GNU bug report logs - #17330
files.el cd-absolute overcome false negative from file-executable-p

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From: Philip Hodges <philip.hodges <at> bluewin.ch>
To: 17330 <at> debbugs.gnu.org
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:
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=10257

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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