Re: [PATCH 13/19] mingw: outsmart MSYS2's path substitution in t1508

From: Eric Sunshine <sunshine@sunshineco.com>
Date: Jan 25, 2016, 06:22 UTC
Message-ID: <CAPig+cQfbJUtqu2F497Usaz3Ysf-5-vQgtg-0y8qh+FUQ=HeTA@mail.gmail.com>
In-Reply-To: <xmqqa8nubekj.fsf@gitster.mtv.corp.google.com>

On Sun, Jan 24, 2016 at 9:03 PM, Junio C Hamano <gitster@pobox.com> wrote:

Show 25 quoted lines

> Johannes Schindelin <johannes.schindelin@gmx.de> writes:
>> A string of the form "@/abcd" is considered a file path
>> by the msys layer and therefore translated to a Windows path.
>>
>> Here the trick is to double the slashes.
>>
>> The MSYS2 patch translation can be studied by calling
>>
>>       test-path-utils print_path <path>
>
> This feels wrong.
>
> The point of this test is that you can ask to checkout a branch
> whose name is a strangely looking "@/at-test", and a ref whose name
> is "refs/heads/@/at-test" indeed is created.
>
> The current "checkout" may be lazy and not signal an error for a
> branch name with two consecutive slashes, but I wouldn't be
> surprised if we tighten that later, and more importantly, I do not
> think we ever promised users if you asked a branch "a//b" to be
> created, we would create "refs/heads/a/b".
>
> The new test hardcodes and promises such an incompatible behaviour,
> i.e. a request to create "@//b" results in "@/b" created, only to
> users on MINGW, fracturing the expectations of the Git userbase.

What the commit message doesn't explain is that the MSYS2 Bash shell automatically translates command-line arguments which look like absolute POSIX paths into Windows paths on behalf of the program being invoked. For instance, given the command-line:

    myprog /fish

the shell will translate /fish to a Windows pathname, such as C:\fish before invoking myprog.exe. While this is often the desired behavior, sometimes the argument represents something other than a path, and the translation is unwanted. MSYS provides a way to suppress the behavior manually by doubling the slash, so:

    myprog //fish

will invoke myprog.exe with literal argument /fish (notice the single slash).

This commit message is trying to say that MSYS shell undesirably sees @/fish as an absolute path, thus tries translating it to a Windows path, such as @C:\fish. The only way to suppress this unwanted translation is to manually double the slash, hence the patch makes the test use @//fish which, when finally seen by the program, is just @/fish, as was intended in the first place. So, doubling the slash on MINGW is not promising incompatible behavior for MINGW users; it's just working around unwanted path translation of the shell.