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 <ps@pks.im>
---
 Documentation/config/reftable.txt |  6 +++
 refs/reftable-backend.c           |  3 ++
 t/t0613-reftable-write-options.sh | 69 +++++++++++++++++++++++++++++++
 3 files changed, 78 insertions(+)

diff --git a/Documentation/config/reftable.txt b/Documentation/config/reftable.txt
index 16b915c75e..6e4466f3c5 100644
--- a/Documentation/config/reftable.txt
+++ b/Documentation/config/reftable.txt
@@ -31,3 +31,9 @@ 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
index a786143de2..5298fcef6e 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -249,6 +249,9 @@ static int reftable_be_config(const char *var, const char *value,
 			die("reftable block size cannot exceed %u", (unsigned)UINT16_MAX);
 		opts->restart_interval = restart_interval;
 		return 0;
+	} else if (!strcmp(var, "reftable.indexobjects")) {
+		opts->skip_index_objects = !git_config_bool(var, value);
+		return 0;
 	}
 
 	return 0;
diff --git a/t/t0613-reftable-write-options.sh b/t/t0613-reftable-write-options.sh
index e0a5b26f58..e2708e11d5 100755
--- 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
-- 
2.45.0