Václav Šmilauer added the comment:
I found the cause of the behavior (perhaps it is common knowledge, but I am new
to python source); imp.load_dynamic calls the following functions
Python/import.c: imp_load_dynamic
(http://hg.python.org/cpython/file/ad51ed93377c/Python/import.c#l1777)
Python/importdl.c: _PyImport_LoadDynamicModule
(http://hg.python.org/cpython/file/ad51ed93377c/Python/importdl.c#l23)
Python/import.c: _PyImport_FindExtensionObject
(http://hg.python.org/cpython/file/ad51ed93377c/Python/import.c#l525)
where the last one uses the extensions object
(http://hg.python.org/cpython/file/ad51ed93377c/Python/import.c#l32), which is
explained at http://hg.python.org/cpython/file/ad51ed93377c/Python/import.c#l449
Magic for extension modules (built-in as well as dynamically
loaded). To prevent initializing an extension module more than
once, we keep a static dictionary 'extensions' keyed by module name
(for built-in modules) or by filename (for dynamically loaded
modules), containing these modules. A copy of the module's
dictionary is stored by calling _PyImport_FixupExtensionObject()
immediately after the module initialization function succeeds. A
copy can be retrieved from there by calling
_PyImport_FindExtensionObject().
The fact that extensions are keyed by file name explains why opening the .so
through symlink does not return the old extension object:
# foo.so
# bar.so -> foo.so (works for both symlinks and hardlinks)
imp.load_dynamic("foo","foo.so")
imp.load_dynamic("bar","bar.so") # will return the bar module
I will investigate whether marking the module as capable of multiple
initialization could be a workaround for the issue -- since the quoted comment
further says
(http://hg.python.org/cpython/file/ad51ed93377c/Python/import.c#l459):
Modules which do support multiple initialization set their m_size
field to a non-negative number (indicating the size of the
module-specific state). They are still recorded in the extensions
dictionary, to avoid loading shared libraries twice.
To fix the issue, I suggest that the *extensions* dict is keyed by
(filename,modulename) tuple for dynamically loaded modules. This would avoid
any ambiguity. Grepping through the code shows that the *extensions* object is
only accessed from Python/import.c, therefore regressions should be unlikely.
What do you think?
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<http://bugs.python.org/issue16194>
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