Here are some patches to add a --remerge-diff capability to show & log, which works by comparing merge commits to an automatic remerge (note that the automatic remerge tree can contain files with conflict markers). Changes since original submission[1]: * Rebased on top of the version of ns/tmp-objdir that Neeraj submitted (Neeraj's patches were based on v2.34, but ns/tmp-objdir got applied on an old commit and does not even build because of that). * Modify ll-merge API to return a status, instead of printing "Cannot merge binary files" on stdout[2] (as suggested by Peff) * Make conflict messages and other such warnings into diff headers of the subsequent remerge-diff rather than appearing in the diff as file content of some funny looking filenames (as suggested by Peff[3] and Junio[4]) * Sergey ack'ed the diff-merges.c portion of the patches, but that wasn't limited to one patch so not sure where to record that ack. [1] https://lore.kernel.org/git/pull.1080.git.git.1630376800.gitgitgadget@xxxxxxxxx/; GitHub wouldn't let me change the target branch for the PR, so I had to create a new one with the new base and thus the reason for not sending this as v2 even though it is. [2] https://lore.kernel.org/git/YVOZRhWttzF18Xql@xxxxxxxxxxxxxxxxxxxxxxx/, https://lore.kernel.org/git/YVOZty9D7NRbzhE5@xxxxxxxxxxxxxxxxxxxxxxx/ [3] https://lore.kernel.org/git/YVOXPTjsp9lrxmS6@xxxxxxxxxxxxxxxxxxxxxxx/ [4] https://lore.kernel.org/git/xmqqr1d7e4ug.fsf@gitster.g/ === FURTHER BACKGROUND (original cover letter material) == Here are some example commits you can try this out on (with git show --remerge-diff $COMMIT): * git.git conflicted merge: 07601b5b36 * git.git non-conflicted change: bf04590ecd * linux.git conflicted merge: eab3540562fb * linux.git non-conflicted change: 223cea6a4f05 Many more can be found by just running git log --merges --remerge-diff in your repository of choice and searching for diffs (most merges tend to be clean and unmodified and thus produce no diff but a search of '^diff' in the log output tends to find the examples nicely). Some basic high level details about this new option: * This option is most naturally compared to --cc, though the output seems to be much more understandable to most users than --cc output. * Since merges are often clean and unmodified, this new option results in an empty diff for most merges. * This new option shows things like the removal of conflict markers, which hunks users picked from the various conflicted sides to keep or remove, and shows changes made outside of conflict markers (which might reflect changes needed to resolve semantic conflicts or cleanups of e.g. compilation warnings or other additional changes an integrator felt belonged in the merged result). * This new option does not (currently) work for octopus merges, since merge-ort is specific to two-parent merges[1]. * This option will not work on a read-only or full filesystem[2]. * We discussed this capability at Git Merge 2020, and one of the suggestions was doing a periodic git gc --auto during the operation (due to potential new blobs and trees created during the operation). I found a way to avoid that; see [2]. * This option is faster than you'd probably expect; it handles 33.5 merge commits per second in linux.git on my computer; see below. In regards to the performance point above, the timing for running the following command: time git log --min-parents=2 --max-parents=2 $DIFF_FLAG | wc -l in linux.git (with v5.4 checked out, since my copy of linux is very out of date) is as follows: DIFF_FLAG=--cc: 71m 31.536s DIFF_FLAG=--remerge-diff: 31m 3.170s Note that there are 62476 merges in this history. Also, output size is: DIFF_FLAG=--cc: 2169111 lines DIFF_FLAG=--remerge-diff: 2458020 lines So roughly the same amount of output as --cc, as you'd expect. As a side note: git log --remerge-diff, when run in various repositories and allowed to run all the way back to the beginning(s) of history, is a nice stress test of sorts for merge-ort. Especially when users run it for you on their repositories they are working on, whether intentionally or via a bug in a tool triggering that command to be run unexpectedly. Long story short, such a bug in an internal tool existed last December and this command was run on an internal repository and found a platform-specific bug in merge-ort on some really old merge commit from that repo. I fixed that bug (a STABLE_QSORT thing) while upstreaming all the merge-ort patches in the mean time, but it was nice getting extra testing. Having more folks run this on their repositories might be useful extra testing of the new merge strategy. Also, I previously mentioned --remerge-diff-only (a flag to show how cherry-picks or reverts differ from an automatic cherry-pick or revert, in addition to showing how merges differ from an automatic merge). This series does not include the patches to introduce that option; I'll submit them later. Two other things that might be interesting but are not included and which I haven't investigated: * some mechanism for passing extra merge options through (e.g. -Xignore-space-change) * a capability to compare the automatic merge to a second automatic merge done with different merge options. (Not sure if this would be of interest to end users, but might be interesting while developing new a --strategy-option, or maybe checking how changing some default in the merge algorithm would affect historical merges in various repositories). [1] I have nebulous ideas of how an Octopus-centric ORT strategy could be written -- basically, just repeatedly invoking ort and trying to make sure nested conflicts can be differentiated. For now, though, a simple warning is printed that octopus merges are not handled and no diff will be shown. [2] New blobs/trees can be written by the three-way merging step. These are written to a temporary area (via tmp-objdir.c) under the git object store that is cleaned up at the end of the operation, with the new loose objects from the remerge being cleaned up after each individual merge. Elijah Newren (9): tmp_objdir: add a helper function for discarding all contained objects ll-merge: make callers responsible for showing warnings merge-ort: capture and print ll-merge warnings in our preferred fashion merge-ort: mark a few more conflict messages as omittable merge-ort: make path_messages available to external callers diff: add ability to insert additional headers for paths merge-ort: format messages slightly different for use in headers show, log: provide a --remerge-diff capability doc/diff-options: explain the new --remerge-diff option Documentation/diff-options.txt | 8 ++++ apply.c | 5 ++- builtin/checkout.c | 12 ++++-- builtin/log.c | 16 ++++++++ diff-merges.c | 12 ++++++ diff.c | 34 ++++++++++++++++- diff.h | 1 + ll-merge.c | 40 ++++++++++--------- ll-merge.h | 9 ++++- log-tree.c | 70 ++++++++++++++++++++++++++++++++++ merge-blobs.c | 5 ++- merge-ort.c | 49 +++++++++++++++++++++--- merge-ort.h | 10 +++++ merge-recursive.c | 8 +++- merge-recursive.h | 1 + notes-merge.c | 5 ++- rerere.c | 10 +++-- revision.h | 6 ++- t/t6404-recursive-merge.sh | 9 ++++- t/t6406-merge-attr.sh | 9 ++++- tmp-objdir.c | 5 +++ tmp-objdir.h | 6 +++ 22 files changed, 288 insertions(+), 42 deletions(-) base-commit: 4e44121c2d7bced65e25eb7ec5156290132bec94 Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1103%2Fnewren%2Fremerge-diff-v1 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1103/newren/remerge-diff-v1 Pull-Request: https://github.com/gitgitgadget/git/pull/1103 -- gitgitgadget
