[PATCH v2 1/5] sparse-checkout: refactor skip worktree retry logic

From: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Date: Jun 26, 2024, 14:29 UTC
Message-ID: <93d0baed0b0f435e5656cef04cf103b5e2e0f41a.1719412192.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.1754.v2.git.1719412192.gitgitgadget@gmail.com>

From: Derrick Stolee <stolee@gmail.com>

The clear_skip_worktree_from_present_files() method was introduced in af6a51875a (repo_read_index: clear SKIP_WORKTREE bit from files present in worktree, 2022-01-14) to help cases where sparse-checkout is enabled but some paths outside of the sparse-checkout also exist on disk. This operation can be slow as it needs to check path existence in a way not stored in the index, so caching was introduced in d79d299352 (Accelerate clear_skip_worktree_from_present_files() by caching, 2022-01-14).

If users are having trouble with the performance of this operation and don't care about paths outside of the sparse-checkout, they can disable them using the sparse.expectFilesOutsideOfPatterns config option introduced in ecc7c8841d (repo_read_index: add config to expect files outside sparse patterns, 2022-02-25).

This check is particularly confusing in the presence of a sparse index, as a sparse tree entry corresponding to an existing directory must first be expanded to a full index before examining the paths within. This is currently implemented using a 'goto' and a boolean variable to ensure we restart only once.

Even with that caching, it was noticed that this could take a long time to execute. 89aaab11a3 (index: add trace2 region for clear skip worktree, 2022-11-03) introduced trace2 regions to measure this time. Further, the way the loop repeats itself was slightly confusing and prone to breakage, so a BUG() statement was added in 8c7abdc596 (index: raise a bug if the index is materialised more than once, 2022-11-03) to be sure that the second run of the loop does not hit any sparse trees.

One thing that can be confusing about the current setup is that the trace2 regions nest and it is not clear that a second loop is running after a sparse index is expanded. Here is an example of what the regions look like in a typical case:

| region_enter | ... | label:clear_skip_worktree_from_present_files
| region_enter | ... | ..label:update
| region_leave | ... | ..label:update
| region_enter | ... | ..label:ensure_full_index
| region_enter | ... | ....label:update
| region_leave | ... | ....label:update
| region_leave | ... | ..label:ensure_full_index
| data         | ... | ..sparse_path_count:1
| data         | ... | ..sparse_path_count_full:269538
| region_leave | ... | label:clear_skip_worktree_from_present_files

One thing that is particularly difficult to understand about these regions is that most of the time is spent between the close of the ensure_full_index region and the reporting of the end data. This is because of the restart of the loop being within the same region as the first iteration of the loop.

This change refactors the method into two separate methods that are traced separately. This will be more important later when we change other features of the methods, but for now the only functional change is the difference in the structure of the trace regions.

After this change, the same telemetry section is split into three distinct chunks:

| region_enter | ... | label:clear_skip_worktree_from_present_files_sparse
| data         | ... | ..sparse_path_count:1
| region_leave | ... | label:clear_skip_worktree_from_present_files_sparse
| region_enter | ... | label:update
| region_leave | ... | label:update
| region_enter | ... | label:ensure_full_index
| region_enter | ... | ..label:update
| region_leave | ... | ..label:update
| region_leave | ... | label:ensure_full_index
| region_enter | ... | label:clear_skip_worktree_from_present_files_full
| data         | ... | ..full_path_count:269538
| region_leave | ... | label:clear_skip_worktree_from_present_files_full

Here, we see the sparse loop terminating early with its first sparse path being a sparse directory containing a file. Then, that loop's region terminates before ensure_full_index begins (in this case, the cache-tree must also be computed). Then, _after_ the index is expanded, the full loop begins with its own region.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
---
 sparse-index.c | 77 ++++++++++++++++++++++++++++++++++----------------
 1 file changed, 53 insertions(+), 24 deletions(-)

diff --git a/sparse-index.c b/sparse-index.c
index e48e40cae71..e0457c87fff 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -486,49 +486,78 @@ static int path_found(const char *path, const char **dirname, size_t *dir_len,
 	return 0;
 }
 
-void clear_skip_worktree_from_present_files(struct index_state *istate)
+static int clear_skip_worktree_from_present_files_sparse(struct index_state *istate)
 {
 	const char *last_dirname = NULL;
 	size_t dir_len = 0;
 	int dir_found = 1;
 
-	int i;
-	int path_count[2] = {0, 0};
-	int restarted = 0;
+	int path_count = 0;
+	int to_restart = 0;
 
-	if (!core_apply_sparse_checkout ||
-	    sparse_expect_files_outside_of_patterns)
-		return;
-
-	trace2_region_enter("index", "clear_skip_worktree_from_present_files",
+	trace2_region_enter("index", "clear_skip_worktree_from_present_files_sparse",
 			    istate->repo);
-restart:
-	for (i = 0; i < istate->cache_nr; i++) {
+	for (int i = 0; i < istate->cache_nr; i++) {
 		struct cache_entry *ce = istate->cache[i];
 
 		if (ce_skip_worktree(ce)) {
-			path_count[restarted]++;
+			path_count++;
 			if (path_found(ce->name, &last_dirname, &dir_len, &dir_found)) {
 				if (S_ISSPARSEDIR(ce->ce_mode)) {
-					if (restarted)
-						BUG("ensure-full-index did not fully flatten?");
-					ensure_full_index(istate);
-					restarted = 1;
-					goto restart;
+					to_restart = 1;
+					break;
 				}
 				ce->ce_flags &= ~CE_SKIP_WORKTREE;
 			}
 		}
 	}
 
-	if (path_count[0])
-		trace2_data_intmax("index", istate->repo,
-				   "sparse_path_count", path_count[0]);
-	if (restarted)
-		trace2_data_intmax("index", istate->repo,
-				   "sparse_path_count_full", path_count[1]);
-	trace2_region_leave("index", "clear_skip_worktree_from_present_files",
+	trace2_data_intmax("index", istate->repo,
+			   "sparse_path_count", path_count);
+	trace2_region_leave("index", "clear_skip_worktree_from_present_files_sparse",
+			    istate->repo);
+	return to_restart;
+}
+
+static void clear_skip_worktree_from_present_files_full(struct index_state *istate)
+{
+	const char *last_dirname = NULL;
+	size_t dir_len = 0;
+	int dir_found = 1;
+
+	int path_count = 0;
+
+	trace2_region_enter("index", "clear_skip_worktree_from_present_files_full",
 			    istate->repo);
+	for (int i = 0; i < istate->cache_nr; i++) {
+		struct cache_entry *ce = istate->cache[i];
+
+		if (S_ISSPARSEDIR(ce->ce_mode))
+			BUG("ensure-full-index did not fully flatten?");
+
+		if (ce_skip_worktree(ce)) {
+			path_count++;
+			if (path_found(ce->name, &last_dirname, &dir_len, &dir_found))
+				ce->ce_flags &= ~CE_SKIP_WORKTREE;
+		}
+	}
+
+	trace2_data_intmax("index", istate->repo,
+			   "full_path_count", path_count);
+	trace2_region_leave("index", "clear_skip_worktree_from_present_files_full",
+			    istate->repo);
+}
+
+void clear_skip_worktree_from_present_files(struct index_state *istate)
+{
+	if (!core_apply_sparse_checkout ||
+	    sparse_expect_files_outside_of_patterns)
+		return;
+
+	if (clear_skip_worktree_from_present_files_sparse(istate)) {
+		ensure_full_index(istate);
+		clear_skip_worktree_from_present_files_full(istate);
+	}
 }
 
 /*

-- 
gitgitgadget