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 <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/config/reftable.txt b/Documentation/config/reftable.txt
index 2374be71d7df850829cb310ec3aa722c573de1fa..68083876fa556d1d6bf3a3c2d10c09bb22d621b1 100644 (file)
--- 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
index a2880aabce3900ec2bd476765cddf65d2abae3e8..5ffb36770a810f157bd12ab970fb8e89da7cc32d 100644 (file)
--- 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
index e0a5b26f58fc0a995253268b13c1611497beb01c..e2708e11d5b941cf707d3c22466e4cc6957d0302 100755 (executable)
--- 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