An initial implementation of the cache API (#428)

* Initial cache implementation

* Address PR comments

* Address comments

packages/tre/package.json

@@ -41,6 +41,7 @@
     "exit-hook": "^2.2.1",
     "get-port": "^5.1.1",
     "glob-to-regexp": "^0.4.1",
+    "http-cache-semantics": "^4.1.0",
     "kleur": "^4.1.5",
     "stoppable": "^1.1.0",
     "undici": "^5.10.0",
@@ -51,6 +52,7 @@
     "@types/debug": "^4.1.7",
     "@types/estree": "^1.0.0",
     "@types/glob-to-regexp": "^0.4.1",
-    "@types/stoppable": "^1.1.1"
+    "@types/stoppable": "^1.1.1",
+    "@types/http-cache-semantics": "^4.0.1"
   }
 }

packages/tre/src/plugins/cache/README.md

@@ -0,0 +1,42 @@
+# The Cache API
+The Cache API within `workerd` is _extremely_ lenient with errors, and will work with a service that doesn't fully support the expected API. This is what it _should_ behave like:
+
+## .add()
+Unimplemented in the runtime
+
+## .addAll()
+Unimplemented in the runtime
+
+## .match()
+`workerd` guarantees:
+- The method will always be `GET`
+- The request headers will include `Cache-Control: only-if-cached` (which Miniflare ignores)
+- The request headers will include `Cf-Cache-Namespaces` if this is a namespaced cache (i.e. `caches.open(...)`)
+`workerd` expects:
+- The `Cf-Cache-Status` header to be present with the value:
+  - `MISS` if it's a cache miss, in which case the rest of the response is ignored by `workerd`
+  - `HIT` if it's a cache hit, in which case `workerd` sends the response on to the user, including the full headers and full body
+
+## .put()
+`workerd` guarantees:
+- The method will always be `PUT`, and the cache key method will always be `GET`
+- The headers will be the headers of the cache key, and the URL will be the URL of the cache key
+- The headers will include `Cf-Cache-Namespaces` if this is a namespaced cache (i.e. `caches.open(...)`)
+- The body contains the serialised response for storage
+  - The serialised response will never:
+    - Have a `206` status code
+    - Have a `Vary: *` header
+    - Have a `304` status code
+`workerd` expects:
+- A `204` (success) or `413` (failure) response code. It doesn't do anything with either
+
+## .delete()
+`workerd` guarantees:
+- The method will always be `PURGE`, and the cache key method will always be `GET`
+- The headers will include `Cf-Cache-Namespaces` if this is a namespaced cache (i.e. `caches.open(...)`)
+- The header `X-Real-IP` will be set to `127.0.0.1`
+- The remaining headers will be the cache key headers
+`workerd` expects:
+- Status `200` on success
+- Status `404` on failure
+- Status `429` on rate limit (which will throw in the user worker)

packages/tre/src/plugins/cache/errors.ts

@@ -0,0 +1,65 @@
+import { HeadersInit, Response } from "undici";
+import { CfHeader } from "../shared/constants";
+
+enum Status {
+  PayloadTooLarge = 413,
+  NotFound = 404,
+  CacheMiss = 504,
+}
+
+export async function fallible<T>(promise: Promise<T>): Promise<T | Response> {
+  try {
+    return await promise;
+  } catch (e) {
+    if (e instanceof CacheError) {
+      return e.toResponse();
+    }
+    throw e;
+  }
+}
+
+export class CacheError extends Error {
+  constructor(
+    private status: number,
+    message: string,
+    readonly headers: HeadersInit = []
+  ) {
+    super(message);
+    this.name = "CacheError";
+  }
+
+  toResponse() {
+    return new Response(null, {
+      status: this.status,
+      headers: this.headers,
+    });
+  }
+
+  context(info: string) {
+    this.message += ` (${info})`;
+    return this;
+  }
+}
+
+export class StorageFailure extends CacheError {
+  constructor() {
+    super(Status.PayloadTooLarge, "Cache storage failed");
+  }
+}
+
+export class PurgeFailure extends CacheError {
+  constructor() {
+    super(Status.NotFound, "Couldn't find asset to purge");
+  }
+}
+
+export class CacheMiss extends CacheError {
+  constructor() {
+    super(
+      // workerd ignores this, but it's the correct status code
+      Status.CacheMiss,
+      "Asset not found in cache",
+      [[CfHeader.CacheStatus, "MISS"]]
+    );
+  }
+}