refs/reftable: allow disabling writing the object index

Besides the expected "ref" and "log" records, the reftable library also writes "obj" records. These are basically a reverse mapping of object IDs to their respective ref records so that it becomes efficient to figure out which references point to a specific object. The motivation for this data structure is the "uploadpack.allowTipSHA1InWant" config, which allows a client to fetch any object by its hash that has a ref pointing to it. This reverse index is not used by Git at all though, and the expectation is that most hosters nowadays use "uploadpack.allowAnySHA1InWant". It may thus be preferable for many users to disable writing these optional object indices altogether to safe some precious disk space. Add a new config "reftable.indexObjects" that allows the user to disable the object index altogether. Signed-off-by: Patrick Steinhardt <[email protected]> Signed-off-by: Junio C Hamano <[email protected]>
ToBoMi · May 14, 2024 · afbdbfa · afbdbfa
1 parent 90db611
commit afbdbfa
Show file tree

Hide file tree

Showing 3 changed files with 77 additions and 0 deletions.
diff --git a/Documentation/config/reftable.txt b/Documentation/config/reftable.txt
@@ -30,3 +30,9 @@ A maximum of `65535` restart points per block is supported.
 +
 The default value is to create restart points every 16 records. A value of `0`
 will use the default value.
+
+reftable.indexObjects::
+	Whether the reftable backend shall write object blocks. Object blocks
+	are a reverse mapping of object ID to the references pointing to them.
++
+The default value is `true`.
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
@@ -245,6 +245,8 @@ static int reftable_be_config(const char *var, const char *value,
 		if (restart_interval > UINT16_MAX)
 			die("reftable block size cannot exceed %u", (unsigned)UINT16_MAX);
 		opts->restart_interval = restart_interval;
+	} else if (!strcmp(var, "reftable.indexobjects")) {
+		opts->skip_index_objects = !git_config_bool(var, value);
 	}
 
 	return 0;

diff --git a/t/t0613-reftable-write-options.sh b/t/t0613-reftable-write-options.sh
@@ -214,4 +214,73 @@ test_expect_success 'restart interval exceeding maximum supported interval' '
 	)
 '
 
+test_expect_success 'object index gets written by default with ref index' '
+	test_config_global core.logAllRefUpdates false &&
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		for i in $(test_seq 5)
+		do
+			printf "update refs/heads/branch-%d HEAD\n" "$i" ||
+			return 1
+		done >input &&
+		git update-ref --stdin <input &&
+		git -c reftable.blockSize=100 pack-refs &&
+
+		cat >expect <<-EOF &&
+		header:
+		  block_size: 100
+		ref:
+		  - length: 53
+		    restarts: 1
+		  - length: 95
+		    restarts: 1
+		  - length: 71
+		    restarts: 1
+		  - length: 80
+		    restarts: 1
+		obj:
+		  - length: 11
+		    restarts: 1
+		EOF
+		test-tool dump-reftable -b .git/reftable/*.ref >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'object index can be disabled' '
+	test_config_global core.logAllRefUpdates false &&
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		for i in $(test_seq 5)
+		do
+			printf "update refs/heads/branch-%d HEAD\n" "$i" ||
+			return 1
+		done >input &&
+		git update-ref --stdin <input &&
+		git -c reftable.blockSize=100 -c reftable.indexObjects=false pack-refs &&
+
+		cat >expect <<-EOF &&
+		header:
+		  block_size: 100
+		ref:
+		  - length: 53
+		    restarts: 1
+		  - length: 95
+		    restarts: 1
+		  - length: 71
+		    restarts: 1
+		  - length: 80
+		    restarts: 1
+		EOF
+		test-tool dump-reftable -b .git/reftable/*.ref >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done