
 2022-03-18 20:29:57 -04:00
Jan Vrany
a2757c4ed6 gdb/mi: consistently notify user when GDB/MI client uses -thread-select 
GDB notifies users about user selected thread changes somewhat
inconsistently as mentioned on gdb-patches mailing list here:

  https://sourceware.org/pipermail/gdb-patches/2022-February/185989.html

Consider GDB debugging a multi-threaded inferior with both CLI and GDB/MI
interfaces connected to separate terminals.

Assuming inferior is stopped and thread 1 is selected, when a thread
2 is selected using '-thread-select 2' command on GDB/MI terminal:

    -thread-select 2
    ^done,new-thread-id="2",frame={level="0",addr="0x00005555555551cd",func="child_sub_function",args=[],file="/home/jv/Projects/gdb/users_jv_patches/gdb/testsuite/gdb.mi/user-selected-context-sync.c",fullname="/home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c",line="30",arch="i386:x86-64"}
    (gdb)

and on CLI terminal we get the notification (as expected):

    [Switching to thread 2 (Thread 0x7ffff7daa640 (LWP 389659))]
    #0  child_sub_function () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:30
    30        volatile int dummy = 0;

However, now that thread 2 is selected, if thread 1 is selected
using 'thread-select --thread 1 1' command on GDB/MI terminal
terminal:

   -thread-select --thread 1 1
   ^done,new-thread-id="1",frame={level="0",addr="0x0000555555555294",func="main",args=[],file="/home/jv/Projects/gdb/users_jv_patches/gdb/testsuite/gdb.mi/user-selected-context-sync.c",fullname="/home/jv/Projects/gdb/users_jv_patches/gdb/testsuite/gdb.mi/user-selected-context-sync.c",line="66",arch="i386:x86-64"}
   (gdb)

but no notification is printed on CLI terminal, despite the fact
that user selected thread has changed.

The problem is that when `-thread-select --thread 1 1` is executed
then thread is switched to thread 1 before mi_cmd_thread_select () is
called, therefore the condition "inferior_ptid != previous_ptid"
there does not hold.

To address this problem, we have to move notification logic up to
mi_cmd_execute () where --thread option is processed and notify
user selected contents observers there if context changes.

However, this in itself breaks GDB/MI because it would cause context
notification to be sent on MI channel. This is because by the time
we notify, MI notification suppression is already restored (done in
mi_command::invoke(). Therefore we had to lift notification suppression
logic also up to mi_cmd_execute (). This change in made distinction
between mi_command::invoke() and mi_command::do_invoke() unnecessary
as all mi_command::invoke() did (after the change) was to call
do_invoke(). So this patches removes do_invoke() and moves the command
execution logic directly to invoke().

With this change, all gdb.mi tests pass, tested on x86_64-linux.

Co-authored-by: Andrew Burgess <aburgess@redhat.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=20631
 2022-03-16 15:08:22 +00:00
Andrew Burgess
740b42ceb7 gdb/python/mi: create MI commands using python 
This commit allows a user to create custom MI commands using Python
similarly to what is possible for Python CLI commands.

A new subclass of mi_command is defined for Python MI commands,
mi_command_py. A new file, gdb/python/py-micmd.c contains the logic
for Python MI commands.

This commit is based on work linked too from this mailing list thread:

  https://sourceware.org/pipermail/gdb/2021-November/049774.html

Which has also been previously posted to the mailing list here:

  https://sourceware.org/pipermail/gdb-patches/2019-May/158010.html

And was recently reposted here:

  https://sourceware.org/pipermail/gdb-patches/2022-January/185190.html

The version in this patch takes some core code from the previously
posted patches, but also has some significant differences, especially
after the feedback given here:

  https://sourceware.org/pipermail/gdb-patches/2022-February/185767.html

A new MI command can be implemented in Python like this:

  class echo_args(gdb.MICommand):
      def invoke(self, args):
          return { 'args': args }

  echo_args("-echo-args")

The 'args' parameter (to the invoke method) is a list
containing (almost) all command line arguments passed to the MI
command (--thread and --frame are handled before the Python code is
called, and removed from the args list).  This list can be empty if
the MI command was passed no arguments.

