From 7818344f82c84709e4d7314997f8b38be2ae8aed Mon Sep 17 00:00:00 2001
From: Peter Steinberger <steipete@gmail.com>
Date: Mon, 23 Mar 2026 04:40:37 +0000
Subject: [PATCH] fix(ci): harden changed extension diff fallback

---
 .github/workflows/ci.yml            |  8 +++-
 scripts/test-extension.mjs          | 72 ++++++++++++++++++++++++++---
 test/scripts/test-extension.test.ts | 10 ++++
 3 files changed, 82 insertions(+), 8 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c036b5d82bc..cc538dd040e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -117,12 +117,18 @@ jobs:
         id: changed
         env:
           BASE_SHA: ${{ github.event_name == 'push' && github.event.before || github.event.pull_request.base.sha }}
+          BASE_REF: ${{ github.event_name == 'push' && github.ref_name || github.event.pull_request.base.ref }}
         run: |
           node --input-type=module <<'EOF'
           import { appendFileSync } from "node:fs";
           import { listChangedExtensionIds } from "./scripts/test-extension.mjs";
 
-          const extensionIds = listChangedExtensionIds({ base: process.env.BASE_SHA, head: "HEAD" });
+          const extensionIds = listChangedExtensionIds({
+            base: process.env.BASE_SHA,
+            head: "HEAD",
+            fallbackBaseRef: process.env.BASE_REF,
+            unavailableBaseBehavior: "all",
+          });
           const matrix = JSON.stringify({ include: extensionIds.map((extension) => ({ extension })) });
 
           appendFileSync(process.env.GITHUB_OUTPUT, `has_changed_extensions=${extensionIds.length > 0}\n`, "utf8");
diff --git a/scripts/test-extension.mjs b/scripts/test-extension.mjs
index 4d9f7a9575e..b92d54ca70c 100644
--- a/scripts/test-extension.mjs
+++ b/scripts/test-extension.mjs
@@ -11,6 +11,15 @@ const __dirname = path.dirname(__filename);
 const repoRoot = path.resolve(__dirname, "..");
 const pnpm = "pnpm";
 
+function runGit(args, options = {}) {
+  return execFileSync("git", args, {
+    cwd: repoRoot,
+    stdio: ["ignore", "pipe", "pipe"],
+    encoding: "utf8",
+    ...options,
+  });
+}
+
 function normalizeRelative(inputPath) {
   return inputPath.split(path.sep).join("/");
 }
@@ -46,16 +55,53 @@ function collectTestFiles(rootPath) {
   return results.toSorted((left, right) => left.localeCompare(right));
 }
 
+function hasGitCommit(ref) {
+  if (!ref || /^0+$/.test(ref)) {
+    return false;
+  }
+
+  try {
+    runGit(["rev-parse", "--verify", `${ref}^{commit}`]);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+function resolveChangedPathsBase(params = {}) {
+  const base = params.base;
+  const head = params.head ?? "HEAD";
+  const fallbackBaseRef = params.fallbackBaseRef;
+
+  if (hasGitCommit(base)) {
+    return base;
+  }
+
+  if (fallbackBaseRef) {
+    const remoteBaseRef = fallbackBaseRef.startsWith("origin/")
+      ? fallbackBaseRef
+      : `origin/${fallbackBaseRef}`;
+    if (hasGitCommit(remoteBaseRef)) {
+      const mergeBase = runGit(["merge-base", remoteBaseRef, head]).trim();
+      if (hasGitCommit(mergeBase)) {
+        return mergeBase;
+      }
+    }
+  }
+
+  if (!base) {
+    throw new Error("A git base revision is required to list changed extensions.");
+  }
+
+  throw new Error(`Git base revision is unavailable locally: ${base}`);
+}
+
 function listChangedPaths(base, head = "HEAD") {
   if (!base) {
     throw new Error("A git base revision is required to list changed extensions.");
   }
 
-  return execFileSync("git", ["diff", "--name-only", base, head], {
-    cwd: repoRoot,
-    stdio: ["ignore", "pipe", "pipe"],
-    encoding: "utf8",
-  })
+  return runGit(["diff", "--name-only", base, head])
     .split("\n")
     .map((line) => line.trim())
     .filter((line) => line.length > 0);
@@ -107,9 +153,21 @@ export function detectChangedExtensionIds(changedPaths) {
 }
 
 export function listChangedExtensionIds(params = {}) {
-  const base = params.base;
   const head = params.head ?? "HEAD";
-  return detectChangedExtensionIds(listChangedPaths(base, head));
+  const unavailableBaseBehavior = params.unavailableBaseBehavior ?? "error";
+
+  try {
+    const base = resolveChangedPathsBase(params);
+    return detectChangedExtensionIds(listChangedPaths(base, head));
+  } catch (error) {
+    if (unavailableBaseBehavior === "all") {
+      return listAvailableExtensionIds();
+    }
+    if (unavailableBaseBehavior === "empty") {
+      return [];
+    }
+    throw error;
+  }
 }
 
 function resolveExtensionDirectory(targetArg, cwd = process.cwd()) {
diff --git a/test/scripts/test-extension.test.ts b/test/scripts/test-extension.test.ts
index 67adc06d2b2..6b905197062 100644
--- a/test/scripts/test-extension.test.ts
+++ b/test/scripts/test-extension.test.ts
@@ -4,6 +4,7 @@ import { describe, expect, it } from "vitest";
 import {
   detectChangedExtensionIds,
   listAvailableExtensionIds,
+  listChangedExtensionIds,
   resolveExtensionTestPlan,
 } from "../../scripts/test-extension.mjs";
 
@@ -79,6 +80,15 @@ describe("scripts/test-extension.mjs", () => {
     );
   });
 
+  it("can fail safe to all extensions when the base revision is unavailable", () => {
+    const extensionIds = listChangedExtensionIds({
+      base: "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef",
+      unavailableBaseBehavior: "all",
+    });
+
+    expect(extensionIds).toEqual(listAvailableExtensionIds());
+  });
+
   it("dry-run still reports a plan for extensions without tests", () => {
     const plan = readPlan(["copilot-proxy"]);