From: Derrick Stolee <dstolee@xxxxxxxxxxxxx>
In an environment like a fork network, it is helpful to have a
commit-graph chain that spans both the base repo and the fork repo. The
fork is usually a small set of data on top of the large repo, but
sometimes the fork is much larger. For example, git-for-windows/git has
almost double the number of commits as git/git because it rebases its
commits on every major version update.
To allow cross-alternate commit-graph chains, we need a few pieces:
1. When looking for a graph-{hash}.graph file, check all alternates.
2. When merging commit-graph chains, do not merge across alternates.
3. When writing a new commit-graph chain based on a commit-graph file
in another object directory, do not allow success if the base file
has of the name "commit-graph" instead of
"commit-graphs/graph-{hash}.graph".
Signed-off-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx>
---
Documentation/technical/commit-graph.txt | 40 +++++++++++++++++
commit-graph.c | 56 +++++++++++++++++++-----
commit-graph.h | 1 +
t/t5324-split-commit-graph.sh | 37 ++++++++++++++++
4 files changed, 123 insertions(+), 11 deletions(-)
diff --git a/Documentation/technical/commit-graph.txt b/Documentation/technical/commit-graph.txt
index d9c6253b0a..473032e476 100644
--- a/Documentation/technical/commit-graph.txt
+++ b/Documentation/technical/commit-graph.txt
@@ -266,6 +266,42 @@ The merge strategy values (2 for the size multiple, 64,000 for the maximum
number of commits) could be extracted into config settings for full
flexibility.
+## Chains across multiple object directories
+
+In a repo with alternates, we look for the `commit-graph-chain` file starting
+in the local object directory and then in each alternate. The first file that
+exists defines our chain. As we look for the `graph-{hash}` files for
+each `{hash}` in the chain file, we follow the same pattern for the host
+directories.
+
+This allows commit-graphs to be split across multiple forks in a fork network.
+The typical case is a large "base" repo with many smaller forks.
+
+As the base repo advances, it will likely update and merge its commit-graph
+chain more frequently than the forks. If a fork updates their commit-graph after
+the base repo, then it should "reparent" the commit-graph chain onto the new
+chain in the base repo. When reading each `graph-{hash}` file, we track
+the object directory containing it. During a write of a new commit-graph file,
+we check for any changes in the source object directory and read the
+`commit-graph-chain` file for that source and create a new file based on those
+files. During this "reparent" operation, we necessarily need to collapse all
+levels in the fork, as all of the files are invalid against the new base file.
+
+It is crucial to be careful when cleaning up "unreferenced" `graph-{hash}.graph`
+files in this scenario. It falls to the user to define the proper settings for
+their custom environment:
+
+ 1. When merging levels in the base repo, the unreferenced files may still be
+ referenced by chains from fork repos.
+
+ 2. The expiry time should be set to a length of time such that every fork has
+ time to recompute their commit-graph chain to "reparent" onto the new base
+ file(s).
+
+ 3. If the commit-graph chain is updated in the base, the fork will not have
+ access to the new chain until its chain is updated to reference those files.
+ (This may change in the future [5].)
+
Related Links
-------------
[0] https://bugs.chromium.org/p/git/issues/detail?id=8
@@ -292,3 +328,7 @@ Related Links
[4] https://public-inbox.org/git/20180108154822.54829-1-git@xxxxxxxxxxxxxxxxx/T/#u
A patch to remove the ahead-behind calculation from 'status'.
+
+[5] https://public-inbox.org/git/f27db281-abad-5043-6d71-cbb083b1c877@xxxxxxxxx/
+ A discussion of a "two-dimensional graph position" that can allow reading
+ multiple commit-graph chains at the same time.
