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bug#17330: thanks for the comments

From: Philip Hodges
Subject: bug#17330: thanks for the comments
Date: Sun, 4 May 2014 00:43:25 +0200

thanks for all the discussion contributions.

I'm really happy about having my own personal same day lisp fix working for me 
to stop cd-absolute believing the false negative from the check_executable 
function. My best hope for the remaining subprocess cwd cases is to look at the 
samba configuration and see if I can get it improved, and maybe add a user to 
the cygwin passwd file.

Anything involving a rebuild and new release will take months before it is 
available for me to use where I need it.

For completeness, and in the spirit of DRY, whatever uses start-process and 
call-process *could* be refactored to share the same code to offer the same 
override as file-executable-p in the same situation. And while we are about it, 
can we please not ask if a file is executable when we really just want to know 
if it is a searchable directory suitable for cd.

I don't see how we can ever completely rule out false negatives, unless we are 
prepared to change check_executable to actually try to use the directory (cd to 
it, open it, list its entries, return t, whatever). Even the euidaccess man 
page warns against using it: "Generally, it is safer just to attempt the 
desired operation and handle any permission error that occurs". But I do accept 
that there may well be no consensus to follow through with more reliable or 
less gullible code. If it is just a few legacy platforms that lack euidaccess 
and fall back to checking the wrong uid with access, then never mind. If we can 
prove that the native and cygwin builds behave differently, then the offending 
library function can probably be fixed long before emacs can work around it. 
Thank you for pushing that suggestion.

My samba share is also afflicted with false negative writeable checks, as 
described here two years ago:

I connect to the samba filesystem directly in Windows as a user without local 
administrator rights. I'll let you know if mounting it from cygwin with special 
acl options confers more appropriate access permissions or even works at all.

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