transactions: Amend pending transactions

The transaction system contains a necessary optimization where a popped
transaction is combined with later, similar transactions. This breaks
the chronological order of states, and can lead to desynchronized
geometries.

To fix this, we replace the queue with only 2 transactions: current and
pending. If a pending transaction exists, it is updated with new state
instead of creating additional transactions.

As we never have more than a single waiting transaction, we no longer
need the queue optimization that is causing problems.

Closes: https://github.com/swaywm/sway/issues/6012

1 files changed, 17 insertions, 1 deletions

diff --git a/include/sway/server.h b/include/sway/server.h
index 0f5e3ab2..5a2562b3 100644
--- a/include/sway/server.h
+++ b/include/sway/server.h
@@ -23,6 +23,8 @@
 #include "sway/xwayland.h"
 #endif
 
+struct sway_transaction;
+
 struct sway_server {
         struct wl_display *wl_display;
         struct wl_event_loop *wl_event_loop;
@@ -85,8 +87,22 @@ struct sway_server {
         struct wlr_text_input_manager_v3 *text_input;
         struct wlr_foreign_toplevel_manager_v1 *foreign_toplevel_manager;
 
+        // The timeout for transactions, after which a transaction is applied
+        // regardless of readiness.
         size_t txn_timeout_ms;
-        list_t *transactions;
+
+        // Stores a transaction after it has been committed, but is waiting for
+        // views to ack the new dimensions before being applied. A queued
+        // transaction is frozen and must not have new instructions added to it.
+        struct sway_transaction *queued_transaction;
+
+        // Stores a pending transaction that will be committed once the existing
+        // queued transaction is applied and freed. The pending transaction can be
+        // updated with new instructions as needed.
+        struct sway_transaction *pending_transaction;
+
+        // Stores the nodes that have been marked as "dirty" and will be put into
+        // the pending transaction.
         list_t *dirty_nodes;
 };