As discussed in python-dev, st_rdev provides an integer, which 
usually needs to be translated in a (major, minor) pair before it's 
useful. There are no implementations of major() and minor() in the 
current library, and maybe there shouldn't be, since their functionality 
is not portable. 
 
This patch provides a portable way to access this information, by 
providing a new attribute to the os.stat() return tuple named 
st_rdev_pair. 

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Are major/minor guaranteed to exist?
Should a check be added to configure?

There is a missing DECREF.
In the last line of the patch to posixmodule.c,
there is if (PyErr_Occurred()) {

In that block, w needs to be DECREFed.
You can initialize w = NULL in the declaration,
then do an XDECREF in the error block.
That way you don't need to handle the error
creating w, if w != NULL, set the items in the tuple.

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I don't think a test is needed. If st_rdev is available, this information must 
be decodable by the system, and this is done by the major/minor pair, 
AFAIK. 
 
About the DECREF, I don't think it's needed. w's reference is stolen by v 
when PyStructSequence_SET_ITEM is run, and will be destroyed with v 
once it's DECREF'd. If that's not true, the whole logic is wrong in that 
function. 
 

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You are correct about the DECREF,
it is not necessary.

I'm not sure if any standards define
major/minor (that's why I asked).
What platform(s) did you test on?

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I'll redo the patch with a test. 
 
Thanks! 

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I'm not sure if that's really the right way to implement the needed features    
for tarfile. Maybe including major, minor, and makedev could be the way to    
go.   
    
I'm sending the rationale to python-dev for discussion, and attaching a new   
patch with their implementations, instead of st_rdev_pair.   

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Is anything stopping this patch? If not, I'll apply it.

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I'm fine with checking in.

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Thanks for the patch. Committed as 

configure 1.345;
configure.in 1.356;
pyconfig.h.in 1.53;
libos.tex 1.98;
posixmodule.c 2.265;