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Re: Aw: Re: Problematic interpretion of paths starting with double slashes




On 06/12/2018 08:14 AM, Sven Eden wrote:
Gesendet: Dienstag, 12. Juni 2018 um 13:52 Uhr
Von: "Eric Blake" <eblake@xxxxxxxxxx>
Then fix your script to provide 3 slashes instead of 2. Only 2 slashes
has the magic UNC behavior.

It is not my script. *my* scripts are portable by all means.

Good to know!



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}

The script was "fixed" from ${prefix}/${dir} a while ago.
Before that the outcome was "///".

Which is very bad style. Good style is to guarantee, that
not more than one slash is issued.

Good style is nice, but in my book, it is trumped by correct code. Since three slashes always behaves identically to one slash, it's handy to know that bending style gets you a construct where you no longer have to worry about what the user may have passed in through variables, even if it is stylistically a bit more verbose than a form that uses minimal /.



Just because your script "works" on a number of systems doesn't mean it
is portable.

I neither wrote it was my script, nor that it was portable
per se. But thanks for jumping down my throat for nothing.

I won't quote and answer to your further attacks, but
instead concentrate on the relevant stuff, okay?

Hey, let's all assume good intent here. I did not mean what I wrote as an attack against you, nor am I accusing you of writing the said code, only that the code you are trying to use (shorthand "your code") is relying on something non-portable, and is therefore worth improving, regardless of whether Cygwin also makes a change. I apologize if my words have come across in a different tone than intended (email tends to be a lousy medium in that regards). And likewise, I'm not upset at your reaction to my words.



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.

The speed penalties would only apply if
   a) "Something" looks up //foo/bar
   b) "Something" made a mistake and actually wanted
      /foo/bar

So apart from the speed penalty that "Something" has to
suffer, and its their own damn fault, the only real
consequence would be that "Something" does not die from
ENOENT any more.

Indeed, and you may have a valid argument for making that change in Cygwin; patches are certainly welcome (that is, since //tmp is already implementation-defined behavior, we can define it to attempt to resolve to the remove host first, and on ENOENT then attempt to resolve it locally). It does have one potential minor drawback: right now, at least bash hard-codes the assumption that on Cygwin, //foo and //foo/ resolve identically (that is, IF //foo exists, it necessarily behaves as a directory), using that assumption to reduce the need for network queries during certain forms of tab completion. If we add the fallback to trying /foo, that assumption is no longer always the case (it could be a regular file, symlink, socket, ...).



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.

Keeping it, changing it, extending it. It doesn't matter.
All three variants would be fully POSIX compliant.

Yes, you've got that part right.


However, I never asked to actually change the current
behaviour. Only whether it was possible to extend it.

Looking up // as UNC is the default, wanted and expected
behaviour. I got that right from the start and even wrote
that I find that pretty cool.
Doing a simple stat on / if (and only if) the UNC lookup
fails, does not endanger anything. It wouldn't break
anything or do any other damage. Besides from adding an
additional <0.01s lag to any failed access that *really*
meant a network share.

So no. Adding this tiny extra functionality wouldn't break
anything for anybody, but allowed the usage of software that
relies on the non-cygwin behaviour. (And is outside the
users control.)

Am I right that the relevant stuff can be found in
winsup/cygwin?

Yes.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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