Add triggers/* branches to support triggering full runs (#18870)

Handle empty list of commits in `get_extra_jobs`, since that's possible when force pushing.

Fixes #13263.

.azure-pipelines.yml

@@ -236,6 +236,7 @@ jobs:
   displayName: 'all tests: Edge Dev'
   condition: |
     or(eq(variables['Build.SourceBranch'], 'refs/heads/epochs/six_hourly'),
+       eq(variables['Build.SourceBranch'], 'refs/heads/triggers/edge_dev'),
        and(eq(variables['Build.Reason'], 'Manual'), variables['run_all_edge_dev']))
   strategy:
     parallel: 10 # chosen to make runtime ~2h
@@ -271,6 +272,7 @@ jobs:
   displayName: 'all tests: Edge Canary'
   condition: |
     or(eq(variables['Build.SourceBranch'], 'refs/heads/epochs/six_hourly'),
+       eq(variables['Build.SourceBranch'], 'refs/heads/triggers/edge_canary'),
        and(eq(variables['Build.Reason'], 'Manual'), variables['run_all_edge_canary']))
   strategy:
     parallel: 10 # chosen to make runtime ~2h
@@ -306,6 +308,7 @@ jobs:
   displayName: 'all tests: Safari'
   condition: |
     or(eq(variables['Build.SourceBranch'], 'refs/heads/epochs/daily'),
+       eq(variables['Build.SourceBranch'], 'refs/heads/triggers/safari_stable'),
        and(eq(variables['Build.Reason'], 'Manual'), variables['run_all_safari']))
   strategy:
     parallel: 5 # chosen to make runtime ~2h
@@ -340,6 +343,7 @@ jobs:
   displayName: 'all tests: Safari Technology Preview'
   condition: |
     or(eq(variables['Build.SourceBranch'], 'refs/heads/epochs/six_hourly'),
+       eq(variables['Build.SourceBranch'], 'refs/heads/triggers/safari_preview'),
        and(eq(variables['Build.Reason'], 'Manual'), variables['run_all_safari_preview']))
   strategy:
     parallel: 5 # chosen to make runtime ~2h

.taskcluster.yml

@@ -13,7 +13,13 @@ tasks:
               $match: {
                 event.ref == "refs/heads/master": [{name: firefox, channel: nightly}, {name: chrome, channel: dev}],
                 event.ref == "refs/heads/epochs/daily": [{name: firefox, channel: stable}, {name: chrome, channel: stable}],
-                event.ref == "refs/heads/epochs/weekly": [{name: firefox, channel: beta}, {name: chrome, channel: beta}]
+                event.ref == "refs/heads/epochs/weekly": [{name: firefox, channel: beta}, {name: chrome, channel: beta}],
+                event.ref == "refs/heads/triggers/chrome_stable": [{name: chrome, channel: stable}],
+                event.ref == "refs/heads/triggers/chrome_beta": [{name: chrome, channel: beta}],
+                event.ref == "refs/heads/triggers/chrome_dev": [{name: chrome, channel: dev}],
+                event.ref == "refs/heads/triggers/firefox_stable": [{name: firefox, channel: stable}],
+                event.ref == "refs/heads/triggers/firefox_beta": [{name: firefox, channel: beta}],
+                event.ref == "refs/heads/triggers/firefox_nightly": [{name: firefox, channel: nightly}]
                 }
           each(browser):
             $map:

docs/running-tests/from-ci.md

@@ -0,0 +1,32 @@
+# Running Tests on CI
+
+Contributors with write access to the repository can trigger full runs in the
+same CI systems used to produce results for [wpt.fyi](https://wpt.fyi). The runs
+are triggered by pushing to branch names on the form `triggers/$browser_$channel`
+and the results will be automatically submitted to wpt.fyi.
+
+This is useful when making infrastructure changes that could affect very many
+tests, in order to avoid regressions.
+
+Note: Full runs use a lot of CI resources, so please take care to not trigger
+them more than necessary.
+
+Instructions:
+
+ * Base your changes on a commit for which there are already results in wpt.fyi.
+
+ * Determine which branch name to push to by looking for `refs/heads/triggers/`
+   in `.azure-pipelines.yml` and `.taskcluster.yml`. For example, to trigger a
+   full run of Safari Technology Preview, the branch name is
+   `triggers/safari_preview`.
+
+ * Force push to the branch, for example:
+   `git push --force-with-lease origin HEAD:triggers/safari_preview`.
+   The `--force-with-lease` argument is to detect if someone else has just
+   pushed. When this happens wait for the checkout step of their triggered run
+   to finish before you force push again.
+
+You can see if the run started from the commit status on GitHub's commits listing
+([example](https://github.com/web-platform-tests/wpt/commits/triggers/safari_preview))
+and if successful the results will show up on wpt.fyi within 10 minutes
+([example](https://wpt.fyi/runs?product=safari)).

docs/running-tests/index.md

@@ -5,6 +5,7 @@
 
    from-web
    from-local-system
+   from-ci
    custom-runner
    ../tools/certs/README.md
 ```
@@ -15,6 +16,9 @@ that approach is available in [Running tests from the Web](from-web).
 Contributors who are interested in modifying and creating tests should refer to
 [Running Tests from the Local System](from-local-system).
 
+Contributors with write access to the repository can also trigger full runs
+in our CI setups, see [Running Tests on CI](from-ci).
+
 Advanced use cases may call for a customized method of executing the tests.
 Guidelines for writing a custom "runner" are available at [Writing Your Own
 Runner](custom-runner).

tools/ci/run_tc.py

@@ -150,7 +150,7 @@ def start_xvfb():
 def get_extra_jobs(event):
     body = None
     jobs = set()
-    if "commits" in event:
+    if "commits" in event and event["commits"]:
         body = event["commits"][0]["message"]
     elif "pull_request" in event:
         body = event["pull_request"]["body"]