import_module() and multiple modules of same name.
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Key: MODPYTHON-115
URL: http://issues.apache.org/jira/browse/MODPYTHON-115
Project: mod_python
Type: Bug
Components: core
Versions: 3.1.4, 3.2
Reporter: Graham Dumpleton
The "apache.import_module()" function is a thin wrapper over the standard
Python module importing system. This means that modules are still stored in
"sys.modules". As modules in "sys.modules" are keyed by their module name, this
in turn means that there can only be one active instance of a module for a
specific name.
The "import_module()" function tries to work around this by checking the path
name of the location of a module against that being requested and if it is
different will reload the correct module. This check of the path though only
occurs when the "path" argument is actually supplied to the "import_module()"
function. The "path" is only supplied in this way when mod_python.publisher
makes use of the "import_module()" function, it is not supplied when the
"Python*Handler" directives are used because in that circumstance a module may
actually be a system module and supplying "path" would prevent it from being
found.
Even though mod_python.publisher supplies the "path" argument to the
"import_module()" function, the check of the path has bugs, with modules
possibly becoming inaccessible as documented in JIRA as MODPYTHON-9.
The check by mod_python of the path name to the actual code file for a module
to determine if it should be reloaded, can also cause a continual cycle of
module reloading even though the modules on disk may not have changed. This
will occur when successive requests alternate between URLs related to the
distinct modules having the same name. This cyclic reloading is documented in
JIRA as MODPYTHON-10.
That a module is reloaded into the same object space as the existing module
when two modules of the same name are in different locations, can also cause
namespace pollution and security issues if one location for the module was
public and the other private. This cross contamination of modules is as
documented in JIRA as MODPYTHON-11.
In respect of the "Python*Handler" directives where the "path" argument was
never supplied to the "import_module()" function, the result would be that the
first module loaded under the specified name would be used. Thus, any
subsequent module of the same name referred to by a "Python*Handler" directive
found in a different directory but within the same interpreter would in effect
be ignored.
A caveat to this though is that such a "Python*Handler" directive would result
in that handlers directory being inserted at the head of "sys.path". If the
first instance of the module loaded under that name were at some point
modified, the module would be automatically reloaded, but it would load the
version from the different directory.
Now, although these problem as they relate to mod_python.publisher are
addressed in mod_python 3.2.6, the underlying problems in 'import_module()' are
not. As the bug reports as they relate to mod_python.publisher have been closed
off as resolved, am creating this bug report so as to carry on a bug report for
the underlying problem as it applies to "Python*Handler" directive and use of
"import_module()" explicitly.
To illustrate the issue as it applies to "Python*Handler" directive, create two
separate directories with a .htaccess file containing:
AddHandler mod_python .py
PythonHandler index
PythonDebug On
In the "index.py" file in each separate directory put:
import os
from mod_python import apache
def handler(req):
req.content_type = 'text/plain'
print >> req, os.getpid(), __file__
return apache.OK
Assuming these are accessed as:
/~grahamd/mod_python_9/subdir-1/index.py
/~grahamd/mod_python_9/subdir-2/index.py
access the first URL, and the result will be:
10665 /Users/grahamd/Sites/mod_python_9/subdir-1/index.py
now access the second URL and we get:
10665 /Users/grahamd/Sites/mod_python_9/subdir-1/index.py
Note this assumes the same child process got it, so fixing Apache to run one
child process is required for this test.
As one can see, it doesn't actually use the 'subdir-2/index.py" module at all
and still uses the "subdir-1/index.py' module.
If one modifies "subdir-1/index.py' so its timestamp is updated and load the
second URL again, we get:
10665 /Users/grahamd/Sites/mod_python_9/subdir-2/index.py
This occurs because it detects the change in the first module loaded, but
because sys.path had the second handler directory at the head of sys.path now,
when reloaded it picked up the latter.
These issues with same name module in multiple locations is listed as ISSUE 14
in my list of module importer problems. See:
http://www.dscpl.com.au/articles/modpython-003.html
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