[PATCH] subtree: allow duplicate cache entries with same value

[Eric Wendland <eric@wendland.dev>]

When a subtree is added, removed, and later re-added at the same path,
'git subtree split' fails with:

    fatal: cache for <hash> already exists!

This happens because 'find_existing_splits()' scans history and calls
'cache_set()' for each prior split. When the same subtree commit was
added twice at different mainline commits, both map to the same subtree
commit. The second 'cache_set()' call tries to write the same mapping
again, but the old code treated any existing cache entry as a fatal error.

Fix 'cache_set()' to silently succeed when the existing cache entry
already contains the same value we're trying to write.

Add a regression test for this scenario.

Signed-off-by: Eric Wendland <eric@wendland.dev>
Closes: <1d8fa22c-eb13-4cdb-8499-e19f0dea6b42@tionis.dev>
---
 contrib/subtree/git-subtree.sh     |  5 +++++
 contrib/subtree/t/t7900-subtree.sh | 29 +++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh
index 791fd8260c..f0f2a47d53 100755
--- a/contrib/subtree/git-subtree.sh
+++ b/contrib/subtree/git-subtree.sh
@@ -343,6 +343,11 @@ cache_set () {
 		test "$oldrev" != "latest_new" &&
 		test -e "$cachedir/$oldrev"
 	then
+		read oldcache <"$cachedir/$oldrev"
+		if test "$oldcache" = "$newrev"
+		then
+			return
+		fi
 		die "fatal: cache for $oldrev already exists!"
 	fi
 	echo "$newrev" >"$cachedir/$oldrev"
diff --git a/contrib/subtree/t/t7900-subtree.sh b/contrib/subtree/t/t7900-subtree.sh
index 18d2b56448..85fe4362fb 100755
--- a/contrib/subtree/t/t7900-subtree.sh
+++ b/contrib/subtree/t/t7900-subtree.sh
@@ -528,6 +528,35 @@ do
 	'
 done
 
+test_expect_success 'split after add, remove, and re-add at same path' '
+	subtree_test_create_repo "$test_count" &&
+	subtree_test_create_repo "$test_count/sub proj" &&
+	test_create_commit "$test_count" main1 &&
+	test_create_commit "$test_count/sub proj" sub1 &&
+	(
+		cd "$test_count" &&
+		git fetch ./"sub proj" HEAD &&
+		git subtree add --prefix="sub dir" FETCH_HEAD
+	) &&
+	test_create_commit "$test_count" "sub dir"/main-sub1 &&
+	(
+		cd "$test_count" &&
+		git rm -rf "sub dir" &&
+		git commit -m "remove sub dir"
+	) &&
+	(
+		cd "$test_count" &&
+		git fetch ./"sub proj" HEAD &&
+		git subtree add --prefix="sub dir" FETCH_HEAD
+	) &&
+	test_create_commit "$test_count" "sub dir"/main-sub2 &&
+	(
+		cd "$test_count" &&
+		git subtree split --prefix="sub dir" --branch=subproj-br &&
+		test "$(git rev-parse --verify subproj-br)"
+	)
+'
+
 # Usually,
 #
 #    git subtree merge -P subA --squash f00...
-- 
2.47.3