ci: skip containers & build jobs when disabling a farm

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25032>

.gitlab-ci.yml

@@ -173,8 +173,9 @@ stages:
       # Source code
       - include/**/*
       - src/**/*
-      - .ci-farms/*
       when: on_success
+    # Run when re-enabling a disabled farm, but not when disabling it
+    - !reference [.re-enable-farm-rules, rules]
     # Otherwise, build/test jobs won't run because no rule matched.
 
 
@@ -199,6 +200,8 @@ stages:
       - .gitlab-ci.yml
       - .gitlab-ci/**/*
       when: on_success
+    # Run when re-enabling a disabled farm, but not when disabling it
+    - !reference [.re-enable-farm-rules, rules]
     # Run pipeline by default if it was triggered by Marge Bot, is for a
     # merge request, and any files affecting the pipeline were changed
     - if: *is-pre-merge-for-marge

.gitlab-ci/farm-rules.yml

@@ -239,3 +239,39 @@
       when: never
     - !reference [.freedreno-farm-rules, rules]
 
+
+# Skip container & build jobs when disabling any farm, and run them if any farm
+# gets re-enabled.
+.re-enable-farm-rules:
+  rules:
+    # changes(disabled) + exists(disabled) = disabling the farm
+    - changes: [ .ci-farms-disabled/microsoft ]
+      exists: [ .ci-farms-disabled/microsoft ]
+      when: never
+    - changes: [ .ci-farms-disabled/collabora ]
+      exists: [ .ci-farms-disabled/collabora ]
+      when: never
+    - changes: [ .ci-farms-disabled/igalia ]
+      exists: [ .ci-farms-disabled/igalia ]
+      when: never
+    - changes: [ .ci-farms-disabled/lima ]
+      exists: [ .ci-farms-disabled/lima ]
+      when: never
+    - changes: [ .ci-farms-disabled/anholt ]
+      exists: [ .ci-farms-disabled/anholt ]
+      when: never
+    - changes: [ .ci-farms-disabled/valve-mupuf ]
+      exists: [ .ci-farms-disabled/valve-mupuf ]
+      when: never
+    - changes: [ .ci-farms-disabled/valve-kws ]
+      exists: [ .ci-farms-disabled/valve-kws ]
+      when: never
+    - changes: [ .ci-farms-disabled/austriancoder ]
+      exists: [ .ci-farms-disabled/austriancoder ]
+      when: never
+    - changes: [ .ci-farms-disabled/freedreno ]
+      exists: [ .ci-farms-disabled/freedreno ]
+      when: never
+    # Any other change to ci-farms-disabled/* means some farm is getting re-enabled
+    - changes: [ .ci-farms-disabled/* ]
+      when: on_success