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filter-branch: resolve $commit^{tree} in no-index case
Commit 348d4f2 (filter-branch: skip index read/write when
possible, 2015-11-06) taught filter-branch to optimize out
the final "git write-tree" when we know we haven't touched
the tree with any of our filters. It does by simply putting
the literal text "$commit^{tree}" into the "$tree" variable,
avoiding a useless rev-parse call.
However, when we pass this to git_commit_non_empty_tree(),
it gets confused; it resolves "$commit^{tree}" itself, and
compares our string to the 40-hex sha1, which obviously
doesn't match. As a result, "--prune-empty" (or any custom
filter using git_commit_non_empty_tree) will fail to drop
an empty commit (when filter-branch is used without a tree
or index filter).
Let's resolve $tree to the 40-hex ourselves, so that
git_commit_non_empty_tree can work. Unfortunately, this is a
bit slower due to the extra process overhead:
$ cd t/perf && ./run 348d4f2 HEAD p7000-filter-branch.sh
[...]
Test 348d4f2 HEAD
--------------------------------------------------------------
7000.2: noop filter 3.76(0.24+0.26) 4.54(0.28+0.24) +20.7%
We could try to make git_commit_non_empty_tree more clever.
However, the value of $tree here is technically
user-visible. The user can provide arbitrary shell code at
this stage, which could itself have a similar assumption to
what is in git_commit_non_empty_tree. So the conservative
choice to fix this regression is to take the 20% hit and
give the pre-348d4f2 behavior. We still end up much faster
than before the optimization:
$ cd t/perf && ./run 348d4f2^ HEAD p7000-filter-branch.sh
[...]
Test 348d4f2^ HEAD
--------------------------------------------------------------
7000.2: noop filter 9.51(4.32+0.40) 4.51(0.28+0.23) -52.6%
Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
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