Signals discard one level of exception handling

[d-maurer]

BPO	771429
Nosy	@mwhudson, @facundobatista, @d-maurer
Files	771429.py: mwh's failed attempt to reproduce

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

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2005-01-15.21:26:40.000>
created_at = <Date 2003-07-15.06:57:57.000>
labels = ['interpreter-core']
title = 'Signals discard one level of exception handling'
updated_at = <Date 2005-01-15.21:26:40.000>
user = 'https://github.com/d-maurer'

bugs.python.org fields:

activity = <Date 2005-01-15.21:26:40.000>
actor = 'facundobatista'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2003-07-15.06:57:57.000>
creator = 'dmaurer'
dependencies = []
files = ['966']
hgrepos = []
issue_num = 771429
keywords = []
message_count = 5.0
messages = ['17064', '17065', '17066', '17067', '17068']
nosy_count = 3.0
nosy_names = ['mwh', 'facundobatista', 'dmaurer']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue771429'
versions = []

[d-maurer]

Signals handled by Python (especially KeyboardInterrupt)
may discard one level of exception handling.

Consider the following code fragment:

try: # outer exception handling level
try: # inner exception handling level
blocking_external_extension_function()
except:
print "inner exception handling"
except:
print "outer exception handling"

Assume that "blocking_external_extension_function"
is an external function (implemented in "C") that
blocks until interrupted by a signal (EINTR) and
then returns with an exception but without handling
the signal. In our concrete case,
"blocking_external_extension_function" has been
"psycopg.connect" (the database "connect" from the
Python-PostgreSQL bridge "psycopg").

In this case, when you interrupt the execution,
while "blocking_external_extension_function"
waits, the "KeyboardInterrupt" is not caught in
the inner "try: ...except: ..." but only in the outer
"try: ... except: ..."

The following happens in detail in Python's main
interpreter loop:

• "blocking_external_extension_function" returns with
  an exception.

• Python sets the next instruction to the exception
  handler of the inner exception handling

• Python begins a new run through its main loop.
  At the start, it detects the "KeyboardInterrupt"
  signal. It now thinks, the "KeyboardInterrupt" were
  caused inside the inner exception handler
  and pops this exception handling level without
  executing it.

  The interpreter has lost one level of exception
  handling.

The behaviour would probably better fit with expectations,
when the interpreter would check for signals at the end of
its main loop and before it sets its instruction pointer to the
exception handler.

[facundobatista]

Logged In: YES
user_id=752496

Please, could you verify if this problem persists in Python 2.3.4
or 2.4?

If yes, in which version? Can you provide a test case?

If the problem is solved, from which version?

Note that if you fail to answer in one month, I'll close this bug
as "Won't fix".

Thank you!

. Facundo

[mwhudson]

Logged In: YES
user_id=6656

In addition, attaching any code samples rather than including them
would seem a good idea. I can't make head or tail of the code in
the summary.

[mwhudson]

Logged In: YES
user_id=6656

Oh, after some judicious view-sourcing and query-replacing, I get
the problem. However, I can't reproduce it (see attached script).

So we're back to needing a failing example and more platform
details.

[facundobatista]

Logged In: YES
user_id=752496

Deprecated. Reopen only if still happens in 2.3 or newer.

. Facundo