odb/source: support writing alternates when creating the database

Add the ability to write alternates when creating the object database.
This change allows us to remove the `write_alternates()` callback in a
subsequent patch.

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

odb/source-files.c

@@ -18,6 +18,7 @@
 #include "run-command.h"
 #include "strbuf.h"
 #include "string-list.h"
+#include "strmap.h"
 #include "strvec.h"
 #include "tree.h"
 #include "write-or-die.h"
@@ -51,9 +52,14 @@ static void odb_source_files_close(struct odb_source *source)
 	odb_source_close(&files->packed->base);
 }
 
-static int odb_source_files_create_on_disk(struct odb_source *source)
+static int odb_source_files_create_on_disk(struct odb_source *source,
+					   const struct odb_create_on_disk_options *opts)
 {
+	struct lock_file alternates_lock = LOCK_INIT;
 	struct strbuf path = STRBUF_INIT;
+	struct strset seen = STRSET_INIT;
+	struct strbuf line = STRBUF_INIT;
+	int ret;
 
 	safe_create_dir(source->odb->repo, source->path, 1);
 
@@ -64,8 +70,74 @@ static int odb_source_files_create_on_disk(struct odb_source *source)
 	strbuf_addf(&path, "%s/info", source->path);
 	safe_create_dir(source->odb->repo, path.buf, 1);
 
+	if (opts->alternates && opts->alternates->nr) {
+		FILE *alternates, *orig;
+
+		strbuf_reset(&path);
+		strbuf_addf(&path, "%s/info/alternates", source->path);
+
+		repo_hold_lock_file_for_update(source->odb->repo, &alternates_lock,
+					       path.buf, LOCK_DIE_ON_ERROR);
+
+		alternates = fdopen_lock_file(&alternates_lock, "w");
+		if (!alternates) {
+			ret = error_errno(_("unable to fdopen alternates lockfile"));
+			goto out;
+		}
+
+		/*
+		 * The alternates file may already exist, e.g. when it has been
+		 * seeded from a template directory. Read any preexisting
+		 * entries so that we don't end up writing duplicates.
+		 */
+		orig = fopen(path.buf, "r");
+		if (orig) {
+			while (strbuf_getline(&line, orig) != EOF) {
+				strset_add(&seen, line.buf);
+				fprintf(alternates, "%s\n", line.buf);
+			}
+
+			if (ferror(orig)) {
+				ret = error_errno(_("unable to read alternates file"));
+				fclose(orig);
+				goto out;
+			}
+
+			fclose(orig);
+		} else if (errno != ENOENT) {
+			ret = error_errno(_("unable to read alternates file"));
+			goto out;
+		}
+
+		for (size_t i = 0; i < opts->alternates->nr; i++) {
+			const char *alternate = opts->alternates->v[i];
+			if (!strset_add(&seen, alternate))
+				continue;
+			fprintf(alternates, "%s\n", alternate);
+		}
+
+		if (ferror(alternates)) {
+			ret = error_errno(_("unable to write alternates file"));
+			goto out;
+		}
+
+		if (commit_lock_file(&alternates_lock)) {
+			ret = error_errno(_("unable to commit alternates file"));
+			goto out;
+		}
+	}
+
+	/* Reprepare the object database to activate alternates. */
+	odb_reprepare(source->odb);
+
+	ret = 0;
+
+out:
+	rollback_lock_file(&alternates_lock);
+	strbuf_release(&line);
 	strbuf_release(&path);
-	return 0;
+	strset_clear(&seen);
+	return ret;
 }
 
 static void odb_source_files_prepare(struct odb_source *source,

odb/source.h

@@ -36,6 +36,15 @@ struct object_id;
 struct odb_stream;
 struct strvec;
 
+struct odb_create_on_disk_options {
+	/*
+	 * Alternates that shall be written into the newly created object
+	 * database. Whether or not this option can be handled is specific to
+	 * the backend.
+	 */
+	const struct strvec *alternates;
+};
+
 /*
  * The source is the part of the object database that stores the actual
  * objects. It thus encapsulates the logic to read and write the specific
@@ -106,7 +115,8 @@ struct odb_source {
 	 * This callback may be NULL in case the source does not need any
 	 * on-disk setup.
 	 */
-	int (*create_on_disk)(struct odb_source *source);
+	int (*create_on_disk)(struct odb_source *source,
+			      const struct odb_create_on_disk_options *opts);
 
 	/*
 	 * This callback is expected to prepare the source so that it becomes
@@ -356,11 +366,12 @@ static inline void odb_source_close(struct odb_source *source)
  * Create on-disk data structures that are required for this source to operate
  * correctly. Returns 0 on success, a negative error code otherwise.
  */
-static inline int odb_source_create_on_disk(struct odb_source *source)
+static inline int odb_source_create_on_disk(struct odb_source *source,
+					    const struct odb_create_on_disk_options *opts)
 {
 	if (!source->create_on_disk)
 		return 0;
-	return source->create_on_disk(source);
+	return source->create_on_disk(source, opts);
 }
 
 /*

setup.c

@@ -2649,6 +2649,8 @@ static int create_default_files(struct repository *repo,
 
 void create_object_database(struct repository *repo)
 {
+	struct odb_create_on_disk_options opts = { 0 };
+
 	/*
 	 * Create the "objects" directory in the common directory. This is done
 	 * so that the repository can be discovered regardless of the backend
@@ -2668,7 +2670,7 @@ void create_object_database(struct repository *repo)
 
 	repo->objects = odb_new(repo, ODB_NEW_HONOR_ENV);
 
-	if (odb_source_create_on_disk(repo->objects->sources) < 0)
+	if (odb_source_create_on_disk(repo->objects->sources, &opts) < 0)
 		die(_("failed creating object database"));
 }