ci/farm-rules: re-add "run every container and build job when a farm gets re-enabled"

Turns out I was right the first time, I was just missing that this
should only be done in MRs, which @daniels added in 16527f6ffd.

Without this, a "farm re-enable" pipeline will run all the jobs for that
farm, but will have none of the container & build jobs to support these
tests.

This reverts commit 1c3097225a.

Fixes: 1c3097225a ("ci: don't run everything just because a farm gets re-enabled")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25243>

.gitlab-ci.yml

@@ -156,7 +156,7 @@ include:
 # When to automatically run the CI for build jobs
 .build-rules:
   rules:
-    # Don't run when disabling a farm
+    # Run when re-enabling a disabled farm, but not when disabling it
     - !reference [.disable-farm-mr-rules, rules]
     # If any files affecting the pipeline are changed, build/test jobs run
     # automatically once all dependency jobs have passed
@@ -200,7 +200,7 @@ include:
 
 .container-rules:
   rules:
-    # Don't run when disabling a farm
+    # Run when re-enabling a disabled farm, but not when disabling it
     - !reference [.disable-farm-mr-rules, rules]
     # Run pipeline by default in the main project if any CI pipeline
     # configuration files were changed, to ensure docker images are up to date

.gitlab-ci/farm-rules.yml

@@ -238,13 +238,18 @@
     - !reference [.freedreno-farm-rules, rules]
 
 
-# Skip container & build jobs when disabling any farm
+# Skip container & build jobs when disabling any farm, and run them if any
+# farm gets re-enabled.
 # Only apply these rules in MR context, because otherwise we get a false
 # positive on files being 'created' when pushing to a new branch, and break
 # our pipeline
 .disable-farm-mr-rules:
   rules:
     # changes(disabled) + exists(disabled) = disabling the farm
+    # Note: this cannot be simplified into a single `.ci-farms-disabled/*` rule
+    # because if there are more than one disabled farm and we only re-enable
+    # one, the exits(.ci-farms-disabled/*) would match and what should be
+    # a farm re-enable pipeline will be detected as a farm disable pipeline.
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
       changes: [ .ci-farms-disabled/microsoft ]
       exists: [ .ci-farms-disabled/microsoft ]
@@ -281,3 +286,8 @@
       changes: [ .ci-farms-disabled/freedreno ]
       exists: [ .ci-farms-disabled/freedreno ]
       when: never
+    # Any other change to ci-farms/* means some farm is getting re-enabled.
+    # Run jobs in Marge pipelines (and let it fallback to manual otherwise)
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event" && $GITLAB_USER_LOGIN == "marge-bot"'
+      changes: [ .ci-farms/* ]
+      when: on_success