[PATCH v3 1/3] attr.c: read attributes in a sparse directory

[Shuqi Liang <cheskaqiqi@gmail.com>]

'git check-attr' cannot currently find attributes of a file within a
sparse directory. This is due to .gitattributes files are irrelevant in
sparse-checkout cone mode, as the file is considered sparse only if all
paths within its parent directory are also sparse. In addition,
searching for a .gitattributes file causes expansion of the sparse
index, which is avoided to prevent potential performance degradation.

However, this behavior can lead to missing attributes for files inside
sparse directories, causing inconsistencies in file handling.

To resolve this, revise 'git check-attr' to allow attribute reading for
files in sparse directories from the corresponding .gitattributes files:

1.Utilize path_in_cone_mode_sparse_checkout() and index_name_pos_sparse
to check if a path falls within a sparse directory.

2.If path is inside a sparse directory, employ the value of
index_name_pos_sparse() to find the sparse directory containing path and
path relative to sparse directory. Proceed to read attributes from the
tree OID of the sparse directory using read_attr_from_blob().

3.If path is not inside a sparse directory，ensure that attributes are
fetched from the index blob with read_blob_data_from_index().

Helped-by: Victoria Dye <vdye@github.com>
Signed-off-by: Shuqi Liang <cheskaqiqi@gmail.com>
---
 attr.c | 47 ++++++++++++++++++++++++++++-------------------
 1 file changed, 28 insertions(+), 19 deletions(-)

diff --git a/attr.c b/attr.c
index 7d39ac4a29..be06747b0d 100644
--- a/attr.c
+++ b/attr.c
@@ -808,35 +808,44 @@ static struct attr_stack *read_attr_from_blob(struct index_state *istate,
 static struct attr_stack *read_attr_from_index(struct index_state *istate,
 					       const char *path, unsigned flags)
 {
+	struct attr_stack *stack = NULL;
 	char *buf;
 	unsigned long size;
+	int pos;
 
 	if (!istate)
 		return NULL;
 
 	/*
-	 * The .gitattributes file only applies to files within its
-	 * parent directory. In the case of cone-mode sparse-checkout,
-	 * the .gitattributes file is sparse if and only if all paths
-	 * within that directory are also sparse. Thus, don't load the
-	 * .gitattributes file since it will not matter.
-	 *
-	 * In the case of a sparse index, it is critical that we don't go
-	 * looking for a .gitattributes file, as doing so would cause the
-	 * index to expand.
+	 * If the pos value is negative, it means the path is not in the index. 
+	 * However, the absolute value of pos minus 1 gives us the position where the path 
+	 * would be inserted in lexicographic order. By subtracting another 1 from this 
+	 * value (pos = -pos - 2), we find the position of the last index entry 
+	 * which is lexicographically smaller than the provided path. This would be 
+	 * the sparse directory containing the path.
 	 */
-	if (!path_in_cone_mode_sparse_checkout(path, istate))
-		return NULL;
+	pos = index_name_pos_sparse(istate, path, strlen(path));
+	pos = - pos - 2;
 
-	buf = read_blob_data_from_index(istate, path, &size);
-	if (!buf)
-		return NULL;
-	if (size >= ATTR_MAX_FILE_SIZE) {
-		warning(_("ignoring overly large gitattributes blob '%s'"), path);
-		return NULL;
-	}
+	if (!path_in_cone_mode_sparse_checkout(path, istate) && 0 <= pos) {
+		if (!S_ISSPARSEDIR(istate->cache[pos]->ce_mode))
+			return NULL;
 
-	return read_attr_from_buf(buf, path, flags);
+		if (strncmp(istate->cache[pos]->name, path, ce_namelen(istate->cache[pos])) == 0) {
+			const char *relative_path = path + ce_namelen(istate->cache[pos]);  
+			stack = read_attr_from_blob(istate, &istate->cache[pos]->oid, relative_path, flags);
+		}
+	} else {
+		buf = read_blob_data_from_index(istate, path, &size);
+		if (!buf)
+			return NULL;
+		if (size >= ATTR_MAX_FILE_SIZE) {
+			warning(_("ignoring overly large gitattributes blob '%s'"), path);
+			return NULL;
+		}
+		stack = read_attr_from_buf(buf, path, flags);
+	}
+	return stack;
 }
 
 static struct attr_stack *read_attr(struct index_state *istate,
-- 
2.39.0