From 67f9786c148c2f62772853ec3fee374a72b83399 Mon Sep 17 00:00:00 2001
From: Derrick Stolee <stolee@gmail.com>
Date: Mon, 30 Sep 2024 16:59:07 -0400
Subject: [PATCH] treewide: custom reasons for expanding index

These cases that call ensure_full_index() are likely to be due to a data
shape issue on a user's machine, so take the extra time to format a
message that can be placed in their trace2 output and hopefully identify
the problem that is leading to this slow behavior.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/update-index.c                   |  4 +++-
 merge-ort.c                              |  3 ++-
 read-cache.c                             |  4 +++-
 sparse-index.c                           |  4 +++-
 t/t1092-sparse-checkout-compatibility.sh | 10 +++++++++-
 unpack-trees.c                           |  6 ++++--
 6 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/builtin/update-index.c b/builtin/update-index.c
index c838c8e6a30073..a1e20bfce9ac37 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -711,7 +711,9 @@ static int do_reupdate(const char **paths,
 		 * to process each path individually
 		 */
 		if (S_ISSPARSEDIR(ce->ce_mode)) {
-			ensure_full_index(the_repository->index);
+			const char *fmt = "update-index:modified sparse dir '%s'";
+			ensure_full_index_with_reason(the_repository->index,
+						      fmt, ce->name);
 			goto redo;
 		}
 
diff --git a/merge-ort.c b/merge-ort.c
index 11029c10be3d2b..143bf85475cb55 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -4533,7 +4533,8 @@ static int record_conflicted_index_entries(struct merge_options *opt)
 	 */
 	strmap_for_each_entry(&opt->priv->conflicted, &iter, e) {
 		if (!path_in_sparse_checkout(e->key, index)) {
-			ensure_full_index(index);
+			const char *fmt = "merge-ort: path outside sparse checkout (%s)";
+			ensure_full_index_with_reason(index, fmt, e->key);
 			break;
 		}
 	}
diff --git a/read-cache.c b/read-cache.c
index 85ddd222a314fd..33e9fa6c44a2c2 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -554,7 +554,9 @@ static int index_name_stage_pos(struct index_state *istate,
 		if (S_ISSPARSEDIR(ce->ce_mode) &&
 		    ce_namelen(ce) < namelen &&
 		    !strncmp(name, ce->name, ce_namelen(ce))) {
-			ensure_full_index(istate);
+			const char *fmt = "searching for '%s' and found parent dir '%s'";
+			ensure_full_index_with_reason(istate, fmt,
+						      name, ce->name);
 			return index_name_stage_pos(istate, name, namelen, stage, search_mode);
 		}
 	}
diff --git a/sparse-index.c b/sparse-index.c
index 51e6eefa02ba38..f4ea5cded939aa 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -757,7 +757,9 @@ void expand_to_path(struct index_state *istate,
 			 * in the index, perhaps it exists within this
 			 * sparse-directory.  Expand accordingly.
 			 */
-			ensure_full_index(istate);
+			const char *fmt = "found index entry for '%s'";
+			ensure_full_index_with_reason(istate, fmt,
+						      path_mutable.buf);
 			break;
 		}
 
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index 683158d56c5519..894ece6fe4e8ca 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -2505,7 +2505,15 @@ test_expect_success 'ensure_full_index_with_reason' '
 
 	GIT_TRACE2_EVENT="$(pwd)/ls-files-trace" \
 		git -C sparse-index ls-files --no-sparse HEAD &&
-	test_trace2_data "sparse-index" "expansion-reason" "ls-files" <ls-files-trace
+	test_trace2_data "sparse-index" "expansion-reason" "ls-files" <ls-files-trace &&
+
+	mkdir -p sparse-index/folder2 &&
+	echo >sparse-index/folder2/a &&
+	GIT_TRACE2_EVENT="$(pwd)/status-trace" \
+		git -C sparse-index status &&
+	test_trace2_data "sparse-index" "skip-worktree sparsedir" "folder2/" <status-trace &&
+	test_trace2_data "sparse-index" "expansion-reason" \
+		"failed to clear skip-worktree while sparse" <status-trace
 '
 
 test_done
diff --git a/unpack-trees.c b/unpack-trees.c
index 344b899b8a88a7..b2fff2cb6db5cd 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1890,8 +1890,10 @@ static void update_sparsity_for_prefix(const char *prefix,
 	 *   the 'ensure_full_index(...)' below.
 	 */
 	if (!path_in_cone_mode_sparse_checkout(ce_prefix.buf, istate) &&
-	    index_name_pos(istate, ce_prefix.buf, ce_prefix.len) >= 0)
-		ensure_full_index(istate);
+	    index_name_pos(istate, ce_prefix.buf, ce_prefix.len) >= 0) {
+		const char *fmt = "could not find '%s' in index";
+		ensure_full_index_with_reason(istate, fmt, ce_prefix.buf);
+	}
 
 	strbuf_release(&ce_prefix);
 }