[ci] Compare merge-branch base ref against the default branch (#18385)

.github/scripts/auto-label-pr/detectors.js

@@ -70,6 +70,7 @@ async function isStackedPr(github, context) {
 async function detectMergeBranch(github, context) {
   const labels = new Set();
   const baseRef = context.payload.pull_request.base.ref;
+  const defaultBranch = context.payload.repository.default_branch;
 
   if (baseRef === 'release') {
     labels.add('merging-to-release');
@@ -78,7 +79,7 @@ async function detectMergeBranch(github, context) {
   } else if (await isStackedPr(github, context)) {
     // GitHub manages the merge order for a stack, so these are not blocked.
     labels.add('stacked-pr');
-  } else if (baseRef !== 'dev') {
+  } else if (baseRef !== defaultBranch) {
     // A chain built by hand: it must not merge until its base branch does.
     labels.add('chained-pr');
   }

.github/scripts/auto-label-pr/tests/detectors.test.js

@@ -43,14 +43,14 @@ const WITHOUT_SCHEMA = 'CODEOWNERS = ["@esphome/core"]';
 
 // Builds a fresh context for detectMergeBranch tests instead of mutating the
 // shared CONTEXT fixture above (which other describe blocks rely on).
-function makeMergeContext(baseRef, { stack } = {}) {
+function makeMergeContext(baseRef, { stack, defaultBranch = 'dev' } = {}) {
   const pull_request = { number: 1, base: { ref: baseRef } };
   if (stack !== undefined) {
     pull_request.stack = stack;
   }
   return {
     repo: { owner: 'esphome', repo: 'esphome' },
-    payload: { pull_request }
+    payload: { pull_request, repository: { default_branch: defaultBranch } }
   };
 }
 
@@ -136,6 +136,21 @@ describe('detectMergeBranch', () => {
     assert.deepEqual(Array.from(labels).sort(), ['chained-pr']);
     assert.equal(state.calls, 1);
   });
+
+  it('base ref matches default branch adds no labels', async () => {
+    const { github } = makeStackGithub({ stack: null });
+    const context = makeMergeContext('other', { defaultBranch: 'other' });
+    const labels = await detectMergeBranch(github, context);
+    assert.deepEqual(Array.from(labels).sort(), []);
+  });
+
+  it('base ref dev when the default branch is main adds chained-pr', async () => {
+    const { github } = makeStackGithub({ stack: null });
+    const context = makeMergeContext('dev', { defaultBranch: 'main' });
+    const labels = await detectMergeBranch(github, context);
+    assert.deepEqual(Array.from(labels).sort(), ['chained-pr']);
+  });
+
 });
 
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