odb/transaction: propagate begin errors

When `odb_transaction_begin()` is invoked, the function returns the
transaction pointer directly. There is no way for the backend to
signal that it failed to set up its state, such as when creating the
temporary object directory backing the transaction.

In a subsequent commit, git-receive-pack(1) starts using ODB
transactions and needs to be able to report such failures rather
than silently ignore them. Refactor `odb_transaction_begin()` to
return an int error code and write the resulting transaction into an
out parameter. Also introduce `odb_transaction_begin_or_die()` as a
convenience for callsites that do not need to handle errors
explicitly.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/add.c

@@ -581,7 +581,7 @@ int cmd_add(int argc,
 		string_list_clear(&only_match_skip_worktree, 0);
 	}
 
-	transaction = odb_transaction_begin(repo->objects);
+	odb_transaction_begin_or_die(repo->objects, &transaction);
 
 	ps_matched = xcalloc(pathspec.nr, 1);
 	if (add_renormalize)

builtin/unpack-objects.c

@@ -596,7 +596,7 @@ static void unpack_all(void)
 		progress = start_progress(the_repository,
 					  _("Unpacking objects"), nr_objects);
 	CALLOC_ARRAY(obj_list, nr_objects);
-	transaction = odb_transaction_begin(the_repository->objects);
+	odb_transaction_begin_or_die(the_repository->objects, &transaction);
 	for (i = 0; i < nr_objects; i++) {
 		unpack_one(i);
 		display_progress(progress, i + 1);

builtin/update-index.c

@@ -1124,7 +1124,7 @@ int cmd_update_index(int argc,
 	 * Allow the object layer to optimize adding multiple objects in
 	 * a batch.
 	 */
-	transaction = odb_transaction_begin(the_repository->objects);
+	odb_transaction_begin_or_die(the_repository->objects, &transaction);
 	while (ctx.argc) {
 		if (parseopt_state != PARSE_OPT_DONE)
 			parseopt_state = parse_options_step(&ctx, options,

cache-tree.c

@@ -490,7 +490,7 @@ int cache_tree_update(struct index_state *istate, int flags)
 
 	trace_performance_enter();
 	trace2_region_enter("cache_tree", "update", istate->repo);
-	transaction = odb_transaction_begin(the_repository->objects);
+	odb_transaction_begin_or_die(the_repository->objects, &transaction);
 	i = update_one(istate->cache_tree, istate->cache, istate->cache_nr,
 		       "", 0, &skip, flags);
 	odb_transaction_commit(transaction);

object-file.c

@@ -1387,8 +1387,9 @@ int index_fd(struct index_state *istate, struct object_id *oid,
 
 		if (flags & INDEX_WRITE_OBJECT) {
 			struct object_database *odb = the_repository->objects;
-			struct odb_transaction *transaction = odb_transaction_begin(odb);
+			struct odb_transaction *transaction;
 
+			odb_transaction_begin_or_die(odb, &transaction);
 			ret = odb_transaction_write_object_stream(odb->transaction,
 								  &stream,
 								  xsize_t(st->st_size),

odb/transaction.c

@@ -2,14 +2,20 @@
 #include "odb/source.h"
 #include "odb/transaction.h"
 
-struct odb_transaction *odb_transaction_begin(struct object_database *odb)
+int odb_transaction_begin(struct object_database *odb,
+			  struct odb_transaction **out)
 {
-	if (odb->transaction)
-		return NULL;
+	int ret;
 
-	odb_source_begin_transaction(odb->sources, &odb->transaction);
+	if (odb->transaction) {
+		*out = NULL;
+		return 0;
+	}
 
-	return odb->transaction;
+	ret = odb_source_begin_transaction(odb->sources, out);
+	odb->transaction = *out;
+
+	return ret;
 }
 
 void odb_transaction_commit(struct odb_transaction *transaction)

odb/transaction.h

@@ -1,6 +1,8 @@
 #ifndef ODB_TRANSACTION_H
 #define ODB_TRANSACTION_H
 
+#include "git-compat-util.h"
+#include "gettext.h"
 #include "odb.h"
 #include "odb/source.h"
 
@@ -33,11 +35,20 @@ struct odb_transaction {
 };
 
 /*
- * Starts an ODB transaction. Subsequent objects are written to the transaction
- * and not committed until odb_transaction_commit() is invoked on the
- * transaction. If the ODB already has a pending transaction, NULL is returned.
+ * Starts an ODB transaction and returns it via `out`. Subsequent objects are
+ * written to the transaction and not committed until odb_transaction_commit()
+ * is invoked on the transaction. Returns 0 on success and a negative value on
+ * error. If the ODB already has a pending transaction, `out` is set to NULL.
  */
-struct odb_transaction *odb_transaction_begin(struct object_database *odb);
+int odb_transaction_begin(struct object_database *odb,
+			  struct odb_transaction **out);
+
+static inline void odb_transaction_begin_or_die(struct object_database *odb,
+						struct odb_transaction **out)
+{
+	if (odb_transaction_begin(odb, out))
+		die(_("failed to start ODB transaction"));
+}
 
 /*
  * Commits an ODB transaction making the written objects visible. If the

read-cache.c

@@ -4042,7 +4042,7 @@ int add_files_to_cache(struct repository *repo, const char *prefix,
 	 * This function is invoked from commands other than 'add', which
 	 * may not have their own transaction active.
 	 */
-	transaction = odb_transaction_begin(repo->objects);
+	odb_transaction_begin_or_die(repo->objects, &transaction);
 	run_diff_files(&rev, DIFF_RACY_IS_MODIFIED);
 	odb_transaction_commit(transaction);