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54fd324 ("rebase -i: reread the todo list if `exec` touched it", 2017-04-26) sought to reread the todo list after running an exec command only if it had been changed. To accomplish this it checks the stat data of the todo list after running an exec command to see if it has changed. Unfortunately there are two problems, firstly the implementation is buggy we actually reread the list after each exec which is quadratic in the number of commit lookups and secondly the design is predicated on using nanosecond time stamps which are not the default. The implementation bug stems from the fact that we write a new todo list to disk before running each command but do not update the stat data to reflect this[1]. The design problem is that it is possible for the user to edit the todo list without changing its size or inode which means we have to rely on the mtime to tell us if it has changed. Unfortunately unless git is built with USE_NSEC it is possible for the original and edited list to share the same mtime. Ideally "git rebase --edit-todo" would set a flag that we would then check in sequencer.c. Unfortunately this is approach will not work as there are scripts in the wild that write to the todo list directly without running "git rebase --edit-todo". Instead of relying on stat data this patch simply reads the possibly edited todo list and compares it to the original with memcmp(). This is much faster than reparsing the todo list each time. This patch reduces the time to run git rebase -r -xtrue v2.32.0~100 v2.32.0 which runs 419 exec commands by 6.6%. For comparison fixing the implementation bug in stat based approach reduces the time by a further 1.4% and is indistinguishable from never rereading the todo list. [1] https://lore.kernel.org/git/20191125131833.GD23183@szeder.dev/ Reported-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
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