A few months ago, directly after sending a patch to fix a performance regression due to a mis-use of test_cmp [https://lore.kernel.org/git/b9203ea247776332e4b6f519aa27d541207adc2f.1659097724.git.gitgitgadget@xxxxxxxxx/], I got curious to see whether Git for Windows had the same issue. And it did not: it passes t5351 in 22 seconds, even while using test_cmp to compare pack files [https://github.com/git-for-windows/git/blob/3922f62f0d5991e9fe0a0817ebf89a91339c7705/t/t5351-unpack-large-objects.sh#L90]. The explanation is of course that Git for Windows uses a test helper for test_cmp that is written in C, instead of the Bash function. And C is much faster than a Bash function, especially on Windows. This is especially sad when said Bash code is only used on Windows. So I originally had pulled out this helper from the years-long effort to let Git for Windows use BusyBox' ash to run the test suite. The result was a single-patch contribution of a change that had been in Git for Windows since June 2018. Unfortunately, this tried-and-tested code was rejected by the Git maintainer. Let's fall back to the next-best solution: git diff --no-index, which the Git maintainer seems to like. The downside is that the diff machinery does a lot more than a simple cmp clone, and therefore a lot more things can go wrong that might make it look like a test case is failing when the fault is somewhere else entirely. There is one way to find out whether this is a valid concern. Changes since v3: * Fixed the subject of the cover letter (which should have been adjusted in v3) * Elaborated the paragraph about the historical context of this patch Changes since v2: * Dropped the test helper, using diff --no-index instead. Changes since v1: * Fixed double "with" in the commit message. * Renamed the test helper to text-cmp. * Made the diff --no-index call more robust by using a double-dash separator. Johannes Schindelin (2): t0021: use Windows-friendly `pwd` tests(mingw): avoid very slow `mingw_test_cmp` t/t0021-conversion.sh | 4 +-- t/test-lib-functions.sh | 66 ----------------------------------------- t/test-lib.sh | 2 +- 3 files changed, 3 insertions(+), 69 deletions(-) base-commit: 23b219f8e3f2adfb0441e135f0a880e6124f766c Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1309%2Fdscho%2Fmingw-test-cmp-v4 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1309/dscho/mingw-test-cmp-v4 Pull-Request: https://github.com/gitgitgadget/git/pull/1309 Range-diff vs v3: 1: b38b8fb5a85 = 1: b38b8fb5a85 t0021: use Windows-friendly `pwd` 2: a7f4265ceb2 ! 2: 128b1f348d8 tests(mingw): avoid very slow `mingw_test_cmp` @@ Commit message `mingw_test_cmp` code with MSYS2's Bash, i.e. the Bash that Git for Windows uses. And a lot more readable. - Note: Earlier attempts at fixing this involved a test helper that avoids - the overhead of the diff machinery, in favor of implementing a behavior - that is more in line with what `mingw_test_cmp` does now, but that - attempt saw a lot of backlash and distractions during review and was - therefore abandoned. + The original reason why Git's test suite needs the `mingw_test_cmp` + function at all (and why `cmp` is not good enough) is that Git's test + suite is not actually trying to compare binary files when it calls + `test_cmp`, but it compares text files. And those text files can contain + CR/LF line endings depending on the circumstances. + + Note: The original fix in the Git for Windows project implemented a test + helper that avoids the overhead of the diff machinery, in favor of + implementing a behavior that is more in line with what `mingw_test_cmp` + does now. This was done to minimize the risk in using something as + complex as the diff machinery to perform something as simple as + determining whether text output is identical to the expected output or + not. This approach has served Git for Windows well for years, but the + attempt to upstream this saw a lot of backlash and distractions during + the review, was disliked by the Git maintainer and was therefore + abandoned. For full details, see the thread at + https://lore.kernel.org/git/pull.1309.git.1659106382128.gitgitgadget@xxxxxxxxx/t Signed-off-by: Johannes Schindelin <johannes.schindelin@xxxxxx> -- gitgitgadget
