use string_print() in gdb

[vstinner]

BPO	3632
Nosy	@amauryfa, @vstinner, @devdanzin
Files	pyobject_dump.patch: Call PyGILState_Ensure() in _PyObject_Dump()

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2008-12-15.22:29:29.165>
created_at = <Date 2008-08-21.12:39:39.593>
labels = ['library', 'performance']
title = 'use string_print() in gdb'
updated_at = <Date 2008-12-15.22:29:29.164>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2008-12-15.22:29:29.164>
actor = 'amaury.forgeotdarc'
assignee = 'none'
closed = True
closed_date = <Date 2008-12-15.22:29:29.165>
closer = 'amaury.forgeotdarc'
components = ['Library (Lib)']
creation = <Date 2008-08-21.12:39:39.593>
creator = 'vstinner'
dependencies = []
files = ['11193']
hgrepos = []
issue_num = 3632
keywords = ['patch']
message_count = 8.0
messages = ['71628', '71630', '71631', '72806', '72822', '73156', '77637', '77886']
nosy_count = 3.0
nosy_names = ['amaury.forgeotdarc', 'vstinner', 'ajaksu2']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue3632'
versions = ['Python 2.6']

[vstinner]

"pyo" macro from gdbinit (see bpo-3631) uses _PyObject_Dump() to display
an object. This function calls (indirectly) string_print() to display
one line of text. But if the GIL is released (I guess it's the GIL or
is it called the "thread state"?), gdb crashs Python:

object : Fatal Python error: PyEval_SaveThread: NULL tstate
Program received signal SIGABRT, Aborted.
0xffffe410 in __kernel_vsyscall ()

Workaround: ensure GIL before Py_BEGIN_ALLOW_THREADS... That sounds
ugly but it works :-) So I propose to enable it in debug mode (#ifdef
Py_DEBUG) with a patch.

I guess that the issue is very specific to (gdb) debugging and should
not affect normal usage of Python. That's why I choosed to enable it
only in debug mode.

[amauryfa]

I once fell into the same issue, but the patch should modify
_PyObject_Dump(), around the call to PyObject_Print.

string_print() is not the only function called by _PyObject_Dump, by far...

And beware that many people routinely run python in "debug mode" outside
gdb, for example during development.
That's why it's better to modify _PyObject_Dump only; we can be sure
that it only affects a debugger session.

[vstinner]

Oh! I have a better idea: why not patching _PyObject_Dump() instead of
string_print() :-) So here is a new patch.

[vstinner]

Can anyone review my new patch?

[amauryfa]

The patch is fine.
I don't know if it can make the 2.6 release, but it is very simple, and
affect only a function used in debugger macros.

[devdanzin]

I would love to have this patch, along with those of bpo-3631 and bpo-3610,
included in Misc as diffs. This would make it easier to get the improved
functionality in a new development box, besides allowing distributions
to apply them at will.

[vstinner]

Can anyone apply this patch? Or explain me why it can not be applied?

[amauryfa]

Committed in r67802.