[issue39395] The os module should unset() environment variable at exit

[Eric Snow]

Eric Snow  added the comment:

FTR, #39376 is related (avoid the process-global env vars in the first place).

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nosy: +eric.snow

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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

STINNER Victor  added the comment:

On non-Windows platforms, Python now requires setenv() and unsetenv()
functions to build.

setenv() and unsetenv() should be available on all platforms supported by 
Python. If it's not the case, someone can maintain a downstream patch which is 
a revert of commit b8d1262e8afe7b907b4a394a191739571092acdb.

If someone asks to support a platform which doesn't provide setenv() and 
unsetenv(), we can revisit the idea of unsetting variables set by Python at 
exit, to workaround putenv() issue: something like my (abandonned) PR 18078.

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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

Change by STINNER Victor :


--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

STINNER Victor  added the comment:


New changeset b8d1262e8afe7b907b4a394a191739571092acdb by Victor Stinner in 
branch 'master':
bpo-39395: putenv() and unsetenv() always available (GH-18135)
https://github.com/python/cpython/commit/b8d1262e8afe7b907b4a394a191739571092acdb


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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

STINNER Victor  added the comment:

Clearing environment variables at exit has a drawback: it changes the behavior 
in code executed after Python finalization (Py_FinalizeEx() call). It may cause 
regression.

So I proposed PR 18135 to fix this issue differently: on non-Windows platforms, 
Python now requires setenv() to build. This PR has no backward compatibility 
issue: it doesn't unset environment variables at exit.

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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

Change by STINNER Victor :


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pull_requests: +17521
pull_request: https://github.com/python/cpython/pull/18135

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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

STINNER Victor  added the comment:

I closed bpo-39406: os.putenv() is now implemented with setenv() if available. 
The internal putenv_dict is no longer used on Windows and if setenv() is 
available.

Now only non-Windows platforms without setenv() are affected by this issue.

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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

STINNER Victor  added the comment:

> Issue39406 is a new feature. In any case we should fix potential crash in 
> older Python versions.

Python 3.8 and older are not affected by this issue since they never destroy 
the posix_putenv_garbage dictionary: memory is never released.

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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

Change by STINNER Victor :


--
versions:  -Python 3.7, Python 3.8

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[issue39395] The os module should unset() environment variable at exit

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Issue39406 is a new feature. In any case we should fix potential crash in older 
Python versions.

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nosy: +serhiy.storchaka

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[issue39395] The os module should unset() environment variable at exit

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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type:  -> crash
versions: +Python 3.7, Python 3.8

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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

STINNER Victor  added the comment:

I proposed bpo-39406 which avoids to have to clear environment variables set by 
Python at exit on platforms providing setenv().

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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

STINNER Victor  added the comment:

Fedora downstream issue, crash in the "elements" package: 
https://bugzilla.redhat.com/show_bug.cgi?id=1791761

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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

Change by STINNER Victor :


--
keywords: +patch
pull_requests: +17471
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/18078

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[issue39395] The os module should unset() environment variable at exit

[STINNER Victor]

New submission from STINNER Victor :

os.environ[key] = value has to keep internally the string "key=value\0" after 
putenv("key=value\0") has been called, since the glibc doesn't copy the string. 
Python has to manage the string memory.

Internally, the posix module uses a "posix_putenv_garbage" dictionary mapping 
key (bytes) to value (bytes). Values are "key=value\0" strings.

The bpo-35381 issue converted the os ("posix" in practice) module PEP 384: 
"Remove all static state from posixmodule": commit 
b3966639d28313809774ca3859a347b9007be8d2. The _posix_clear() function is now 
called by _PyImport_Cleanup().

Problem: the glibc is not aware that Python is exiting and that the memory of 
the environment variable has been released. Next access to environment 
variables ("environ" C variable, putenv, setenv, unsetenv, ...) can crash. 
Sometimes, it doesn't crash even if the memory has been released, because 
free() does not always dig immediately holes in the heap memory (the complex 
problelm of memory fragmentation).

The posix module should notify the glibc that the memory will be released 
before releasing the memory, to avoid keeping dangling pointers in the 
"environ" C variable.

The following crash in the Elements module is an example of crash introduced by 
commit b3966639d28313809774ca3859a347b9007be8d2 which introduced this issue:
https://bugzilla.redhat.com/show_bug.cgi?id=1791761

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components: Interpreter Core
messages: 360309
nosy: vstinner
priority: normal
severity: normal
status: open
title: The os module should unset() environment variable at exit
versions: Python 3.9

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