Fix size_mult option in fetch.writeCommitGraph

From: Derrick Stolee <[email protected]>

In 50f26bd ("fetch: add fetch.writeCommitGraph config setting", 2019-09-02), the fetch builtin added the capability to write a commit-graph using the "--split" feature. This feature creates multiple commit-graph files, and those can merge based on a set of "split options" including a size multiple. The default size multiple is 2, which intends to provide a log_2 N depth of the commit-graph chain where N is the number of commits.

However, I noticed during dogfooding that my commit-graph chains were becoming quite large when left only to builds by 'git fetch'. It turns out that in split_graph_merge_strategy(), we default the size_mult variable to 2 except we override it with the context's split_opts if they exist. In builtin/fetch.c, we create such a split_opts, but do not populate it with values.

This problem is due to two failures:

1. It is unclear that we can add the flag COMMIT_GRAPH_WRITE_SPLIT with a NULL split_opts.
2. If we have a non-NULL split_opts, then we override the default values even if a zero value is given.

Correct both of these issues. First, do not override size_mult when the options provide a zero value. Second, stop creating a split_opts in the fetch builtin.

This problem is due to two failures:

1. It is unclear that we can add the flag COMMIT_GRAPH_WRITE_SPLIT with a NULL split_opts.
2. If we have a non-NULL split_opts, then we override the default values even if a zero value is given.

Correct both of these issues. First, do not override size_mult when the options provide a zero value. Second, stop creating a split_opts in the fetch builtin.

Signed-off-by Derrick Stolee <[email protected]>

---
 builtin/fetch.c | 4 +---
 commit-graph.c  | 4 +++-
 2 files changed, 4 insertions(+), 4 deletions(-)

builtin/fetch.c +1 -3

index f8765b385b..b4c6d921d0 100644
@@ -1866,15 +1866,13 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 	    (fetch_write_commit_graph < 0 &&
 	     the_repository->settings.fetch_write_commit_graph)) {
 		int commit_graph_flags = COMMIT_GRAPH_WRITE_SPLIT;
-		struct split_commit_graph_opts split_opts;
-		memset(&split_opts, 0, sizeof(struct split_commit_graph_opts));
 
 		if (progress)
 			commit_graph_flags |= COMMIT_GRAPH_WRITE_PROGRESS;
 
 		write_commit_graph_reachable(get_object_directory(),
 					     commit_graph_flags,
-					     &split_opts);
+					     NULL);
 	}
 
 	close_object_store(the_repository->objects);

commit-graph.c +3 -1

index e771394aff..b205e65ed1 100644
@@ -1542,7 +1542,9 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
 
 	if (ctx->split_opts) {
 		max_commits = ctx->split_opts->max_commits;
-		size_mult = ctx->split_opts->size_multiple;
+
+		if (ctx->split_opts->size_multiple)
+			size_mult = ctx->split_opts->size_multiple;
 	}
 
 	g = ctx->r->objects->commit_graph;

Quoted 7 lines · Derrick Stolee via GitGitGadget

This problem is due to two failures:

1. It is unclear that we can add the flag COMMIT_GRAPH_WRITE_SPLIT with a NULL split_opts.
2. If we have a non-NULL split_opts, then we override the default values even if a zero value is given.

Quoted 3 lines

Correct both of these issues. First, do not override size_mult when the options provide a zero value. Second, stop creating a split_opts in the fetch builtin.

OK, so there is the hardcoded default 2 in the code, and split_opts structure *can* override it, but 0 in the field of the structure is meant to signal "no, I do not have any value to override the default", not "I do want to set the multiple to 0"?

From: Derrick Stolee <[email protected]>

In 50f26bd ("fetch: add fetch.writeCommitGraph config setting", 2019-09-02), the fetch builtin added the capability to write a commit-graph using the "--split" feature. This feature creates multiple commit-graph files, and those can merge based on a set of "split options" including a size multiple. The default size multiple is 2, which intends to provide a log_2 N depth of the commit-graph chain where N is the number of commits.

However, I noticed during dogfooding that my commit-graph chains were becoming quite large when left only to builds by 'git fetch'. It turns out that in split_graph_merge_strategy(), we default the size_mult variable to 2 except we override it with the context's split_opts if they exist. In builtin/fetch.c, we create such a split_opts, but do not populate it with values.

This problem is due to two failures:

1. It is unclear that we can add the flag COMMIT_GRAPH_WRITE_SPLIT with a NULL split_opts.
2. If we have a non-NULL split_opts, then we override the default values even if a zero value is given.

Correct both of these issues. First, do not override size_mult when the options provide a zero value. Second, stop creating a split_opts in the fetch builtin.

---
 builtin/fetch.c | 4 +---
 commit-graph.c  | 4 +++-