Add a plugin API for reading HTTP response bodies

Plugins read bodies by response id through ctx.httpResponse.body()
instead of opening HttpResponse.bodyPath themselves. The accessors are
named after fetch's, minus the single-use semantics, since the bytes are
durable and re-reading should work.

Underneath is a chunked pull over the existing plugin protocol, so the
host can move bodies off the filesystem without plugins noticing.
text() now decodes with the response's charset rather than assuming
UTF-8, and the buffering accessors refuse past 32 MiB and point at
chunks().

packages/plugin-runtime-types/src/plugins/Context.ts

@@ -6,6 +6,7 @@ import type {
   GetCookieValueResponse,
   GetHttpRequestByIdRequest,
   GetHttpRequestByIdResponse,
+  GetHttpResponseBodyInfoRequest,
   JsonPrimitive,
   ListCookieNamesResponse,
   ListFoldersRequest,
@@ -65,6 +66,56 @@ type DynamicPromptFormRequest = Omit<PromptFormRequest, "inputs"> & {
 
 export type WorkspaceHandle = Pick<WorkspaceInfo, "id" | "name">;
 
+export interface ReadHttpResponseBodyOptions {
+  /**
+   * Refuse to buffer a body larger than this, in bytes. Defaults to 32 MiB.
+   * Pass `Infinity` to read whatever is there.
+   */
+  maxBytes?: number;
+
+  /** Bytes to pull from the host at a time. Defaults to 1 MiB. */
+  chunkSize?: number;
+}
+
+/**
+ * A response body, read back from wherever the host stored it.
+ *
+ * The accessors are named after `fetch`'s, but unlike `fetch` the body is not
+ * used up by reading it: these bytes are in durable storage, so every accessor
+ * can be called as many times as you like, in any order.
+ */
+export interface HttpResponseBody {
+  /** The response these bytes belong to. */
+  readonly responseId: string;
+
+  /**
+   * How many bytes are stored, which is not necessarily what the
+   * `Content-Length` header claimed. Zero when the response has no body.
+   */
+  readonly contentLength: number;
+
+  /** The response's `Content-Type` header, verbatim, or null if it had none. */
+  readonly contentType: string | null;
+
+  /**
+   * The body decoded to a string, using the charset from `contentType` and
+   * falling back to UTF-8. Throws if the body is over `maxBytes`.
+   */
+  text(options?: ReadHttpResponseBodyOptions): Promise<string>;
+
+  /** `text()`, parsed as JSON. */
+  json<T = unknown>(options?: ReadHttpResponseBodyOptions): Promise<T>;
+
+  /** The raw bytes. Throws if the body is over `maxBytes`. */
+  arrayBuffer(options?: ReadHttpResponseBodyOptions): Promise<ArrayBuffer>;
+
+  /**
+   * The raw bytes, a chunk at a time, so a body of any size can be read
+   * without holding all of it at once. Not subject to `maxBytes`.
+   */
+  chunks(options?: Pick<ReadHttpResponseBodyOptions, "chunkSize">): AsyncIterable<Uint8Array>;
+}
+
 export interface Context {
   clipboard: {
     copyText(text: string): Promise<void>;
@@ -129,6 +180,12 @@ export interface Context {
   };
   httpResponse: {
     find(args: FindHttpResponsesRequest): Promise<FindHttpResponsesResponse["httpResponses"]>;
+    /**
+     * Read a response's body by id. Where the host keeps the bytes — files on
+     * a desktop, rows in a database, somewhere else later — is not something a
+     * plugin sees or should depend on.
+     */
+    body(args: GetHttpResponseBodyInfoRequest): Promise<HttpResponseBody>;
   };
   templates: {
     render<T extends JsonValue>(args: TemplateRenderRequest & { data: T }): Promise<T>;

packages/plugin-runtime-types/src/plugins/index.ts

@@ -13,7 +13,12 @@ import type { WorkspaceActionPlugin } from "./WorkspaceActionPlugin";
 
 export type { Context };
 export type { DynamicAuthenticationArg } from "./AuthenticationPlugin";
-export type { CallPromptFormDynamicArgs, DynamicPromptFormArg } from "./Context";
+export type {
+  CallPromptFormDynamicArgs,
+  DynamicPromptFormArg,
+  HttpResponseBody,
+  ReadHttpResponseBodyOptions,
+} from "./Context";
 export type { DynamicTemplateFunctionArg } from "./TemplateFunctionPlugin";
 export type { TemplateFunctionPlugin };
 export type { FolderActionPlugin } from "./FolderActionPlugin";