Don't use bitfield to test for similar transactions

When there's multiple transactions in the queue, sway can take a
shortcut by checking if they all operate on the same set of containers.
If they do, it can skip all but the last transaction. The way we tested
for transactions which used the same containers was to exclusive-or
their con IDs together, but this has proved not only to be ineffective
but also has the potential to make sway crash.

This patch replaces the exclusive-or with a loop and container
comparison.

1 files changed, 17 insertions, 3 deletions

diff --git a/sway/desktop/transaction.c b/sway/desktop/transaction.c
index c18529fb..1316a80d 100644
--- a/sway/desktop/transaction.c
+++ b/sway/desktop/transaction.c
@@ -22,7 +22,6 @@ struct sway_transaction {
 	list_t *instructions;   // struct sway_transaction_instruction *
 	size_t num_waiting;
 	size_t num_configures;
-	uint32_t con_ids;       // Bitwise XOR of view container IDs
 	struct timespec commit_time;
 };
 
@@ -218,6 +217,22 @@ static void transaction_apply(struct sway_transaction *transaction) {
 
 static void transaction_commit(struct sway_transaction *transaction);
 
+// Return true if both transactions operate on the same containers
+static bool transaction_same_containers(struct sway_transaction *a,
+		struct sway_transaction *b) {
+	if (a->instructions->length != b->instructions->length) {
+		return false;
+	}
+	for (int i = 0; i < a->instructions->length; ++i) {
+		struct sway_transaction_instruction *a_inst = a->instructions->items[i];
+		struct sway_transaction_instruction *b_inst = b->instructions->items[i];
+		if (a_inst->container != b_inst->container) {
+			return false;
+		}
+	}
+	return true;
+}
+
 static void transaction_progress_queue() {
 	if (!server.transactions->length) {
 		return;
@@ -242,7 +257,7 @@ static void transaction_progress_queue() {
 	while (server.transactions->length >= 2) {
 		struct sway_transaction *a = server.transactions->items[0];
 		struct sway_transaction *b = server.transactions->items[1];
-		if (a->con_ids == b->con_ids) {
+		if (transaction_same_containers(a, b)) {
 			list_del(server.transactions, 0);
 			transaction_destroy(a);
 		} else {
@@ -294,7 +309,6 @@ static void transaction_commit(struct sway_transaction *transaction) {
 					instruction->state.view_width,
 					instruction->state.view_height);
 			++transaction->num_waiting;
-			transaction->con_ids ^= con->id;
 
 			// From here on we are rendering a saved buffer of the view, which
 			// means we can send a frame done event to make the client redraw it