fix(ci): harden changed extension diff fallback

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()) {