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commit-graph: allow cross-alternate chains
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@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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‎Documentation/technical/commit-graph.txt‎

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Original file line numberDiff line numberDiff line change
@@ -266,6 +266,42 @@ The merge strategy values (2 for the size multiple, 64,000 for the maximum
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number of commits) could be extracted into config settings for full
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flexibility.
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## Chains across multiple object directories
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In a repo with alternates, we look for the `commit-graph-chain` file starting
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in the local object directory and then in each alternate. The first file that
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exists defines our chain. As we look for the `graph-{hash}` files for
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each `{hash}` in the chain file, we follow the same pattern for the host
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directories.
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This allows commit-graphs to be split across multiple forks in a fork network.
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The typical case is a large "base" repo with many smaller forks.
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As the base repo advances, it will likely update and merge its commit-graph
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chain more frequently than the forks. If a fork updates their commit-graph after
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the base repo, then it should "reparent" the commit-graph chain onto the new
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chain in the base repo. When reading each `graph-{hash}` file, we track
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the object directory containing it. During a write of a new commit-graph file,
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we check for any changes in the source object directory and read the
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`commit-graph-chain` file for that source and create a new file based on those
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files. During this "reparent" operation, we necessarily need to collapse all
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levels in the fork, as all of the files are invalid against the new base file.
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It is crucial to be careful when cleaning up "unreferenced" `graph-{hash}.graph`
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files in this scenario. It falls to the user to define the proper settings for
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their custom environment:
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1. When merging levels in the base repo, the unreferenced files may still be
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referenced by chains from fork repos.
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2. The expiry time should be set to a length of time such that every fork has
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time to recompute their commit-graph chain to "reparent" onto the new base
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file(s).
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3. If the commit-graph chain is updated in the base, the fork will not have
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access to the new chain until its chain is updated to reference those files.
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(This may change in the future [5].)
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Related Links
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-------------
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[0] https://bugs.chromium.org/p/git/issues/detail?id=8
@@ -292,3 +328,7 @@ Related Links
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[4] https://public-inbox.org/git/20180108154822.54829-1-git@jeffhostetler.com/T/#u
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A patch to remove the ahead-behind calculation from 'status'.
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[5] https://public-inbox.org/git/f27db281-abad-5043-6d71-cbb083b1c877@gmail.com/
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A discussion of a "two-dimensional graph position" that can allow reading
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multiple commit-graph chains at the same time.

