ci: avoid building from the same commit in parallel

At times, we may need to push the same commit to multiple branches
in the same push.  Rewinding 'next' to rebuild on top of 'master'
soon after a release is such an occasion.  Making sure 'main' stays
in sync with 'master' to help those who expect that primary branch
of the project is named either of these is another.

We already use the branch name as a "concurrency group" key, but
that does not address the situation illustrated above.

Let's introduce another `concurrency` attribute, using the commit
hash as the concurrency group key, on the workflow run level, to
address this. This will hold any workflow run in the queued state
when there is already a workflow run targeting the same commit,
until that latter run completed. The `skip-if-redundant` check of
the second run will then have a chance to see whether the first
run succeeded.

The only caveat with this strategy is that only one workflow run
will be kept in the queued state by the `concurrency` feature: if
another run targeting the same commit is triggered, the
previously-queued run will be canceled. Considering the benefit,
this seems the smaller price to pay than to overload Git's build
agent pool with undesired workflow runs.

Helped-by: Junio C Hamano <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

.github/workflows/main.yml

@@ -5,6 +5,19 @@ on: [push, pull_request]
 env:
   DEVELOPER: 1
 
+# If more than one workflow run is triggered for the very same commit hash
+# (which happens when multiple branches pointing to the same commit), only
+# the first one is allowed to run, the second will be kept in the "queued"
+# state. This allows a successful completion of the first run to be reused
+# in the second run via the `skip-if-redundant` logic in the `config` job.
+#
+# The only caveat is that if a workflow run is triggered for the same commit
+# hash that another run is already being held, that latter run will be
+# canceled. For more details about the `concurrency` attribute, see:
+# https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency
+concurrency:
+  group: ${{ github.sha }}
+
 jobs:
   ci-config:
     name: config