Problematic interpretion of paths starting with double slashes

Tue Jun 12 16:28:00 GMT 2018

On 06/12/2018 03:53 AM, Sven Eden wrote:
> Hello everybody,
>> I have a problem with a script that wants to copy a file,
> which has a variable paths build from various variables.
>> The first variable is a possible prefix, the second an
> absolute path. If no prefix is needed, the first consists of
> just a slash.

Then fix your script to provide 3 slashes instead of 2. Only 2 slashes 
has the magic UNC behavior.
That is, if you have a script that is concatenating:
${prefix}/${dir}
where ${prefix} might be empty, you can always rewrite it to be:
${prefix}///${dir}
to get portable behavior that always resolves to the root dir instead of 
trying to dereference remote machines under //.
>> This puzzled me, because this works perfectly well on every
> GNU/Linux (plus WSL) system I tried.

Just because your script "works" on a number of systems doesn't mean it 
is portable.
> I am completely aware that Â§4.13 of the Open Group Base
> Specification Issue 7 (IEEE Std 1003.1-2017) reads:
> "If a pathname begins with two successive <slash>
> characters, the first component following the leading
> <slash> characters may be interpreted in an
> implementation-defined manner, although more than two
> leading <slash> characters shall be treated as a single
> <slash> character."

Yes, that's exactly why your script is not portable. So even though 
Linux has chosen to make "/" and "//" synonymous, Cygwin has not.
>> My question therefore is, whether the behavior can be gotten
> nearer what every other GNU/Linux system does.
> Maybe, if said first component can not be resolved as an smb
> host, try an absolute path instead?

That won't work as nicely as you want, because you will introduce long 
timeouts for every time that Cygwin first has to ascertain that '//tmp' 
does not exist as a remote host. Maybe we could indeed make '//tmp' 
resolve to '/tmp' if there is no remote '//tmp' available, but the speed 
penalties in doing so will not make it pleasant. Better is to fix your 
script to not try accessing '//tmp' in the first place.
> I have searched the cygwin mailing list, but all I could find
> was some discussion about UNC paths from 1997.

Yes, we've had special support for // as UNC for a LOOONG time, and 
changing it now would break existing users that expect it to work as 
allowed by POSIX.
> P.S An example of a failing script line is:
> https://github.com/gentoo/gentoo/blob/331976f/eclass/toolchain.eclass#L2136

Well, then report that as a bug to the author of that script.
-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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