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authorLibravatar SZEDER Gábor <szeder.dev@gmail.com>2018-10-11 11:43:08 +0200
committerLibravatar Junio C Hamano <gitster@pobox.com>2018-10-12 07:23:29 +0900
commite3d837989ef6b98ce6822145322d9e3d250a5b6f (patch)
treedca7cce32a35c0804304647eb798ca886c70c571 /blame.c
parentsplit-index: count the number of deleted entries (diff)
downloadtgif-e3d837989ef6b98ce6822145322d9e3d250a5b6f.tar.xz
split-index: don't compare cached data of entries already marked for split index
When unpack_trees() constructs a new index, it copies cache entries from the original index [1]. prepare_to_write_split_index() has to deal with this, and it has a dedicated code path for copied entries that are present in the shared index, where it compares the cached data in the corresponding copied and original entries. If the cached data matches, then they are considered the same; if it differs, then the copied entry will be marked for inclusion as a replacement entry in the just about to be written split index by setting the CE_UPDATE_IN_BASE flag. However, a cache entry already has its CE_UPDATE_IN_BASE flag set upon reading the split index, if the entry already has a replacement entry there, or upon refreshing the cached stat data, if the corresponding file was modified. The state of this flag is then preserved when unpack_trees() copies a cache entry from the shared index. So modify prepare_to_write_split_index() to check the copied cache entries' CE_UPDATE_IN_BASE flag first, and skip the thorough comparison of cached data if the flag is already set. Those couple of lines comparing the cached data would then have too many levels of indentation, so extract them into a helper function. Note that comparing the cached data in copied and original entries in the shared index might actually be entirely unnecessary. In theory all code paths refreshing the cached stat data of an entry in the shared index should set the CE_UPDATE_IN_BASE flag in that entry, and unpack_trees() should preserve this flag when copying cache entries. This means that the cached data is only ever changed if the CE_UPDATE_IN_BASE flag is set as well. Our test suite seems to confirm this: instrumenting the conditions in question and running the test suite repeatedly with 'GIT_TEST_SPLIT_INDEX=yes' showed that the cached data in a copied entry differs from the data in the shared entry only if its CE_UPDATE_IN_BASE flag is indeed set. In practice, however, our test suite doesn't have 100% coverage, GIT_TEST_SPLIT_INDEX is inherently random, and I certainly can't claim to possess complete understanding of what goes on in unpack_trees()... Therefore I kept the comparison of the cached data when CE_UPDATE_IN_BASE is not set, just in case that an unnoticed or future code path were to accidentally miss setting this flag upon refreshing the cached stat data or unpack_trees() were to drop this flag while copying a cache entry. [1] Note that when unpack_trees() constructs the new index and decides that a cache entry should now refer to different content than what was recorded in the original index (e.g. 'git read-tree -m HEAD^'), then that can't really be considered a copy of the original, but rather the creation of a new entry. Notably and pertinent to the split index feature, such a new entry doesn't have a reference to the original's shared index entry anymore, i.e. its 'index' field is set to 0. Consequently, such an entry is treated by prepare_to_write_split_index() as an entry not present in the shared index and it will be added to the new split index, while the original entry will be marked as deleted, and neither the above discussion nor the changes in this patch apply to them. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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