odb/source: make read_alternates() function pluggable

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

odb.c

@@ -131,10 +131,10 @@ static bool odb_is_source_usable(struct object_database *o, const char *path)
 	return usable;
 }
 
-static void parse_alternates(const char *string,
-			     int sep,
-			     const char *relative_base,
-			     struct strvec *out)
+void parse_alternates(const char *string,
+		      int sep,
+		      const char *relative_base,
+		      struct strvec *out)
 {
 	struct strbuf pathbuf = STRBUF_INIT;
 	struct strbuf buf = STRBUF_INIT;
@@ -198,24 +198,6 @@ static void parse_alternates(const char *string,
 	strbuf_release(&buf);
 }
 
-static void odb_source_read_alternates(struct odb_source *source,
-				       struct strvec *out)
-{
-	struct strbuf buf = STRBUF_INIT;
-	char *path;
-
-	path = xstrfmt("%s/info/alternates", source->path);
-	if (strbuf_read_file(&buf, path, 1024) < 0) {
-		warn_on_fopen_errors(path);
-		free(path);
-		return;
-	}
-	parse_alternates(buf.buf, '\n', source->path, out);
-
-	strbuf_release(&buf);
-	free(path);
-}
-
 static struct odb_source *odb_add_alternate_recursively(struct object_database *odb,
 							const char *source,
 							int depth)

odb.h

@@ -500,4 +500,9 @@ int odb_write_object_stream(struct object_database *odb,
 			    struct odb_write_stream *stream, size_t len,
 			    struct object_id *oid);
 
+void parse_alternates(const char *string,
+		      int sep,
+		      const char *relative_base,
+		      struct strvec *out);
+
 #endif /* ODB_H */

odb/source-files.c

@@ -2,9 +2,11 @@
 #include "abspath.h"
 #include "chdir-notify.h"
 #include "object-file.h"
+#include "odb.h"
 #include "odb/source.h"
 #include "odb/source-files.h"
 #include "packfile.h"
+#include "strbuf.h"
 
 static void odb_source_files_reparent(const char *name UNUSED,
 				      const char *old_cwd,
@@ -117,6 +119,25 @@ static int odb_source_files_write_object_stream(struct odb_source *source,
 	return odb_source_loose_write_stream(source, stream, len, oid);
 }
 
+static int odb_source_files_read_alternates(struct odb_source *source,
+					    struct strvec *out)
+{
+	struct strbuf buf = STRBUF_INIT;
+	char *path;
+
+	path = xstrfmt("%s/info/alternates", source->path);
+	if (strbuf_read_file(&buf, path, 1024) < 0) {
+		warn_on_fopen_errors(path);
+		free(path);
+		return 0;
+	}
+	parse_alternates(buf.buf, '\n', source->path, out);
+
+	strbuf_release(&buf);
+	free(path);
+	return 0;
+}
+
 struct odb_source_files *odb_source_files_new(struct object_database *odb,
 					      const char *path,
 					      bool local)
@@ -137,6 +158,7 @@ struct odb_source_files *odb_source_files_new(struct object_database *odb,
 	files->base.freshen_object = odb_source_files_freshen_object;
 	files->base.write_object = odb_source_files_write_object;
 	files->base.write_object_stream = odb_source_files_write_object_stream;
+	files->base.read_alternates = odb_source_files_read_alternates;
 
 	/*
 	 * Ideally, we would only ever store absolute paths in the source. This

odb/source.h

@@ -54,6 +54,7 @@ struct object_id;
 struct object_info;
 struct odb_read_stream;
 struct odb_write_stream;
+struct strvec;
 
 /*
  * A callback function that can be used to iterate through objects. If given,
@@ -231,6 +232,19 @@ struct odb_source {
 	int (*write_object_stream)(struct odb_source *source,
 				   struct odb_write_stream *stream, size_t len,
 				   struct object_id *oid);
+
+	/*
+	 * This callback is expected to read the list of alternate object
+	 * database sources connected to it and write them into the `strvec`.
+	 *
+	 * The result is expected to be paths to the alternates. All paths must
+	 * be resolved to absolute paths.
+	 *
+	 * The callback is expected to return 0 on success, a negative error
+	 * code otherwise.
+	 */
+	int (*read_alternates)(struct odb_source *source,
+			       struct strvec *out);
 };
 
 /*
@@ -384,4 +398,18 @@ static inline int odb_source_write_object_stream(struct odb_source *source,
 	return source->write_object_stream(source, stream, len, oid);
 }
 
+/*
+ * Read the list of alternative object database sources from the given backend
+ * and populate the `strvec` with them. The listing is not recursive -- that
+ * is, if any of the yielded alternate sources has alternates itself, those
+ * will not be yielded as part of this function call.
+ *
+ * Return 0 on success, a negative error code otherwise.
+ */
+static inline int odb_source_read_alternates(struct odb_source *source,
+					     struct strvec *out)
+{
+	return source->read_alternates(source, out);
+}
+
 #endif