fix(hosting): never serve stale placeholder config.json after deploy (#174)

Author: fossamagna

.changeset/stale-placeholder-config-json.md

@@ -0,0 +1,5 @@
+---
+"@aws-blocks/core": patch
+---
+
+Never serve a stale placeholder `config.json` after a deploy. The build-time placeholder (`{"_placeholder":true}`) was uploaded with the 1-year mutable cache-control (`public, s-maxage=31536000, max-age=0, must-revalidate`), and the post-deploy CloudFront invalidation targeted `/.blocks-sandbox/*` — which never matches the real cache key, because the skew-protection viewer-request function rewrites the URI to `/builds/<buildId>/.blocks-sandbox/config.json` before the cache lookup. An edge that cached the placeholder during the deploy window could keep serving it for up to a year, making `getApiUrl()` throw and every client API call fail. The placeholder is now registered as a no-cache path (`no-cache, no-store, must-revalidate`) so edges never cache it long-term, and the config deployment now also invalidates the post-rewrite key `/builds/<buildId>/.blocks-sandbox/*`.

packages/core/src/hosting.test.ts

@@ -1507,4 +1507,111 @@ describe('Hosting', () => {
       );
     });
   });
+
+  describe('config.json stale-placeholder guard (#173)', () => {
+    // Helper: pull every BucketDeployment custom resource's Properties.
+    const bucketDeployments = (stack: Stack) => {
+      const crs = Template.fromStack(stack).findResources(
+        'Custom::CDKBucketDeployment',
+      );
+      return Object.values(crs).map(
+        (cr) => (cr as { Properties: Record<string, unknown> }).Properties,
+      );
+    };
+
+    it('uploads the placeholder config.json with a no-cache directive, never the 1-year mutable cache-control', () => {
+      // The build-time placeholder (`{_placeholder:true}`) is written into the
+      // static dir. If it inherits the mutable asset tier's
+      // `s-maxage=31536000` and an edge caches it during the deploy window, the
+      // edge serves the placeholder for up to a year — breaking every client
+      // API call. It must instead be uploaded as a no-cache path so the edge
+      // never caches it long-term.
+      createSpaBuildOutput(tmpDir);
+      const app = new App();
+      const stack = new Stack(app, 'PlaceholderCacheStack');
+      new Hosting(stack, 'Hosting', { root: tmpDir, api: MOCK_API });
+
+      const deployments = bucketDeployments(stack);
+      const cc = (p: Record<string, unknown>) =>
+        (p.SystemMetadata as Record<string, string> | undefined)?.['cache-control'];
+      const includes = (p: Record<string, unknown>) =>
+        (p.Include as string[] | undefined) ?? [];
+      const excludes = (p: Record<string, unknown>) =>
+        (p.Exclude as string[] | undefined) ?? [];
+
+      // A deployment must upload `.blocks-sandbox/config.json` with the
+      // no-cache directive (this is the placeholder upload).
+      const noCacheDeploy = deployments.find(
+        (p) =>
+          includes(p).includes('.blocks-sandbox/config.json') &&
+          cc(p) === 'no-cache, no-store, must-revalidate',
+      );
+      assert.ok(
+        noCacheDeploy,
+        'placeholder .blocks-sandbox/config.json must be uploaded with ' +
+          '"no-cache, no-store, must-revalidate"',
+      );
+
+      // No deployment may cover the placeholder with the 1-year mutable
+      // cache-control: the mutable/other tier must EXCLUDE it.
+      const leaks = deployments.filter((p) => {
+        const directive = cc(p);
+        return (
+          typeof directive === 'string' &&
+          directive.includes('s-maxage=31536000') &&
+          !excludes(p).includes('.blocks-sandbox/config.json')
+        );
+      });
+      assert.strictEqual(
+        leaks.length,
+        0,
+        'no mutable-tier deployment may apply s-maxage=31536000 to ' +
+          '.blocks-sandbox/config.json',
+      );
+    });
+
+    it('registers the placeholder as a no-cache path even for a static-only site (no api)', () => {
+      // The placeholder is always written (step 5), so its no-cache
+      // registration must not depend on `props.api`. This guards against a
+      // regression that moves the registration inside an `if (props.api)`
+      // block, which would reopen the stale-placeholder window for
+      // static-only sites.
+      createSpaBuildOutput(tmpDir);
+      const app = new App();
+      const stack = new Stack(app, 'StaticOnlyPlaceholderStack');
+      new Hosting(stack, 'Hosting', { root: tmpDir });
+
+      Template.fromStack(stack).hasResourceProperties(
+        'Custom::CDKBucketDeployment',
+        Match.objectLike({
+          Include: Match.arrayWith(['.blocks-sandbox/config.json']),
+          SystemMetadata: Match.objectLike({
+            'cache-control': 'no-cache, no-store, must-revalidate',
+          }),
+        }),
+      );
+    });
+
+    it('invalidates the post-rewrite cache key (/builds/<id>/.blocks-sandbox/*)', () => {
+      // The viewer-request skew-protection function rewrites the URI to
+      // `/builds/<buildId>/.blocks-sandbox/config.json` BEFORE the cache
+      // lookup, so the real edge cache key lives under `/builds/<id>/`.
+      // Invalidating only `/.blocks-sandbox/*` never matches it.
+      createSpaBuildOutput(tmpDir);
+      const app = new App();
+      const stack = new Stack(app, 'InvalidationPathStack');
+      new Hosting(stack, 'Hosting', { root: tmpDir, api: MOCK_API });
+
+      // Assert via Template + Match so a CDK property rename fails loudly here
+      // rather than silently skipping a structural-heuristic lookup.
+      Template.fromStack(stack).hasResourceProperties(
+        'Custom::CDKBucketDeployment',
+        Match.objectLike({
+          DistributionPaths: Match.arrayWith([
+            Match.stringLikeRegexp('^/builds/.+/\\.blocks-sandbox/\\*$'),
+          ]),
+        }),
+      );
+    });
+  });
 });

