Add a CQ group watching all branch heads with no verifiers.

The project settings for chromium/src requires all changes to be landed
by submitting to CQ, but nothing happens if there is no CQ configured
for the branch.

Bug: 1044808
Change-Id: I21f007029c3dd6fbae95a157245eb81a7896d62e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2019704
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740386}

infra/config/generated/commit-queue.cfg

@@ -1363,3 +1363,23 @@ config_groups: <
     >
   >
 >
+config_groups: <
+  name: "fallback-empty-cq"
+  gerrit: <
+    url: "https://chromium-review.googlesource.com"
+    projects: <
+      name: "chromium/src"
+      ref_regexp: "refs/branch-heads/.+"
+    >
+  >
+  verifiers: <
+    gerrit_cq_ability: <
+      committer_list: "project-chromium-committers"
+      dry_run_access_list: "project-chromium-tryjob-access"
+    >
+    tree_status: <
+      url: "https://chromium-status.appspot.com/"
+    >
+  >
+  fallback: YES
+>

infra/config/generators/cq-fallback.star

+def _find_empty_cq_group(ctx):
+  cq_cfg = ctx.output['commit-queue.cfg']
+
+  for c in cq_cfg.config_groups:
+    if c.name == 'fallback-empty-cq':
+      return c
+
+  fail('Could not find empty CQ group')
+
+def _generate_cq_group_fallback(ctx):
+  cq_group = _find_empty_cq_group(ctx)
+  cq_group.fallback = 1  # YES
+
+
+lucicfg.generator(_generate_cq_group_fallback)

infra/config/main.star

@@ -59,6 +59,31 @@ luci.cq(
     status_host = 'chromium-cq-status.appspot.com',
 )
 
+# Declare a CQ group that watches all branch heads
+# We won't add any builders, but SUBMIT TO CQ fails on Gerrit if there is no CQ
+# group
+luci.cq_group(
+    name = 'fallback-empty-cq',
+    # TODO(crbug/959436): enable it.
+    cancel_stale_tryjobs = False,
+    retry_config = cq.RETRY_ALL_FAILURES,
+    tree_status_host = 'chromium-status.appspot.com/',
+    watch = cq.refset(
+        repo = 'https://chromium.googlesource.com/chromium/src',
+        refs = ['refs/branch-heads/.+'],
+    ),
+    acls = [
+        acl.entry(
+            acl.CQ_COMMITTER,
+            groups = 'project-chromium-committers',
+        ),
+        acl.entry(
+            acl.CQ_DRY_RUNNER,
+            groups = 'project-chromium-tryjob-access',
+        ),
+    ],
+)
+
 luci.logdog(
     gs_bucket = 'chromium-luci-logdog',
 )
@@ -122,6 +147,10 @@ exec('//consoles/tryserver.chromium.win.star')
 exec('//notifiers.star')
 
 exec('//generators/cq-builders-md.star')
+# TODO(https://crbug.com/966115) Run the generator to set the fallback field for
+# the empty CQ group until it's exposed in lucicfg or there is a better way to
+# create a CQ group for all of the canary branches
+exec('//generators/cq-fallback.star')
 # TODO(https://crbug.com/819899) There are a number of noop jobs for dummy
 # builders defined due to legacy requirements that trybots mirror CI bots
 # no-op scheduler jobs are not supported by the lucicfg libraries, so this