[PATCH v3 10/14] commit-graph: allow cross-alternate chains
- From
Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
- Date
- Jun 3, 2019, 16:03 UTC
- Message-ID
- <5cfd653d24edbb5320ecde53271fa957a773a729.1559577826.git.gitgitgadget@gmail.com>
- In-Reply-To
- <pull.184.v3.git.gitgitgadget@gmail.com>
From: Derrick Stolee <dstolee@microsoft.com>
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/graoh-{hash}.graph".Signed-off-by: Derrick Stolee <dstolee@microsoft.com> --- Documentation/technical/commit-graph.txt | 40 +++++++++++++++++++++ commit-graph.c | 46 ++++++++++++++++++------ commit-graph.h | 1 + t/t5323-split-commit-graph.sh | 37 +++++++++++++++++++ 4 files changed, 114 insertions(+), 10 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@jeffhostetler.com/T/#u A patch to remove the ahead-behind calculation from 'status'. + +[5] https://public-inbox.org/git/f27db281-abad-5043-6d71-cbb083b1c877@gmail.com/ + 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 bfef1b6960..0b7c186a5d 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; } @@ -385,8 +388,7 @@ static struct commit_graph *load_commit_graph_chain(struct repository *r, const oids = xcalloc(st.st_size / (the_hash_algo->hexsz + 1), sizeof(struct object_id)); while (strbuf_getline_lf(&line, fp) != EOF && valid) { - char *graph_name; - struct commit_graph *g; + struct object_directory *odb; if (get_oid_hex(line.buf, &oids[i])) { warning(_("invalid commit-graph chain: line '%s' not a hash"), @@ -395,14 +397,23 @@ 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); + 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; + else + valid = 0; + + break; + } + } } free(oids); @@ -1411,7 +1422,7 @@ 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]); @@ -1482,6 +1493,9 @@ 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; @@ -1490,6 +1504,18 @@ 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 5c48c4f66a..10466bc064 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/t5323-split-commit-graph.sh b/t/t5323-split-commit-graph.sh index 5cb5663a30..fad27dc6e3 100755 --- a/t/t5323-split-commit-graph.sh +++ b/t/t5323-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 "$TRASH_DIRECTORY/.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 $graphdir && + echo "$TRASH_DIRECTORY/.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
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