When used within gdb the above command produced output like this:

  (gdb)
  -echo-args a b c
  ^done,args=["a","b","c"]
  (gdb)

The 'invoke' method of the new command must return a dictionary.  The
keys of this dictionary are then used as the field names in the mi
command output (e.g. 'args' in the above).

The values of the result returned by invoke can be dictionaries,
lists, iterators, or an object that can be converted to a string.
These are processed recursively to create the mi output.  And so, this
is valid:

  class new_command(gdb.MICommand):
      def invoke(self,args):
          return { 'result_one': { 'abc': 123, 'def': 'Hello' },
                   'result_two': [ { 'a': 1, 'b': 2 },
                                   { 'c': 3, 'd': 4 } ] }

Which produces output like:

  (gdb)
  -new-command
  ^done,result_one={abc="123",def="Hello"},result_two=[{a="1",b="2"},{c="3",d="4"}]
  (gdb)

I have required that the fields names used in mi result output must
match the regexp: "^[a-zA-Z][-_a-zA-Z0-9]*$" (without the quotes).
This restriction was never written down anywhere before, but seems
sensible to me, and we can always loosen this rule later if it proves
to be a problem.  Much harder to try and add a restriction later, once
people are already using the API.

What follows are some details about how this implementation differs
from the original patch that was posted to the mailing list.

In this patch, I have changed how the lifetime of the Python
gdb.MICommand objects is managed.  In the original patch, these object
were kept alive by an owned reference within the mi_command_py object.
As such, the Python object would not be deleted until the
mi_command_py object itself was deleted.

This caused a problem, the mi_command_py were held in the global mi
command table (in mi/mi-cmds.c), which, as a global, was not cleared
until program shutdown.  By this point the Python interpreter has
already been shutdown.  Attempting to delete the mi_command_py object
at this point was causing GDB to try and invoke Python code after
finalising the Python interpreter, and we would crash.

To work around this problem, the original patch added code in
python/python.c that would search the mi command table, and delete the
mi_command_py objects before the Python environment was finalised.

In contrast, in this patch, I have added a new global dictionary to
the gdb module, gdb._mi_commands.  We already have several such global
data stores related to pretty printers, and frame unwinders.

The MICommand objects are placed into the new gdb.mi_commands
dictionary, and it is this reference that keeps the objects alive.
When GDB's Python interpreter is shut down gdb._mi_commands is deleted,
and any MICommand objects within it are deleted at this point.

This change avoids having to make the mi_cmd_table global, and walk
over it from within GDB's python related code.

This patch handles command redefinition entirely within GDB's python
code, though this does impose one small restriction which is not
present in the original code (detailed below), I don't think this is a
big issue.  However, the original patch relied on being able to
finish executing the mi_command::do_invoke member function after the
mi_command object had been deleted.  Though continuing to execute a
member function after an object is deleted is well defined, it is
also (IMHO) risky, its too easy for someone to later add a use of the
object without realising that the object might sometimes, have been
deleted.  The new patch avoids this issue.

The one restriction that is added to avoid this, is that an MICommand
object can't be reinitialised with a different command name, so:

  (gdb) python cmd = MyMICommand("-abc")
  (gdb) python cmd.__init__("-def")
  can't reinitialize object with a different command name

This feels like a pretty weird edge case, and I'm happy to live with
this restriction.

I have also changed how the memory is managed for the command name.
In the most recently posted patch series, the command name is moved
into a subclass of mi_command, the python mi_command_py, which
inherits from mi_command is then free to use a smart pointer to manage
the memory for the name.

In this patch, I leave the mi_command class unchanged, and instead
hold the memory for the name within the Python object, as the lifetime
of the Python object always exceeds the c++ object stored in the
mi_cmd_table.  This adds a little more complexity in py-micmd.c, but
leaves the mi_command class nice and simple.

Next, this patch adds some extra functionality, there's a
MICommand.name read-only attribute containing the name of the command,
and a read-write MICommand.installed attribute that can be used to
install (make the command available for use) and uninstall (remove the
command from the mi_cmd_table so it can't be used) the command.  This
attribute will be automatically updated if a second command replaces
an earlier command.

This patch adds additional error handling, and makes more use the
gdbpy_handle_exception function.

Co-Authored-By: Jan Vrany <jan.vrany@labware.com>
 2022-03-14 14:09:09 +00:00