canonicalizePath is no longer canonical for directories (trailing slashes)

[dcoutts]

It used to be the case that canonicalizePath for directories would return the directory without the trailing slash, irrespective of whether the directory was given with or without the trailing slash. As of some version, but certainly including 1.2.6 and still in 1.2.7.1 the result sometimes includes a trailing slash and sometimes does not. This violates the property that the result be canonical.

$ ghci -package directory-1.2.7.1
> :m System.Directory
> canonicalizePath "./"
"/home/duncan/"
> canonicalizePath "."
"/home/duncan"
> canonicalizePath "./foo/"
"/home/duncan/foo/"
> canonicalizePath "./foo"
"/home/duncan/foo"

This canonical property is important, lots of code relies on it. As an example of the kind of bug it causes:

cabal new-build ./
Unknown build target './'.
The project has no package directory './'.
Perhaps you meant the package directory './'?

This happens because this code relies on the canonical property to match up packages that are known to live at various locations with a directory given by the user. Since one code path happens to call canonicalizePath with a trailing slash and another without then there is no match and the confusing error.

[dcoutts]

Possibly related to #42

[Rufflewind]

Sorry, I don't think there's a sensible way to fix this. The idea of “canonicalization” has always been quite vague and there does not seem to be an agreement on whether trailing slashes carry significant meaning.

The current behavior aligns with that of directory 1.2.2.1 or older. Between 1.2.3.0 and 1.2.5.0 the behavior was different, which was an unintentional result (#42) of fixing #23. The current behavior is now fully documented:

Similar to normalise, passing an empty path is equivalent to passing the current directory. The function preserves the presence or absence of the trailing path separator unless the path refers to the root directory /.

I suggest running normalise dropTrailingPathSeparator on it if you don't want the trailing slash.

[dcoutts]

Sorry, I don't think there's a sensible way to fix this.

Surely it's just a matter of integrating dropTrailingPathSeparator, no?

The idea of “canonicalization” has always been quite vague

The canonicalization is not that vague. Everyone accepts that file systems have multiple names that refer to the same file system object (hard links etc), but programmers and users have reasonable expectations about things that are really the same name.

there does not seem to be an agreement on whether trailing slashes carry significant meaning

There doesn't need to be any such agreement. It's clearly not canonical to preserve them. They should either always be there or never (the old behaviour was never). I'm fully aware about the vagaries of whether trailing slashes should carry meaning or not, but that really ought to be irrelevant for a function that is there to produce a canonical result.

The current behavior aligns with that of directory 1.2.2.1 or older.

No it does not. In particular the following versions have the correct behaviour (ie same result for input paths given with or without trailing slash):

• 1.2.2.0 (shipped with ghc-7.10.3)
• 1.2.1.0 (shipped with ghc-7.8.4)
• 1.2.0.1 (shipped with ghc-7.6.3)
• 1.1.0.2 (shipped with ghc-7.4.1)
• 1.0.0.0 (shipped with ghc-6.8.2)

It is only the version shipped with ghc-8.0.1 that has the new incorrect behaviour of non-canonical results.

So it's quite clear the old behaviour is possible, and well established.

We need canonical results. Lots of apps rely on this. We need a form where we can compare strings and have those refer to the same name. To the best of my knowledge the old canonicalizePath was always fine for this. Sure being able to use it on paths that don't even exist is a bonus and there best effort is ok, but for ones that do exist it seems reasonably clear that there's a sensible meaning: and for trailing slashes that's plain and obvious.

It's not ok to just say "oh the behaviour changed, please add a call to dropTrailingPathSeparator", when there's no good reason for the change and particularly because it's a silent behaviour change that subtly breaks an important property. If we really thought that it was unsustainable to have a function that tries to produce canonical names then it'd be fine to add a few replacements and deprecate the old one. But I've not yet seen any argument that this is the situation here.

[Rufflewind]

No it does not. In particular the following versions have the correct behaviour (ie same result for input paths given with or without trailing slash):

You're right. The quirky behavior did not exist prior to 1.2.2.1. Sorry, I did not verify the old behavior (I took #42 for granted). My guess is that the quirk had existed only on Windows systems.

In that case I will revert the fix for #42 and bump the major version.

[dcoutts]

Ok great, but presumably we don't want to re-introduce the odd behaviour described in #42. It should still be the case that canonicalizePath "." returns a path without the trailing slash.

BTW, I noticed this in the implementation of prependCurrentDirectory which is used by canonicalizePath:

      | isRelative path ->
          (</> path) . addTrailingPathSeparator <$> getCurrentDirectory

Specifically, addTrailingPathSeparator ought to be redundant there. The </> will add it if necessary. Or am I missing something?

[Rufflewind]

Specifically, addTrailingPathSeparator ought to be redundant there. The </> will add it if necessary. Or am I missing something?

It used to matter, but now it's redundant because of the case path of { "" -> …; _ -> … }.

[Rufflewind]

Let me know if Rufflewind@43488ba fixes it. In particular, the tests have been updated to reflect the change in behavior.

[bgamari]

@dcoutts any update on this? I'd like to be able to get a new release candidate out soon.