ci/github-script/check-target-branch: simplify policy decisions

This commit hopefully improves readability: encapsulates some
intermediary decisions in variable, plus adopts early return decision
tree.

Assisted-by: Codex gpt-5.6-sol medium

ci/github-script/check-target-branch-policy.ts

@@ -19,10 +19,14 @@ type TargetBranchReviewDecision =
 function getTargetBranchPolicy({ base, head }: { base: string; head: string }) {
   const baseClassification = classify(base)
   const headClassification = classify(head)
+  const isPrimaryBase = baseClassification.type.includes('primary')
+  const shouldSkipDevelopmentMerge =
+    headClassification.type.includes('development')
 
   return {
-    shouldSkipDevelopmentMerge: headClassification.type.includes('development'),
-    shouldCheckMassRebuild: baseClassification.type.includes('primary'),
+    shouldSkipDevelopmentMerge,
+    shouldCheckMassRebuild: !shouldSkipDevelopmentMerge && isPrimaryBase,
+    shouldCheckNixosRebuild: !shouldSkipDevelopmentMerge && isPrimaryBase,
   }
 }
 
@@ -34,34 +38,34 @@ function decideTargetBranchReview({
   isExemptKernelUpdate,
   isExemptHomeAssistantUpdate,
 }: TargetBranchPolicyFacts): TargetBranchReviewDecision {
-  const baseClassification = classify(base)
-  const headClassification = classify(head)
-
-  if (headClassification.type.includes('development')) {
-    return 'skip-development-merge'
-  }
-
-  if (!baseClassification.type.includes('primary')) {
-    return 'dismiss'
-  }
-
-  if (
+  const {
+    shouldSkipDevelopmentMerge,
+    shouldCheckMassRebuild,
+    shouldCheckNixosRebuild,
+  } = getTargetBranchPolicy({ base, head })
+  const isMassRebuild =
     maxRebuildCount >= 1000 &&
-    !isExemptHomeAssistantUpdate &&
-    !isExemptKernelUpdate
-  ) {
-    return 'mass-rebuild'
-  } else if (rebuildsAllTests && !isExemptKernelUpdate) {
-    return 'nixos-rebuild'
-  } else if (
-    maxRebuildCount >= 500 &&
     !isExemptKernelUpdate &&
     !isExemptHomeAssistantUpdate
-  ) {
-    return 'possible-mass-rebuild'
-  } else {
-    return 'dismiss'
+
+  if (shouldCheckMassRebuild && isMassRebuild) {
+    return 'mass-rebuild'
   }
+
+  if (shouldCheckNixosRebuild && rebuildsAllTests && !isExemptKernelUpdate) {
+    return 'nixos-rebuild'
+  }
+
+  const isPossibleMassRebuild =
+    maxRebuildCount >= 500 &&
+    !isMassRebuild &&
+    !isExemptKernelUpdate &&
+    !isExemptHomeAssistantUpdate
+  if (shouldCheckMassRebuild && isPossibleMassRebuild) {
+    return 'possible-mass-rebuild'
+  }
+
+  return shouldSkipDevelopmentMerge ? 'skip-development-merge' : 'dismiss'
 }
 
 module.exports = { decideTargetBranchReview, getTargetBranchPolicy }

ci/github-script/check-target-branch.ts

@@ -156,41 +156,7 @@ async function checkTargetBranch({
   ).data
   const base = prInfo.base.ref
   const head = prInfo.head.ref
-  const { shouldSkipDevelopmentMerge, shouldCheckMassRebuild } =
-    getTargetBranchPolicy({ base, head })
-
-  // Don't run on, e.g., staging-nixos to master merges.
-  if (shouldSkipDevelopmentMerge) {
-    core.info(
-      `Skipping checkTargetBranch: PR is from a development branch (${head})`,
-    )
-
-    await dismissReviews({
-      github,
-      context,
-      core,
-      dry,
-      reviewKey,
-    })
-
-    return
-  }
-  // Don't run on PRs against staging branches, wip branches, haskell-updates, etc.
-  if (!shouldCheckMassRebuild) {
-    core.info(
-      `Skipping checkTargetBranch: PR is against a non-primary base branch (${base})`,
-    )
-
-    await dismissReviews({
-      github,
-      context,
-      core,
-      dry,
-      reviewKey,
-    })
-
-    return
-  }
+  const { shouldCheckMassRebuild } = getTargetBranchPolicy({ base, head })
 
   const maxRebuildCount = Math.max(
     ...Object.values(changed.rebuildCountByKernel),
@@ -202,7 +168,7 @@ async function checkTargetBranch({
   // https://github.com/NixOS/nixpkgs/pull/521157
   // These should go to master and release-xx.xx when backported
   let isExemptKernelUpdate = false
-  if (prInfo.changed_files === 1) {
+  if (shouldCheckMassRebuild && prInfo.changed_files === 1) {
     const changedFiles = (
       await github.rest.pulls.listFiles({
         ...context.repo,
@@ -248,21 +214,34 @@ async function checkTargetBranch({
 
   if (decision === 'mass-rebuild') {
     await postMassRebuildReview(reviewFacts)
-  } else if (decision === 'nixos-rebuild') {
+    return
+  }
+
+  if (decision === 'nixos-rebuild') {
     await postNixosRebuildReview(reviewFacts)
-  } else if (decision === 'possible-mass-rebuild') {
+    return
+  }
+
+  if (decision === 'possible-mass-rebuild') {
     await postPossibleMassRebuildReview(reviewFacts)
+    return
+  }
+
+  if (decision === 'skip-development-merge') {
+    core.info(
+      `Skipping checkTargetBranch: PR merges the development branch ${head} into ${base}`,
+    )
   } else {
     core.info('checkTargetBranch: this PR is against an appropriate branch.')
-
-    await dismissReviews({
-      github,
-      context,
-      core,
-      dry,
-      reviewKey,
-    })
   }
+
+  await dismissReviews({
+    github,
+    context,
+    core,
+    dry,
+    reviewKey,
+  })
 }
 
 module.exports = checkTargetBranch