refs.c: do not permit err == NULL

Some functions that take a strbuf argument to append an error treat
!err as an indication that the message should be suppressed (e.g.,
ref_update_reject_duplicates).  Others write the message to stderr on
!err (e.g., repack_without_refs).  Others crash (e.g.,
ref_transaction_update).

Some of these behaviors are for historical reasons and others were
accidents.  Luckily no callers pass err == NULL any more.  Simplify
by consistently requiring the strbuf argument.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -2646,6 +2646,8 @@ int repack_without_refs(const char **refnames, int n, struct strbuf *err)
 	struct string_list_item *ref_to_delete;
 	int i, ret, removed = 0;
 
+	assert(err);
+
 	/* Look for a packed ref */
 	for (i = 0; i < n; i++)
 		if (get_packed_ref(refnames[i]))
@@ -2656,14 +2658,9 @@ int repack_without_refs(const char **refnames, int n, struct strbuf *err)
 		return 0; /* no refname exists in packed refs */
 
 	if (lock_packed_refs(0)) {
-		if (err) {
-			unable_to_lock_message(git_path("packed-refs"), errno,
-					       err);
+		unable_to_lock_message(git_path("packed-refs"), errno, err);
 		return -1;
 	}
-		unable_to_lock_error(git_path("packed-refs"), errno);
-		return error("cannot delete '%s' from packed refs", refnames[i]);
-	}
 	packed = get_packed_refs(&ref_cache);
 
 	/* Remove refnames from the cache */
@@ -2688,7 +2685,7 @@ int repack_without_refs(const char **refnames, int n, struct strbuf *err)
 
 	/* Write what remains */
 	ret = commit_packed_refs();
-	if (ret && err)
+	if (ret)
 		strbuf_addf(err, "unable to overwrite old ref-pack file: %s",
 			    strerror(errno));
 	return ret;
@@ -2696,6 +2693,8 @@ int repack_without_refs(const char **refnames, int n, struct strbuf *err)
 
 static int delete_ref_loose(struct ref_lock *lock, int flag, struct strbuf *err)
 {
+	assert(err);
+
 	if (!(flag & REF_ISPACKED) || flag & REF_ISSYMREF) {
 		/*
 		 * loose.  The loose file name is the same as the
@@ -3551,6 +3550,8 @@ struct ref_transaction {
 
 struct ref_transaction *ref_transaction_begin(struct strbuf *err)
 {
+	assert(err);
+
 	return xcalloc(1, sizeof(struct ref_transaction));
 }
 
@@ -3590,6 +3591,8 @@ int ref_transaction_update(struct ref_transaction *transaction,
 {
 	struct ref_update *update;
 
+	assert(err);
+
 	if (transaction->state != REF_TRANSACTION_OPEN)
 		die("BUG: update called for transaction that is not open");
 
@@ -3622,6 +3625,8 @@ int ref_transaction_create(struct ref_transaction *transaction,
 {
 	struct ref_update *update;
 
+	assert(err);
+
 	if (transaction->state != REF_TRANSACTION_OPEN)
 		die("BUG: create called for transaction that is not open");
 
@@ -3653,6 +3658,8 @@ int ref_transaction_delete(struct ref_transaction *transaction,
 {
 	struct ref_update *update;
 
+	assert(err);
+
 	if (transaction->state != REF_TRANSACTION_OPEN)
 		die("BUG: delete called for transaction that is not open");
 
@@ -3715,13 +3722,14 @@ static int ref_update_reject_duplicates(struct ref_update **updates, int n,
 					struct strbuf *err)
 {
 	int i;
+
+	assert(err);
+
 	for (i = 1; i < n; i++)
 		if (!strcmp(updates[i - 1]->refname, updates[i]->refname)) {
-			const char *str =
-				"Multiple updates for ref '%s' not allowed.";
-			if (err)
-				strbuf_addf(err, str, updates[i]->refname);
-
+			strbuf_addf(err,
+				    "Multiple updates for ref '%s' not allowed.",
+				    updates[i]->refname);
 			return 1;
 		}
 	return 0;
@@ -3735,6 +3743,8 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 	int n = transaction->nr;
 	struct ref_update **updates = transaction->updates;
 
+	assert(err);
+
 	if (transaction->state != REF_TRANSACTION_OPEN)
 		die("BUG: commit called for transaction that is not open");
 
@@ -3771,7 +3781,6 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 			ret = (errno == ENOTDIR)
 				? TRANSACTION_NAME_CONFLICT
 				: TRANSACTION_GENERIC_ERROR;
-			if (err)
 			strbuf_addf(err, "Cannot lock the ref '%s'.",
 				    update->refname);
 			goto cleanup;
@@ -3786,7 +3795,6 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 			if (write_ref_sha1(update->lock, update->new_sha1,
 					   update->msg)) {
 				update->lock = NULL; /* freed by write_ref_sha1 */
-				if (err)
 				strbuf_addf(err, "Cannot update the ref '%s'.",
 					    update->refname);
 				ret = TRANSACTION_GENERIC_ERROR;