Re: Add a predicate for canonical file name

[Philipp Stephani <p.stephani2@gmail.com>]

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Stefan Monnier <monnier@iro.umontreal.ca> schrieb am Mo., 12. Sep. 2016 um
22:09 Uhr:

> > Using string= here will cause false negatives, e.g. with Windows file
> > names that use backslashes vs forward slashes, or due to letter-case
> > differences on case-insensitive file systems.  Did you really mean
> > that?
>
> Indeed, such notions have already been requested and discussed here, and
> it's not clear exactly what is needed and when.
>
> I can see several different meanings of "canonical", i.e. representative
> member of an equivalence class (and I'd be inclined to prefer a function
> that checks for "equivalence" between two file names rather than
> a function that tries to find one canonical name).  The equivalence
> classes could be:
>
> - equivalent regardless of the actual on-disk data.  I.e. this can't
>   take symlinks or hard links into account.  Questions remain about
>   whether it could presume the "normal semantics of the most common
>   file-system".  E.g. should it assume case-insensitive names in
>   MacOS/Windows and case-sensitive in GNU/Linux?
>

You couldn't even do that because case-folding on Windows depends on the
file system. The only possibilities I see here are converting backslashes
into slashes (or vice versa), and collapsing "//" and "/./". You couldn't
even collapse "/../" because of symlinks. (Or you could ignore directory
symlinks, as in https://golang.org/pkg/path/#Clean).


>
> - equivalent in practice for the current state of the file-system.
>   You can test this equivalence by comparing the output of
>   `file-attributes', except when the name corresponds to a file that
>   doesn't exist (yet?).
>

Like all filesystem operations, this easily introduces race conditions if
decisions are made based on it: What might be an equivalent file name now
(pointing to the same inode), might not be a nanosecond later.
Over all, such an operation sounds more useful than it actually is.

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