python-gdb.py fails with TypeError("'FakeRepr' object is not subscriptable") is gdb fails to read debug symbols

[vstinner]

BPO	34989
Nosy	@vstinner, @miss-islington
PRs	bpo-34989: python-gdb.py: fix current_line_num() #9889 [3.7] bpo-34989: python-gdb.py: fix current_line_num() (GH-9889) #9897 [3.6] bpo-34989: python-gdb.py: fix current_line_num() (GH-9889) #9898 [2.7] bpo-34989: python-gdb.py: fix current_line_num() (GH-9889) #9899

^{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 2018-10-16.10:38:22.027>
created_at = <Date 2018-10-15.11:11:42.908>
labels = ['3.7', '3.8']
title = 'python-gdb.py fails with TypeError("\'FakeRepr\' object is not subscriptable") is gdb fails to read debug symbols'
updated_at = <Date 2018-10-16.10:38:22.008>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2018-10-16.10:38:22.008>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2018-10-16.10:38:22.027>
closer = 'vstinner'
components = ['Demos and Tools']
creation = <Date 2018-10-15.11:11:42.908>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 34989
keywords = ['patch']
message_count = 7.0
messages = ['327742', '327744', '327788', '327791', '327792', '327796', '327818']
nosy_count = 2.0
nosy_names = ['vstinner', 'miss-islington']
pr_nums = ['9889', '9897', '9898', '9899']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue34989'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7', 'Python 3.8']

[vstinner]

On Fedora 28, gdb fails to read (some?) debug symbols:
"Dwarf Error: could not find partial DIE containing offset 0x316 [in module /usr/lib/debug/usr/bin/python3.6-3.6.6-1.fc28.x86_64.debug]"
https://bugzilla.redhat.com/show_bug.cgi?id=1613614

In this case, the "py-bt" command of python-gdb.py fails with a TypeError:

(gdb) py-bt
Traceback (most recent call first):
  <unknown at remote 0x7ffff7e5f7e0>
Python Exception <class 'TypeError'> 'FakeRepr' object is not subscriptable: 
Error occurred in Python command: 'FakeRepr' object is not subscriptable

python-gdb.py shouldn't fail on such case, but handle the error.

Attached PR fix this issue.

[vstinner]

The bug can be reproduced using this change:

diff --git a/Tools/gdb/libpython.py b/Tools/gdb/libpython.py
index bf4047419e..f973d4d4bd 100755
--- a/Tools/gdb/libpython.py
+++ b/Tools/gdb/libpython.py
@@ -402,6 +402,7 @@ class PyObjectPtr(object):
         the pointer accordingly.
         '''
         try:
+            raise RuntimeError
             p = PyObjectPtr(gdbval)
             cls = cls.subclass_from_type(p.type())
             return cls(gdbval, cast_to=cls.get_gdb_type())

(Don't forget to run "make" again to copy Tools/gdb/libpython.py to python-gdb.py.)

[vstinner]

New changeset 2e438cc by Victor Stinner in branch 'master':
bpo-34989: python-gdb.py: fix current_line_num() (GH-9889)
2e438cc

[miss-islington]

New changeset fcea3dd by Miss Islington (bot) in branch '3.7':
bpo-34989: python-gdb.py: fix current_line_num() (GH-9889)
fcea3dd

[miss-islington]

New changeset 71e601e by Miss Islington (bot) in branch '3.6':
bpo-34989: python-gdb.py: fix current_line_num() (GH-9889)
71e601e

[vstinner]

New changeset aadb44e by Victor Stinner in branch '2.7':
bpo-34989: python-gdb.py: fix current_line_num() (GH-9889) (GH-9899)
aadb44e

[vstinner]

Thanks Łukasz Langa for the review!