‎commit-graph.c‎

Lines changed: 45 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -320,6 +320,9 @@ static struct commit_graph *load_commit_graph_v1(struct repository *r, const cha
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struct commit_graph *g = load_commit_graph_one(graph_name);
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free(graph_name);
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if (g)
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g->obj_dir = obj_dir;
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return g;
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}
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@@ -379,9 +382,10 @@ static struct commit_graph *load_commit_graph_chain(struct repository *r, const
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count = st.st_size / (the_hash_algo->hexsz + 1);
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oids = xcalloc(count, sizeof(struct object_id));
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for (i = 0; i < count && valid; i++) {
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char *graph_name;
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struct commit_graph *g;
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prepare_alt_odb(r);
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for (i = 0; i < count; i++) {
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struct object_directory *odb;
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if (strbuf_getline_lf(&line, fp) == EOF)
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break;
@@ -393,14 +397,29 @@ static struct commit_graph *load_commit_graph_chain(struct repository *r, const
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break;
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}
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graph_name = get_split_graph_filename(obj_dir, line.buf);
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g = load_commit_graph_one(graph_name);
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free(graph_name);
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valid = 0;
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for (odb = r->objects->odb; odb; odb = odb->next) {
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char *graph_name = get_split_graph_filename(odb->path, line.buf);
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struct commit_graph *g = load_commit_graph_one(graph_name);
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if (g && add_graph_to_chain(g, graph_chain, oids, i))
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graph_chain = g;
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else
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valid = 0;
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free(graph_name);
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if (g) {
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g->obj_dir = odb->path;
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if (add_graph_to_chain(g, graph_chain, oids, i)) {
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graph_chain = g;
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valid = 1;
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}
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break;
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}
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}
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if (!valid) {
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warning(_("unable to find all commit-graph files"));
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break;
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}
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}
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free(oids);
@@ -1418,7 +1437,7 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
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if (ctx->split && ctx->base_graph_name && ctx->num_commit_graphs_after > 1) {
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char *new_base_hash = xstrdup(oid_to_hex(&ctx->new_base_graph->oid));
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char *new_base_name = get_split_graph_filename(ctx->obj_dir, new_base_hash);
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char *new_base_name = get_split_graph_filename(ctx->new_base_graph->obj_dir, new_base_hash);
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free(ctx->commit_graph_filenames_after[ctx->num_commit_graphs_after - 2]);
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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)
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while (g && (g->num_commits <= split_strategy_size_mult * num_commits ||
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num_commits > split_strategy_max_commits)) {
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if (strcmp(g->obj_dir, ctx->obj_dir))
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break;
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num_commits += g->num_commits;
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g = g->base_graph;
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@@ -1501,6 +1523,18 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
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ctx->new_base_graph = g;
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if (ctx->num_commit_graphs_after == 2) {
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char *old_graph_name = get_commit_graph_filename(g->obj_dir);
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if (!strcmp(g->filename, old_graph_name) &&
1530+
strcmp(g->obj_dir, ctx->obj_dir)) {
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ctx->num_commit_graphs_after = 1;
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ctx->new_base_graph = NULL;
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}
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free(old_graph_name);
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}
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ALLOC_ARRAY(ctx->commit_graph_filenames_after, ctx->num_commit_graphs_after);
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ALLOC_ARRAY(ctx->commit_graph_hash_after, ctx->num_commit_graphs_after);
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‎commit-graph.h‎

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@@ -48,6 +48,7 @@ struct commit_graph {
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uint32_t num_commits;
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struct object_id oid;
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char *filename;
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const char *obj_dir;
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uint32_t num_commits_in_base;
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struct commit_graph *base_graph;

‎t/t5324-split-commit-graph.sh‎

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@@ -90,6 +90,21 @@ test_expect_success 'add more commits, and write a new base graph' '
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graph_read_expect 12
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'
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test_expect_success 'fork and fail to base a chain on a commit-graph file' '
94+
test_when_finished rm -rf fork &&
95+
git clone . fork &&
96+
(
97+
cd fork &&
98+
rm .git/objects/info/commit-graph &&
99+
echo "$(pwd)/../.git/objects" >.git/objects/info/alternates &&
100+
test_commit new-commit &&
101+
git commit-graph write --reachable --split &&
102+
test_path_is_file $graphdir/commit-graph-chain &&
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test_line_count = 1 $graphdir/commit-graph-chain &&
104+
verify_chain_files_exist $graphdir
105+
)
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'
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93108
test_expect_success 'add three more commits, write a tip graph' '
94109
git reset --hard commits/3 &&
95110
git merge merge/1 &&
@@ -132,4 +147,26 @@ test_expect_success 'add one commit, write a merged graph' '
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133148
graph_git_behavior 'merged commit-graph: commit 12 vs 6' commits/12 commits/6
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150+
test_expect_success 'create fork and chain across alternate' '
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git clone . fork &&
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(
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cd fork &&
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git config core.commitGraph true &&
155+
rm -rf $graphdir &&
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echo "$(pwd)/../.git/objects" >.git/objects/info/alternates &&
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test_commit 13 &&
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git branch commits/13 &&
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git commit-graph write --reachable --split &&
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test_path_is_file $graphdir/commit-graph-chain &&
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test_line_count = 3 $graphdir/commit-graph-chain &&
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ls $graphdir/graph-*.graph >graph-files &&
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test_line_count = 1 graph-files &&
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git -c core.commitGraph=true rev-list HEAD >expect &&
165+
git -c core.commitGraph=false rev-list HEAD >actual &&
166+
test_cmp expect actual
167+
)
168+
'
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graph_git_behavior 'alternate: commit 13 vs 6' commits/13 commits/6
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135172
test_done

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