Short-circuit when change set has no changes

- Don't call ExecuteChangeSet on stack start when there are no
  changes in the change set. Fix a spurious error when using a
  change set with no changes on a stack in an
  `UPDATE_ROLLBACK_COMPLETE` state.

Short-circuit when changes is empty. This fixes a
mistaken error when stack-start! believes that we
made changes and throws on seeing
`UPDATE_ROLLBACK_COMPLETE` because it thinks that
we caused it. This also avoids an unnecessary
ExecuteChangeSet operation.

Author: john-shaffer

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## Unreleased
 
+- Don't call ExecuteChangeSet on stack start when there are no
+  changes in the change set. Fix a spurious error when using a
+  change set with no changes on a stack in an
+  `UPDATE_ROLLBACK_COMPLETE` state.
+
 ## v0.25.0 (2025-02-26)
 
 - Add :template-url option to change-set and stack components.

src/salmon/cloudformation.clj

@@ -419,13 +419,15 @@
                  (aws/client {:api :cloudformation :region region}))]
     (if inst-client
       instance
-      (let [{:keys [stack-id]} change-set
+      (let [{:keys [changes stack-id]} change-set
             ex! (if change-set
                   (fn []
-                    (let [r (execute-change-set! client name change-set)]
-                      (if (u/anomaly? r)
-                        [stack-id false]
-                        [stack-id true])))
+                    (if (seq changes)
+                      (let [r (execute-change-set! client name change-set)]
+                        (if (u/anomaly? r)
+                          [stack-id false]
+                          [stack-id true]))
+                      [stack-id false]))
                   #(cou-stack! client signal (template-data config :template template :validate? false)))]
         (validate! signal)
         (loop [[r updated?] (ex!)]