[PATCH v3 03/26] read-cache: expand on query into sparse-directory entry

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <dstolee@microsoft.com>

Callers to index_name_pos() or index_name_stage_pos() have a specific
path in mind. If that happens to be a path with an ancestor being a
sparse-directory entry, it can lead to unexpected results.

In the case that we did not find the requested path, check to see if the
position _before_ the inserted position is a sparse directory entry that
matches the initial segment of the input path (including the directory
separator at the end of the directory name). If so, then expand the
index to be a full index and search again. This expansion will only
happen once per index read.

Future enhancements could be more careful to expand only the necessary
sparse directory entry, but then we would have a special "not fully
sparse, but also not fully expanded" mode that could affect writing the
index to file. Since this only occurs if a specific file is requested
outside of the sparse checkout definition, this is unlikely to be a
common situation.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 read-cache.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/read-cache.c b/read-cache.c
index 3ad94578095e..3698bc7bf77d 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -567,6 +567,27 @@ static int index_name_stage_pos(struct index_state *istate, const char *name, in
 		}
 		first = next+1;
 	}
+
+	if (istate->sparse_index &&
+	    first > 0) {
+		/* Note: first <= istate->cache_nr */
+		struct cache_entry *ce = istate->cache[first - 1];
+
+		/*
+		 * If we are in a sparse-index _and_ the entry before the
+		 * insertion position is a sparse-directory entry that is
+		 * an ancestor of 'name', then we need to expand the index
+		 * and search again. This will only trigger once, because
+		 * thereafter the index is fully expanded.
+		 */
+		if (S_ISSPARSEDIR(ce->ce_mode) &&
+		    ce_namelen(ce) < namelen &&
+		    !strncmp(name, ce->name, ce_namelen(ce))) {
+			ensure_full_index(istate);
+			return index_name_stage_pos(istate, name, namelen, stage);
+		}
+	}
+
 	return -first-1;
 }
 
-- 
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