[libc++] Add a wait step in the BuildKite pipeline to shield macOS builders

We don't have many of those and they are rather slow, so we'd rather not run
those jobs if we know other jobs in the pipeline failed anyway.

diff --git a/libcxx/utils/ci/buildkite-pipeline.yml b/libcxx/utils/ci/buildkite-pipeline.yml
index cd6ff77..f502364 100644 (file)
--- a/libcxx/utils/ci/buildkite-pipeline.yml
+++ b/libcxx/utils/ci/buildkite-pipeline.yml
@@ -277,6 +277,12 @@ steps:
         - exit_status: -1  # Agent was lost
           limit: 2
 
+  # All jobs defined before this `wait` will run whenever a CI job is started.
+  # Jobs defined after the `wait` will run only if all the jobs above succeeded.
+  # We use this to reduce the load on testers that have more constrained resources
+  # and avoid running builds that we know fail anyway.
+  - wait
+
   # Build with the configuration we use to generate libc++.dylib on Apple platforms
   - label: "Apple system"
     command: "libcxx/utils/ci/run-buildbot x86_64-apple-system"