[PATCH 4/6] odb/transaction: propagate commit errors

[Justin Tobler <jltobler@gmail.com>]

When `odb_transaction_commit()` is invoked, the return value of the
backend commit callback is silently discarded. A backend has no way
to signal that committing failed, such as when the "files" backend
cannot migrate its temporary object directory into the permanent
ODB.

In a subsequent commit, git-receive-pack(1) starts using ODB transaction
to stage objects and consequently cares about such failures so it can
handle the error appropriately. Change the commit callback signature to
return an int error code and have `odb_transaction_commit()` forward it
accordingly.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
---
 odb/transaction.c | 13 ++++++++++---
 odb/transaction.h |  2 +-
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/odb/transaction.c b/odb/transaction.c
index d3de01db50..b20d6a16f8 100644
--- a/odb/transaction.c
+++ b/odb/transaction.c
@@ -18,19 +18,26 @@ int odb_transaction_begin(struct object_database *odb,
 	return ret;
 }
 
-void odb_transaction_commit(struct odb_transaction *transaction)
+int odb_transaction_commit(struct odb_transaction *transaction)
 {
+	int ret;
+
 	if (!transaction)
-		return;
+		return 0;
 
 	/*
 	 * Ensure the transaction ending matches the pending transaction.
 	 */
 	ASSERT(transaction == transaction->source->odb->transaction);
 
-	transaction->commit(transaction);
+	ret = transaction->commit(transaction);
+	if (ret)
+		return ret;
+
 	transaction->source->odb->transaction = NULL;
 	free(transaction);
+
+	return 0;
 }
 
 int odb_transaction_write_object_stream(struct odb_transaction *transaction,
diff --git a/odb/transaction.h b/odb/transaction.h
index cd6d50f2e5..7898770071 100644
--- a/odb/transaction.h
+++ b/odb/transaction.h
@@ -54,7 +54,7 @@ static inline void odb_transaction_begin_or_die(struct object_database *odb,
  * Commits an ODB transaction making the written objects visible. If the
  * specified transaction is NULL, the function is a no-op.
  */
-void odb_transaction_commit(struct odb_transaction *transaction);
+int odb_transaction_commit(struct odb_transaction *transaction);
 
 /*
  * Writes the object in the provided stream into the transaction. The resulting
-- 
2.54.0.105.g59ff4886a5