[issue27387] Thread hangs on str.encode() when locale is not set

[Irit Katriel]

Irit Katriel  added the comment:

Python 2 issue.

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nosy: +iritkatriel
resolution:  -> out of date
stage:  -> resolved
status: open -> closed

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[issue27387] Thread hangs on str.encode() when locale is not set

[Brett Cannon]

Change by Brett Cannon :


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[issue27387] Thread hangs on str.encode() when locale is not set

[R. David Murray]

R. David Murray added the comment:

No, I'm talking about the threading docs, not the encoding docs.  I think 
that's the only place it matters.  Specifically, in the section that I linked 
to, in the bullet point that warns against launching threads on import, it can 
note that even if you try to make your own code avoid the import lock, implicit 
imports such as the one done by encode/decode can trip you up.

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[issue27387] Thread hangs on str.encode() when locale is not set

[STINNER Victor]

STINNER Victor added the comment:

> Maybe it is worth adding a warning to that section of the 2.7 docs about 
> implicit imports in general and encode/decode in particular?

Ok to add a note to str.encode and str.decode methods to explain that
an import is needed the first time that an encoding is used.

I'm not ok for a warning, we should not discourage developers to use
these methods! They are not dangerous by themself.

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[issue27387] Thread hangs on str.encode() when locale is not set

[Brett Cannon]

Brett Cannon added the comment:

Adding a note to the docs sounds reasonable.

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[issue27387] Thread hangs on str.encode() when locale is not set

[R. David Murray]

R. David Murray added the comment:

This situation is warned about explicitly in the threading docs 
(https://docs.python.org/2/library/threading.html#importing-in-threaded-code).  
The import deadlock is fixed in python3, but it is still a really bad idea to 
launch threads on module import.

What isn't obvious, of course, is that calling encode for the first time for a 
given encoding does an implicit import of the relevant encoding.  I don't think 
encodings is the only stdlib module that does implicit imports, but it is 
probably the most used case.  Maybe it is worth adding a warning to that 
section of the 2.7 docs about implicit imports in general and encode/decode in 
particular?

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assignee:  -> docs@python
components: +Documentation -Interpreter Core, Unicode
nosy: +docs@python, r.david.murray

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[issue27387] Thread hangs on str.encode() when locale is not set

[STINNER Victor]

STINNER Victor added the comment:

It is a deadlock on the import lock. You should avoid creating and waiting
for a thread when a module is imported. Defer the creation of the thread.

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[issue27387] Thread hangs on str.encode() when locale is not set

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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nosy: +brett.cannon, eric.snow, ncoghlan

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[issue27387] Thread hangs on str.encode() when locale is not set

[Josh Purvis]

New submission from Josh Purvis:

This bug manifest itself in at least one very specific situation:

1. No locale is set on the machine
2. A file (test1.py) imports a second (test2.py)
3. The second file (test2.py) calls str.encode() from inside a thread
4. Running Python 2.7

[Environment with no locale set]:

# both of these are unset:
$ echo $LC_CTYPE

$ echo $LANG

$

[test1.py]:

import test2

[test2.py]:

from threading import Thread

class TestThread(Thread):
def run(self):
msg = 'Error from server: code=000a'
print msg
msg = msg.encode('utf-8')

t = TestThread()
t.start()
t.join()

print 'done'

[Expected behavior]:

$ python test1.py   
  
Error from server: code=000a
done

[Actual behavior]: 

$ python test1.py   
  
Error from server: code=000a
[script hangs here indefinitely]

Much thanks to Alan Boudreault, a developer of the cassandra-driver Python 
package, for helping me locate this bug and further narrow it down to the 
threading module. The above code snippet was copied from his comment on my 
issue over there (https://datastax-oss.atlassian.net/browse/PYTHON-592).

Another curious behavior is that if you modify test1.py to decode any string 
prior to the import, it implicitly fixes the issue:

[test1.py']:

"any string".decode('utf-8')
import test2

I realize that one should probably always have a locale set, however, this 
proved to be very difficult to isolate, especially given that it works if no 
import occurs or a string is decoded prior to the import.

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components: Interpreter Core, Unicode
messages: 269262
nosy: ezio.melotti, haypo, joshpurvis
priority: normal
severity: normal
status: open
title: Thread hangs on str.encode() when locale is not set
type: behavior
versions: Python 2.7

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