build: avoid conflicts when publishing snapshot builds (#19737)

For the past year, we've had CI failures when multiple PRs
have been merged. Since we run CI for all landed commits, the
snapshot publish job runs multiple times concurrently and could
end up pushing to the snapshot repositories out of order.

This can result in conflicts when multiple jobs push artifacts
at the same time. e.g.
https://circleci.com/gh/angular/components/155407.

We intend to fix this by queuing snapshot publishing across CircleCI
builds. This is done using the CircleCI API. We constantly check the
API to check that there are no previous snapshot jobs running.

Instead of implementing this logic our own, we leverage a CircleCI
orb that provides this functionality.

Author: devversion

.circleci/config.yml

@@ -150,6 +150,15 @@ var_21: &slack_notify_on_failure
 # -----------------------------
 version: 2.1
 
+# Configures CircleCI orbs for this pipeline. Orbs allow consumption of publicly shared
+# CircleCI commands, jobs, configuration elements or executors. Read more about orbs here:
+# https://circleci.com/docs/2.0/orb-intro/
+orbs:
+  # Set up the `queue` orb that allows us to queue up builds across jobs. We use it
+  # to make sure that snapshot builds are not deployed out of order, resulting in Git
+  # push conflicts.
+  queue: eddiewebb/[email protected]
+
 # -----------------------------------------------------------------------------------------
 # Job definitions. Jobs which are defined just here, will not run automatically. Each job
 # must be part of a workflow definition in order to run for PRs and push builds.
@@ -427,6 +436,11 @@ jobs:
       # as part of this job to the docs-content repository. It's not contained in the
       # attached release output, so we need to build it here.
       - run: bazel build src/components-examples:npm_package --config=release
+
+      # Ensures that we do not push the snapshot artifacts upstream until all previous
+      # snapshot build jobs have completed. This helps avoiding conflicts when multiple
+      # commits have been pushed (resulting in multiple concurrent snapshot publish jobs).
+      - queue/until_front_of_line
       - run: ./scripts/circleci/publish-snapshots.sh
       - *slack_notify_on_failure