[PATCH 6/8] odb/source: support writing alternates when creating the database

From

Patrick Steinhardt <ps@pks.im>

Date

Aug 25, 2026, 14:11 UTC

Message-ID

<20260825-pks-odb-write-alternates-at-creation-time-v1-6-911513ba95c3@pks.im>

In-Reply-To

<20260825-pks-odb-write-alternates-at-creation-time-v1-0-911513ba95c3@pks.im>

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 odb/source-files.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
 odb/source.h       | 18 +++++++++++---
 setup.c            |  4 ++-
 3 files changed, 89 insertions(+), 6 deletions(-)

diff --git a/odb/source-files.c b/odb/source-files.c
index b7b3a297bb..5e77b21d9f 100644
--- a/odb/source-files.c
+++ b/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 strbuf path = STRBUF_INIT;
+	struct strset seen = STRSET_INIT;
+	struct strbuf line = STRBUF_INIT;
+	FILE *f = NULL;
+	int ret;
 
 	safe_create_dir(source->odb->repo, source->path, 1);
 
@@ -64,8 +70,71 @@ 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) {
+		strbuf_reset(&path);
+		strbuf_addf(&path, "%s/info/alternates", source->path);
+
+		/*
+		 * 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.
+		 */
+		f = fopen(path.buf, "r");
+		if (f) {
+			while (strbuf_getline(&line, f) != EOF)
+				strset_add(&seen, line.buf);
+
+			if (ferror(f)) {
+				ret = error_errno(_("unable to read alternates file"));
+				goto out;
+			}
+
+			fclose(f);
+		} else if (errno != ENOENT) {
+			ret = error_errno(_("unable to read alternates file"));
+			goto out;
+		}
+
+		f = fopen(path.buf, "a");
+		if (!f) {
+			ret = error_errno(_("unable to open alternates file for writing"));
+			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(f, "%s\n", alternate);
+		}
+
+		if (ferror(f)) {
+			ret = error_errno(_("unable to write alternates file"));
+			goto out;
+		}
+
+		ret = fclose(f);
+		f = NULL;
+		if (ret) {
+			ret = error_errno(_("unable to write alternates file"));
+			goto out;
+		}
+	}
+
+	/* Reprepare the object database to activate alternates. */
+	odb_reprepare(source->odb);
+
+	ret = 0;
+
+out:
+	if (f)
+		fclose(f);
+	strbuf_release(&line);
 	strbuf_release(&path);
-	return 0;
+	strset_clear(&seen);
+	return ret;
 }
 
 static void odb_source_files_prepare(struct odb_source *source,
diff --git a/odb/source.h b/odb/source.h
index ea8675247e..86b82fab3b 100644
--- a/odb/source.h
+++ b/odb/source.h
@@ -5,6 +5,7 @@
 #include "object.h"
 #include "odb.h"
 #include "odb/transaction.h"
+#include "strvec.h"
 
 enum odb_source_type {
 	/*
@@ -26,6 +27,15 @@ enum odb_source_type {
 	ODB_SOURCE_INMEMORY,
 };
 
+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;
+};
+
 /*
  * Convert between the enum and its name. Returns the equivalent of "unknown"
  * for unknown types.
@@ -106,7 +116,8 @@ 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 +367,12 @@ 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);
 }
 
 /*
diff --git a/setup.c b/setup.c
index e654e27d05..426cc7dff8 100644
--- a/setup.c
+++ b/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"));
 }
 

-- 
2.55.0.822.g20453c30eb.dirty