Re: fread reading directories

From: Brandon Casey <drafnel@gmail.com>
Date: Jun 8, 2020, 19:08 UTC
Message-ID: <CA+sFfMcQ+HQPk3SMsBhWjfLiVLzfhHSv9OpzPHAJt5b50TEPeQ@mail.gmail.com>
In-Reply-To: <xmqqlfkxlbn4.fsf@gitster.c.googlers.com>

On Mon, Jun 8, 2020 at 10:18 AM Junio C Hamano <gitster@pobox.com> wrote:

Show 21 quoted lines

>
> It may make sense to do one of the two things:
>
>  - The lighter weight one is to rename the macro to the reflect the
>    trait we are trying to capture more faithfully: "fopen opens
>    directories" and leave the code and performance characteristics
>    as-is.
>
>  - Heavier weight one is to audit callers of fopen() and only let
>    those that know they do not have a directory directly call
>    fopen().  The other callers would call our wrapper under a
>    different name.  This way, the former won't have to pay the
>    overhead of checking for "you gave me a directory but I only take
>    a file" error twice.  This is what Brandon proposed in the
>    thread.
>
> Doing neither would leave this seed of confusion for later readers,
> which is not ideal.  I am tempted to say that we for now should do
> an even lighter variant of the former, which is to give a comment.
>
> Thoughts?

I'd suggest a medium weight approach which would be to introduce a new function with an appropriate name (fopen_file_only()?) that behaves the way we want it to, and replace every existing fopen() call with this new function. We could introduce a new macro, which I think would only be used on Windows, to say "fopen already fails to open directories" (FOPEN_FAILS_ON_DIRECTORIES?) so that fopen_file_only could be simplified to just a bare fopen there. That way it's clear to the reader, at the callsite, that the call does not have the standard behavior of fopen.

Then, FREAD_READS_DIRECTORIES could be removed from all but the 1 or 2 platforms that it was originally set for. I'd imagine that we'd basically just promote the git_fopen() function from compat to become the implementation of the first tier fopen_file_only() function. On the FREAD_READS_DIRECTORIES platforms, a bare fopen would also become fopen_file_only(). The call to fopen() within fopen_file_only() would obviously need to take this into account to ensure that it calls the real fopen().

I think this would put the pieces in place for someone to audit all of the existing uses of fopen_file_only() and potentially replace them with a straight fopen() if appropriate. And it would allow future code to explicitly make the choice between fopen_file_only() or just fopen().

None of this would produce any functional change on any of our platforms, but I think it would make things more clear.

-Brandon