fix: select rollback artifact pairs atomically

scripts/drill-runbook.ts

@@ -292,13 +292,12 @@ export async function verifyRollbackReleaseCoherence(
 ) {
   const readArtifact = options.readArtifact ?? readJsonArtifact;
   const paths = options.paths ?? DEFAULT_ROLLBACK_PATHS;
-  const release = await releaseManifest(readArtifact, paths);
-  const runtimeValue = await readPrimaryOrFallback(
+  const artifactPair = await selectRollbackArtifactPair(
     readArtifact,
-    paths.primaryRuntime,
-    paths.fallbackRuntime,
+    paths,
   );
-  const runtimeArtifact = parseRuntimeConfigArtifact(runtimeValue);
+  const release = parseReleaseArtifact(artifactPair.release);
+  const runtimeArtifact = parseRuntimeConfigArtifact(artifactPair.runtime);
   const runtime = {
     ...runtimeArtifact,
     BUILD_ID: requireIdentity(runtimeArtifact.BUILD_ID, "runtime BUILD_ID"),
@@ -377,6 +376,75 @@ function requireIdentity(value: string | undefined, label: string): string {
   return value;
 }
 
+async function selectRollbackArtifactPair(
+  readArtifact: JsonArtifactReader,
+  paths: RollbackArtifactPaths,
+): Promise<Readonly<{ release: unknown; runtime: unknown }>> {
+  const primary = await readArtifactPair(
+    readArtifact,
+    paths.primaryRelease,
+    paths.primaryRuntime,
+  );
+  if (
+    primary.release.status === "fulfilled" &&
+    primary.runtime.status === "fulfilled"
+  ) {
+    return Object.freeze({
+      release: primary.release.value,
+      runtime: primary.runtime.value,
+    });
+  }
+
+  const releaseMissing =
+    primary.release.status === "rejected" &&
+    hasErrorCode(primary.release.reason, "ENOENT");
+  const runtimeMissing =
+    primary.runtime.status === "rejected" &&
+    hasErrorCode(primary.runtime.reason, "ENOENT");
+  if (releaseMissing && runtimeMissing) {
+    const fallback = await readArtifactPair(
+      readArtifact,
+      paths.fallbackRelease,
+      paths.fallbackRuntime,
+    );
+    if (fallback.release.status === "rejected") {
+      throw fallback.release.reason;
+    }
+    if (fallback.runtime.status === "rejected") {
+      throw fallback.runtime.reason;
+    }
+    return Object.freeze({
+      release: fallback.release.value,
+      runtime: fallback.runtime.value,
+    });
+  }
+
+  if (primary.release.status === "rejected" && !releaseMissing) {
+    throw primary.release.reason;
+  }
+  if (primary.runtime.status === "rejected" && !runtimeMissing) {
+    throw primary.runtime.reason;
+  }
+  throw new Error("primary rollback artifact pair is incomplete");
+}
+
+async function readArtifactPair(
+  readArtifact: JsonArtifactReader,
+  releasePath: string,
+  runtimePath: string,
+): Promise<
+  Readonly<{
+    release: PromiseSettledResult<unknown>;
+    runtime: PromiseSettledResult<unknown>;
+  }>
+> {
+  const [release, runtime] = await Promise.allSettled([
+    Promise.resolve().then(() => readArtifact(releasePath)),
+    Promise.resolve().then(() => readArtifact(runtimePath)),
+  ]);
+  return Object.freeze({ release, runtime });
+}
+
 async function readPrimaryOrFallback(
   readArtifact: JsonArtifactReader,
   primary: string,

tests/unit/release-coherence.test.ts

@@ -100,8 +100,10 @@ async function releaseV2With(
 
 function artifactReader(
   entries: Readonly<Record<string, unknown | Error>>,
+  reads: string[] = [],
 ): (path: string) => Promise<unknown> {
   return async (path) => {
+    reads.push(path);
     const value = entries[path];
     if (value === undefined) {
       throw Object.assign(new Error(`missing ${path}`), { code: "ENOENT" });
@@ -315,11 +317,54 @@ describe("release coherence", () => {
       }),
     ).resolves.toMatchObject({ coherence: { compatible: false } });
 
+    const releaseOnlyMissingReads: string[] = [];
     await expect(
       verifyRollbackReleaseCoherence({
-        readArtifact: artifactReader(validPublic),
+        readArtifact: artifactReader(
+          {
+            "dist/config.json": runtimeV2,
+            ...validPublic,
+          },
+          releaseOnlyMissingReads,
+        ),
+        contractPackages,
+      }),
+    ).rejects.toThrow("primary rollback artifact pair");
+    expect(releaseOnlyMissingReads).toEqual([
+      "dist/release-manifest.json",
+      "dist/config.json",
+    ]);
+
+    const runtimeOnlyMissingReads: string[] = [];
+    await expect(
+      verifyRollbackReleaseCoherence({
+        readArtifact: artifactReader(
+          {
+            "dist/release-manifest.json": exactV2,
+            ...validPublic,
+          },
+          runtimeOnlyMissingReads,
+        ),
+        contractPackages,
+      }),
+    ).rejects.toThrow("primary rollback artifact pair");
+    expect(runtimeOnlyMissingReads).toEqual([
+      "dist/release-manifest.json",
+      "dist/config.json",
+    ]);
+
+    const missingPairReads: string[] = [];
+    await expect(
+      verifyRollbackReleaseCoherence({
+        readArtifact: artifactReader(validPublic, missingPairReads),
         contractPackages,
       }),
     ).resolves.toMatchObject({ coherence: { compatible: true } });
+    expect(missingPairReads).toEqual([
+      "dist/release-manifest.json",
+      "dist/config.json",
+      "public/release-manifest.json",
+      "public/config.json",
+    ]);
   });
 });