Getting Python Extensions to GDB

[CMCDragonkai]

Issue description

GDB supports python extensions: https://devguide.python.org/gdb/

This is not the same as the python support, but this allows access to commands like py-bt.

This means you can actually stacktrace and inspect the Python application source code, not just the Python interpreter.

However there's no information on how to get this in NixOS. The gdb derivation doesn't seem to have any information on this.

[CMCDragonkai]

I want to do something like this: https://web.archive.org/web/20180531125031/http://grapsus.net/blog/post/Low-level-Python-debugging-with-GDB

[deliciouslytyped]

I was able to get this to work by manually sourcing in gdb the appropriate python file contained in the cpython repository (search for "py-bt"), along with a debug build of python (if you don't do that you'll get some sort of message about missing information, I can't remember). I'm leaving a comment so that I come back to this eventually.

For the debug python there was a bit of a mess with dealign with disallowedReferences, but I haven't spent time to figure out how to clean up what I have.

[deliciouslytyped]

I'm going to package this somehow at some point, but here is the link because I always forget how to get there - despite my note above. https://github.com/python/cpython/blob/7bf069b6110278102c8f4719975a5eb5a5af25f9/Tools/gdb/libpython.py

Note also it needs a python with debug symbols but nix-shell -p "(enableDebugging python37)" or such, works. It's so good...

[LWP 22238 exited]

Thread 1 "python" received signal SIGSEGV, Segmentation fault.
0x00007fffeca0919d in clang_getCursorTLSKind () from /nix/store/bwnavb2jpbxz3nzkqa7pjjflfb4261hr-clang-10.0.0-lib/lib/libclang.so
(gdb) py-bt
Traceback (most recent call first):
  File "bindings/python/clang/cindex.py", line 1593, in tls_kind
    self._tls_kind = conf.lib.clang_getCursorTLSKind(self)
  <built-in method getattr of module object at remote 0x7fffefe07d10>
  File "pyclasvi.py", line 968, in _add_attr
    attrData = getattr(obj, attrName)
  File "pyclasvi.py", line 1132, in _add_obj
    res = self._add_attr(objStack, attrName, subFoldNode)
  File "pyclasvi.py", line 1150, in set_cursor
    self._add_obj([c], self.foldTree.get_root())
  File "pyclasvi.py", line 1549, in _set_active_cursor
    self.cursorOutputFrame.set_cursor(cursor)
  File "pyclasvi.py", line 1539, in _on_cursor_selection
    self._set_active_cursor(curCursor)
  File "pyclasvi.py", line 507, in _on_selection
    self.selectCmd()
  File "/nix/store/85a6mbc69gdvk65h0k6zq92snv4c4dqw-python3-3.7.7/lib/python3.7/tkinter/__init__.py", line 1705, in __call__
    return self.func(*args)
  File "/nix/store/85a6mbc69gdvk65h0k6zq92snv4c4dqw-python3-3.7.7/lib/python3.7/tkinter/__init__.py", line 1283, in mainloop
    self.tk.mainloop(n)
  File "pyclasvi.py", line 1808, in main
    app.mainloop()
  File "pyclasvi.py", line 1811, in <module>
    main()

[deliciouslytyped]

Here's an expression

{ pkgs ? import <nixpkgs> {}
, _gdb ? pkgs.gdb
, python3 ? pkgs.python3
, enableDebugging ? pkgs.enableDebugging
, makeWrapper ? pkgs.makeWrapper
, runCommand ? pkgs.runCommand
}: rec {
  unpackSrc = drv: runCommand "${drv.name}-unpacked" { inherit (drv) src; } ''
    unpackPhase
    mv "$sourceRoot" "$out"
    '';

  debugpy = let
      self = enableDebugging (
        #output '/nix/store/fip9hficzysnpa4g3nkykp0p06bfm33v-python3-3.7.6' is not allowed to refer to the following paths:
        #  /nix/store/bs24q7v5hzg92zq5l56r7yhnp5ljzjv0-openssl-1.1.1d-dev
        python3.overrideAttrs (old: { disallowedReferences = []; }) #TODO bit of a hack? debug symbols or something probably ends up referring to openssl
        );
    in
      python3.override { inherit self; };

  gdb = runCommand "gdb-withpy" { buildInputs = [ makeWrapper ]; } ''
    mkdir -p "$out/bin"
    makeWrapper "${_gdb}/bin/gdb" "$out/bin/gdb" \
      --add-flags '-ex "source ${unpackSrc debugpy}/Tools/gdb/libpython.py"'
    '';

  }

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[spacekitteh]

Still valid.

[stale[bot]]

I marked this as stale due to inactivity. → More info

[deliciouslytyped]

I might make a PR, but I'm not sure in what form https://github.com/NixOS/nixpkgs/issues/48809#issuecomment-614224176 would be PR-able.

[stale[bot]]

I marked this as stale due to inactivity. → More info

[vlaci]

For me just adding the extension from the python source worked without the need of debug build of Python on unstable.

[luochen1990]

Still needed

[luochen1990]

@deliciouslytyped Hello, could you please make a PR? so users can use it like gdb.overrideAttrs { pythonSupport = true; } or similar way.