In 652bd0211d (rebase: use 'skip_cache_tree_update' option, 2022-11-10),
we updated `reset_head()` to stop updating the index tree cache. This
was done as a performance optimization: the function is only called by
"sequencer.c" and "rebase.c", both of which assume a clean index before
they perform their operation, so we know that the end result will be a
clean index, too. Consequently, we can skip recomputing the cache as we
can instead use `prime_cache_tree()` directly.

In a subsequent commit we're about to add a new caller though where the
assumption doesn't hold anymore: the index may be dirty before calling
`reset_head()`, and consequently we cannot prime the cache with a given
tree anymore as the index and tree will mismatch.

Adapt the logic so that we only skip the cache tree update in case we're
doing a hard reset. While we could introduce logic that only skips the
update in case the incoming index was dirty already, that doesn't really
feel worth it: after all, the mentioned commit says itself that the
performance improvement was negligible anyway.

Signed-off-by: Patrick Steinhardt <redacted>
---
 reset.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/reset.c b/reset.c
index 5ba9a3a574..2fabc54d9b 100644
--- a/reset.c
+++ b/reset.c

@@ -166,10 +166,11 @@ int reset_head(struct repository *r, const struct reset_head_opts *opts)
 	unpack_tree_opts.dry_run = dry_run;
 	unpack_tree_opts.merge = 1;
 	unpack_tree_opts.preserve_ignored = 0; /* FIXME: !overwrite_ignore */
-	unpack_tree_opts.skip_cache_tree_update = 1;
 	init_checkout_metadata(&unpack_tree_opts.meta, switch_to_branch, oid, NULL);
-	if (reset_hard)
+	if (reset_hard) {
+		unpack_tree_opts.skip_cache_tree_update = 1;
 		unpack_tree_opts.reset = UNPACK_RESET_PROTECT_UNTRACKED;
+	}
 
 	if (!reset_hard && !fill_tree_descriptor(r, &desc[nr++], &head_oid)) {
 		ret = error(_("failed to find tree of %s"),

@@ -196,7 +197,8 @@ int reset_head(struct repository *r, const struct reset_head_opts *opts)
 		goto leave_reset_head;
 	}
 
-	prime_cache_tree(r, r->index, tree);
+	if (reset_hard)
+		prime_cache_tree(r, r->index, tree);
 
 	if (write_locked_index(r->index, &lock, COMMIT_LOCK) < 0) {
 		ret = error(_("could not write index"));

-- 
2.54.0.1136.gdb2ca164c4.dirty