ci: avoid unnecessary builds

Whenever a branch is pushed to a repository which has GitHub Actions
enabled, a bunch of new workflow runs are started.

We sometimes see contributors push multiple branch updates in rapid
succession, which in conjunction with the impressive time swallowed by
even just a single CI build frequently leads to many queued-up runs.

This is particularly problematic in the case of Pull Requests where a
single contributor can easily (inadvertently) prevent timely builds for
other contributors when using a shared repository.

To help with this situation, let's use the `concurrency` feature of
GitHub workflows, essentially canceling GitHub workflow runs that are
obsoleted by more recent runs:

  https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency

For workflows that *do* want the behavior in the pre-image of this
patch, they can use the ci-config feature to disable the new behavior by
adding an executable script on the ci-config branch called
'skip-concurrent' which terminates with a non-zero exit code.

Original-patch-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

3 files changed, 50 insertions, 2 deletions

diff --git a/.github/workflows/check-whitespace.yml b/.github/workflows/check-whitespace.yml
index ad3466ad16..0bcc9cffbd 100644
--- a/.github/workflows/check-whitespace.yml
+++ b/.github/workflows/check-whitespace.yml
@@ -9,6 +9,12 @@ on:
   pull_request:
     types: [opened, synchronize]
 
+# Avoid unnecessary builds. Unlike the main CI jobs, these are not
+# ci-configurable (but could be).
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   check-whitespace:
     runs-on: ubuntu-latest
diff --git a/.github/workflows/l10n.yml b/.github/workflows/l10n.yml
index 27f72f0ff3..51fd46e6af 100644
--- a/.github/workflows/l10n.yml
+++ b/.github/workflows/l10n.yml
@@ -2,6 +2,12 @@ name: git-l10n
 
 on: [push, pull_request_target]
 
+# Avoid unnecessary builds. Unlike the main CI jobs, these are not
+# ci-configurable (but could be).
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   git-po-helper:
     if: >-
diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index bd6f75b8e0..d27cd44584 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -11,6 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     outputs:
       enabled: ${{ steps.check-ref.outputs.enabled }}${{ steps.skip-if-redundant.outputs.enabled }}
+      skip_concurrent: ${{ steps.check-ref.outputs.skip_concurrent }}
     steps:
       - name: try to clone ci-config branch
         run: |
@@ -34,7 +35,15 @@ jobs:
           then
             enabled=no
           fi
+
+          skip_concurrent=yes
+          if test -x config-repo/ci/config/skip-concurrent &&
+             ! config-repo/ci/config/skip-concurrent '${{ github.ref }}'
+          then
+            skip_concurrent=no
+          fi
           echo "::set-output name=enabled::$enabled"
+          echo "::set-output name=skip_concurrent::$skip_concurrent"
       - name: skip if the commit or tree was already tested
         id: skip-if-redundant
         uses: actions/github-script@v3
@@ -82,6 +91,9 @@ jobs:
     needs: ci-config
     if: needs.ci-config.outputs.enabled == 'yes'
     runs-on: windows-latest
+    concurrency:
+      group: windows-build-${{ github.ref }}
+      cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
     - uses: actions/checkout@v2
     - uses: git-for-windows/setup-git-for-windows-sdk@v1
@@ -101,11 +113,14 @@ jobs:
   windows-test:
     name: win test
     runs-on: windows-latest
-    needs: [windows-build]
+    needs: [ci-config, windows-build]
     strategy:
       fail-fast: false
       matrix:
         nr: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+    concurrency:
+      group: windows-test-${{ matrix.nr }}-${{ github.ref }}
+      cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
     - name: download tracked files and build artifacts
       uses: actions/download-artifact@v2
@@ -137,6 +152,9 @@ jobs:
       NO_PERL: 1
       GIT_CONFIG_PARAMETERS: "'user.name=CI' 'user.email=ci@git'"
     runs-on: windows-latest
+    concurrency:
+      group: vs-build-${{ github.ref }}
+      cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
     - uses: actions/checkout@v2
     - uses: git-for-windows/setup-git-for-windows-sdk@v1
@@ -184,11 +202,14 @@ jobs:
   vs-test:
     name: win+VS test
     runs-on: windows-latest
-    needs: vs-build
+    needs: [ci-config, vs-build]
     strategy:
       fail-fast: false
       matrix:
         nr: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+    concurrency:
+      group: vs-test-${{ matrix.nr }}-${{ github.ref }}
+      cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
     - uses: git-for-windows/setup-git-for-windows-sdk@v1
     - name: download tracked files and build artifacts
@@ -218,6 +239,9 @@ jobs:
     name: ${{matrix.vector.jobname}} (${{matrix.vector.pool}})
     needs: ci-config
     if: needs.ci-config.outputs.enabled == 'yes'
+    concurrency:
+      group: ${{ matrix.vector.jobname }}-${{ matrix.vector.pool }}-${{ github.ref }}
+      cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     strategy:
       fail-fast: false
       matrix:
@@ -281,6 +305,9 @@ jobs:
     name: ${{matrix.vector.jobname}} (${{matrix.vector.image}})
     needs: ci-config
     if: needs.ci-config.outputs.enabled == 'yes'
+    concurrency:
+      group: dockerized-${{ matrix.vector.jobname }}-${{ matrix.vector.image }}-${{ github.ref }}
+      cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     strategy:
       fail-fast: false
       matrix:
@@ -316,6 +343,9 @@ jobs:
     env:
       jobname: StaticAnalysis
     runs-on: ubuntu-22.04
+    concurrency:
+      group: static-analysis-${{ github.ref }}
+      cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
     - uses: actions/checkout@v2
     - run: ci/install-dependencies.sh
@@ -327,6 +357,9 @@ jobs:
     env:
       jobname: sparse
     runs-on: ubuntu-20.04
+    concurrency:
+      group: sparse-${{ github.ref }}
+      cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
     - name: Download a current `sparse` package
       # Ubuntu's `sparse` version is too old for us
@@ -345,6 +378,9 @@ jobs:
     name: documentation
     needs: ci-config
     if: needs.ci-config.outputs.enabled == 'yes'
+    concurrency:
+      group: documentation-${{ github.ref }}
+      cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     env:
       jobname: Documentation
     runs-on: ubuntu-latest