packages/core/src/hosting.ts

@@ -35,6 +35,7 @@ import type {
   FrameworkType,
 } from '@aws-blocks/hosting';
 import { BLOCKS_RPC_PREFIX, BLOCKS_AUTH_PREFIX } from './constants.js';
+import { BLOCKS_SANDBOX_DIR } from './common/constants.js';
 import { registerConfig } from './cdk/config-registry.js';
 import { getRegisteredRoutes } from './raw-route.js';
 
@@ -509,6 +510,19 @@ export class Hosting extends Construct {
       JSON.stringify({ _placeholder: true }),
     );
 
+    // ── 5a. Mark config.json as a no-cache path ─────────────────────
+    //    config.json is a fixed-name, mutable runtime-config file: it must
+    //    NOT inherit the content-hashed mutable-asset cache-control
+    //    (`s-maxage=31536000`) applied to `/assets/<hash>.js`. Registering it
+    //    as a no-cache path uploads the build-time placeholder with
+    //    `no-cache, no-store, must-revalidate` so an edge never caches it
+    //    long-term; the real config is deployed in step 8 with `max-age=60`.
+    const configNoCachePath = `${BLOCKS_SANDBOX_DIR}/config.json`;
+    const existingNoCachePaths = manifest.staticAssets.noCachePaths ?? [];
+    manifest.staticAssets.noCachePaths = existingNoCachePaths.includes(configNoCachePath)
+      ? existingNoCachePaths
+      : [...existingNoCachePaths, configNoCachePath];
+
     // ── 5b. Inject static route for .blocks-sandbox config ──────────
     //    Insert a static route for /.blocks-sandbox/* so CloudFront
     //    serves config.json from S3 instead of routing to compute.
@@ -606,7 +620,18 @@ export class Hosting extends Construct {
         destinationKeyPrefix: `builds/${buildId}/.blocks-sandbox`,
         prune: false,
         distribution: hosting.distribution,
-        distributionPaths: ['/.blocks-sandbox/*'],
+        // The skew-protection viewer-request CloudFront function rewrites the
+        // URI to `/builds/<buildId>/.blocks-sandbox/config.json` BEFORE the
+        // cache lookup, so the real edge cache key lives under `/builds/<id>/`.
+        // Invalidating only `/.blocks-sandbox/*` never matches that key and is
+        // a no-op for config.json. Invalidate the post-rewrite key too. (The
+        // primary guard against staleness is step 5a's no-cache placeholder;
+        // this is defense-in-depth so a post-deploy invalidation actually
+        // clears any edge entry at its real key.)
+        distributionPaths: [
+          `/builds/${buildId}/.blocks-sandbox/*`,
+          '/.blocks-sandbox/*',
+        ],
         cacheControl: [s3deploy.CacheControl.fromString('public, max-age=60, must-revalidate')],
       });