Re: [PATCH v5 2/2] tests(mingw): avoid very slow `mingw_test_cmp`

[Taylor Blau <me@ttaylorr.com>]

Hi Johannes,

On Tue, Dec 06, 2022 at 03:07:47PM +0000, Johannes Schindelin via GitGitGadget wrote:
> Learning the lesson to avoid shell scripting wherever possible, the Git
> for Windows project implemented a minimal replacement for
> `mingw_test_cmp` in the form of a `test-tool` subcommand that parses the
> input files line by line, ignoring line endings, and compares them.
> Essentially the same thing as `mingw_test_cmp`, but implemented in
> C instead of Bash. This solution served the Git for Windows project
> well, over years.
>
> However, when this solution was finally upstreamed, the conclusion was
> reached that a change to use `git diff --no-index` instead of
> `mingw_test_cmp` was more easily reviewed and hence should be used
> instead.
>
> The reason why this approach was not even considered in Git for Windows
> is that in 2007, there was already a motion on the table to use Git's
> own diff machinery to perform comparisons in Git's test suite, but it
> was dismissed in https://lore.kernel.org/git/xmqqbkrpo9or.fsf@gitster.g/
> as undesirable because tests might potentially succeed due to bugs in
> the diff machinery when they should not succeed, and those bugs could
> therefore hide regressions that the tests try to prevent.
>
> By the time Git for Windows' `mingw-test-cmp` in C was finally
> contributed to the Git mailing list, reviewers agreed that the diff
> machinery had matured enough and should be used instead.
>
> When the concern was raised that the diff machinery, due to its
> complexity, would perform substantially worse than the test helper
> originally implemented in the Git for Windows project, a test
> demonstrated that these performance differences are well lost within the
> 100+ minutes it takes to run Git's test suite on Windows.

Thanks for this update. I think these five paragraphs are an accurate
and helpful record of the history behind this patch and approach. It is
very much appreciated.

Thanks,
Taylor