diff --git a/commit-graph.c b/commit-graph.c
index fb3100921c..fba705bc51 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -320,6 +320,9 @@ static struct commit_graph *load_commit_graph_v1(struct repository *r, const cha
struct commit_graph *g = load_commit_graph_one(graph_name);
free(graph_name);
+ if (g)
+ g->obj_dir = obj_dir;
+
return g;
}
@@ -379,9 +382,10 @@ static struct commit_graph *load_commit_graph_chain(struct repository *r, const
count = st.st_size / (the_hash_algo->hexsz + 1);
oids = xcalloc(count, sizeof(struct object_id));
- for (i = 0; i < count && valid; i++) {
- char *graph_name;
- struct commit_graph *g;
+ prepare_alt_odb(r);
+
+ for (i = 0; i < count; i++) {
+ struct object_directory *odb;
if (strbuf_getline_lf(&line, fp) == EOF)
break;
@@ -393,14 +397,29 @@ static struct commit_graph *load_commit_graph_chain(struct repository *r, const
break;
}
- graph_name = get_split_graph_filename(obj_dir, line.buf);
- g = load_commit_graph_one(graph_name);
- free(graph_name);
+ valid = 0;
+ for (odb = r->objects->odb; odb; odb = odb->next) {
+ char *graph_name = get_split_graph_filename(odb->path, line.buf);
+ struct commit_graph *g = load_commit_graph_one(graph_name);
- if (g && add_graph_to_chain(g, graph_chain, oids, i))
- graph_chain = g;
- else
- valid = 0;
+ free(graph_name);
+
+ if (g) {
+ g->obj_dir = odb->path;
+
+ if (add_graph_to_chain(g, graph_chain, oids, i)) {
+ graph_chain = g;
+ valid = 1;
+ }
+
+ break;
+ }
+ }
+
+ if (!valid) {
+ warning(_("unable to find all commit-graph files"));
+ break;
+ }
}
free(oids);
@@ -1418,7 +1437,7 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
if (ctx->split && ctx->base_graph_name && ctx->num_commit_graphs_after > 1) {
char *new_base_hash = xstrdup(oid_to_hex(&ctx->new_base_graph->oid));
- char *new_base_name = get_split_graph_filename(ctx->obj_dir, new_base_hash);
+ char *new_base_name = get_split_graph_filename(ctx->new_base_graph->obj_dir, new_base_hash);
free(ctx->commit_graph_filenames_after[ctx->num_commit_graphs_after - 2]);
free(ctx->commit_graph_hash_after[ctx->num_commit_graphs_after - 2]);
@@ -1493,6 +1512,9 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
while (g && (g->num_commits <= split_strategy_size_mult * num_commits ||
num_commits > split_strategy_max_commits)) {
+ if (strcmp(g->obj_dir, ctx->obj_dir))
+ break;
+
num_commits += g->num_commits;
g = g->base_graph;
@@ -1501,6 +1523,18 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
ctx->new_base_graph = g;
+ if (ctx->num_commit_graphs_after == 2) {
+ char *old_graph_name = get_commit_graph_filename(g->obj_dir);
+
+ if (!strcmp(g->filename, old_graph_name) &&
+ strcmp(g->obj_dir, ctx->obj_dir)) {
+ ctx->num_commit_graphs_after = 1;
+ ctx->new_base_graph = NULL;
+ }
+
+ free(old_graph_name);
+ }
+
ALLOC_ARRAY(ctx->commit_graph_filenames_after, ctx->num_commit_graphs_after);
ALLOC_ARRAY(ctx->commit_graph_hash_after, ctx->num_commit_graphs_after);
diff --git a/commit-graph.h b/commit-graph.h
index c321834533..802d35254f 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -48,6 +48,7 @@ struct commit_graph {
uint32_t num_commits;
struct object_id oid;
char *filename;
+ const char *obj_dir;
uint32_t num_commits_in_base;
struct commit_graph *base_graph;
diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index 5cb5663a30..46f0832f68 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -90,6 +90,21 @@ test_expect_success 'add more commits, and write a new base graph' '
graph_read_expect 12
'
+test_expect_success 'fork and fail to base a chain on a commit-graph file' '
+ test_when_finished rm -rf fork &&
+ git clone . fork &&
+ (
+ cd fork &&
+ rm .git/objects/info/commit-graph &&
+ echo "$(pwd)/../.git/objects" >.git/objects/info/alternates &&
+ test_commit new-commit &&
+ git commit-graph write --reachable --split &&
+ test_path_is_file $graphdir/commit-graph-chain &&
+ test_line_count = 1 $graphdir/commit-graph-chain &&
+ verify_chain_files_exist $graphdir
+ )
+'
+
test_expect_success 'add three more commits, write a tip graph' '
git reset --hard commits/3 &&
git merge merge/1 &&
@@ -132,4 +147,26 @@ test_expect_success 'add one commit, write a merged graph' '
graph_git_behavior 'merged commit-graph: commit 12 vs 6' commits/12 commits/6
+test_expect_success 'create fork and chain across alternate' '
+ git clone . fork &&
+ (
+ cd fork &&
+ git config core.commitGraph true &&
+ rm -rf $graphdir &&
+ echo "$(pwd)/../.git/objects" >.git/objects/info/alternates &&
+ test_commit 13 &&
+ git branch commits/13 &&
+ git commit-graph write --reachable --split &&
+ test_path_is_file $graphdir/commit-graph-chain &&
+ test_line_count = 3 $graphdir/commit-graph-chain &&
+ ls $graphdir/graph-*.graph >graph-files &&
+ test_line_count = 1 graph-files &&
+ git -c core.commitGraph=true rev-list HEAD >expect &&
+ git -c core.commitGraph=false rev-list HEAD >actual &&
+ test_cmp expect actual
+ )
+'
+
+graph_git_behavior 'alternate: commit 13 vs 6' commits/13 commits/6
+
test_done
--
gitgitgadget
