deepseek-harness/packages/host/apiproxy/src/fetch/handler.ts

/**
 * Server side of the fetch carrier: maps an ApiProxy onto a pure
 * WHATWG Request->Response function. Two-level parse: full form (type/rpcId/method +
 * path==method) -> payload dispatched per method. HTTP status expresses only the carrier
 * (404 unknown path / 400 non-JSON body / 500 handler crash); business errors are always
 * 200 + ServerResponse.
 */

import { randomUUID } from 'node:crypto'
import type { z } from 'zod'
import type { ApiProxy, MuxFrame, HostFrame } from '../api/index.ts'
import type { RequestPayload, ResponseValue, RpcMethodMap } from '../api/rpc-map.ts'
import type { ClientRequest, RpcError, RpcRequest, RpcResponse, ServerRequest, ServerResponse } from '../api/rpc.ts'
import { RpcId } from '../api/rpc.ts'
import type { Wire } from '../api/rpc.schema.ts'
import { clientRequestSchema, clientResponseSchema } from '../api/rpc.schema.ts'
import {
  sessionCancelRequestSchema,
  sessionCreateRequestSchema,
  sessionHistoryRequestSchema,
  sessionListRequestSchema,
  sessionModelsRequestSchema,
  sessionPromptRequestSchema,
  sessionSelectModelRequestSchema,
} from '../api/sessions.schema.ts'
import { hostDescribeRequestSchema } from '../api/host.schema.ts'
import {
  workspaceCreateRequestSchema,
  workspaceInsertSessionBeforeRequestSchema,
  workspaceListRequestSchema,
  workspaceRenameRequestSchema,
} from '../api/workspace.schema.ts'

/**
 * Unary dispatch table, keyed by (and compiler-locked to) RpcMethodMap: a map row without a
 * route row fails to compile, and each row's schema/invoke pair is checked against that row's
 * payload type — a schema pasted onto the wrong row is a type error, not a runtime surprise.
 * Schemas anchor to the Wire<> widening (the repo-wide exactOptionalPropertyTypes accommodation
 * documented on Wire); the dispatch point carries the one Wire→exact cast.
 */
type UnaryRoutes = {
  [K in keyof RpcMethodMap]: {
    schema: z.ZodType<Wire<RequestPayload<K>>>
    invoke(api: ApiProxy, request: RpcRequest<RequestPayload<K>>): Promise<RpcResponse<ResponseValue<K>>>
  }
}

const UNARY_ROUTES: UnaryRoutes = {
  'session.list': { schema: sessionListRequestSchema, invoke: (api, r) => api.sessions.list(r) },
  'session.create': { schema: sessionCreateRequestSchema, invoke: (api, r) => api.sessions.create(r) },
  'session.history': { schema: sessionHistoryRequestSchema, invoke: (api, r) => api.sessions.history(r) },
  'session.models': { schema: sessionModelsRequestSchema, invoke: (api, r) => api.sessions.models(r) },
  'session.selectModel': { schema: sessionSelectModelRequestSchema, invoke: (api, r) => api.sessions.selectModel(r) },
  'session.prompt': { schema: sessionPromptRequestSchema, invoke: (api, r) => api.sessions.prompt(r) },
  'session.cancel': { schema: sessionCancelRequestSchema, invoke: (api, r) => api.sessions.cancel(r) },
  'host.describe': { schema: hostDescribeRequestSchema, invoke: (api, r) => api.host.describe(r) },
  'workspace.list': { schema: workspaceListRequestSchema, invoke: (api, r) => api.workspace.list(r) },
  'workspace.create': { schema: workspaceCreateRequestSchema, invoke: (api, r) => api.workspace.create(r) },
  'workspace.rename': { schema: workspaceRenameRequestSchema, invoke: (api, r) => api.workspace.rename(r) },
  'workspace.insertSessionBefore': { schema: workspaceInsertSessionBeforeRequestSchema, invoke: (api, r) => api.workspace.insertSessionBefore(r) },
}

/** Route lookup that narrows an arbitrary path segment to a map key (single cast point for the string→key refinement). */
function methodFor(path: string): keyof RpcMethodMap | undefined {
  return Object.hasOwn(UNARY_ROUTES, path) ? path as keyof RpcMethodMap : undefined
}

/**
 * Sentinel rpcId for error responses to envelopes whose own rpcId is unreadable: the response
 * must still be a valid ServerResponse (a self-violating shape would turn the server's explicit
 * bad-request report into a client-side parse failure). Fixed value, documented here as wire contract.
 */
const INVALID_REQUEST_RPC_ID = RpcId('invalid-request')

/** Wrap a business error as a ServerResponse full form (rpcId backfilled; an unreadable rpcId uses the invalid-request sentinel). */
function errorResponse(rpcId: RpcId, error: RpcError): Response {
  const body: ServerResponse = { type: 'server-response', rpcId, result: { ok: false, error } }
  return Response.json(body)
}

/** Complete the impl's narrow form into a ServerResponse full form. */
function fullResponse(narrow: RpcResponse<unknown>): Response {
  const body: ServerResponse = { type: 'server-response', rpcId: narrow.rpcId, result: narrow.result }
  return Response.json(body)
}

/**
 * Parse the payload and invoke one unary route. Generic over the map key so
 * the row's schema/invoke pairing typechecks; the only cast collapses the
 * Wire<> widening back to the exact payload (undefined-valued properties and
 * absent ones are indistinguishable after JSON transport).
 */
// K appears once in the signature but ties the UNARY_ROUTES[K] row lookup to its own
// schema/invoke pairing; a union parameter degrades the row to an uninvokable intersection.
// eslint-disable-next-line @typescript-eslint/no-unnecessary-type-parameters
async function handleUnary<K extends keyof RpcMethodMap>(api: ApiProxy, method: K, message: ClientRequest): Promise<Response> {
  const route = UNARY_ROUTES[method]
  const payload = route.schema.safeParse(message.payload)
  if (!payload.success) {
    return errorResponse(message.rpcId, { code: 'bad-request', message: `invalid payload for ${method}`, details: { issues: payload.error.issues } })
  }
  try {
    return fullResponse(await route.invoke(api, { rpcId: message.rpcId, payload: payload.data }))
  } catch (error: unknown) {
    // The impl never throws business errors; reaching here means the implementation itself crashed — 500, carrier layer.
    return new Response(`handler failure: ${String(error)}`, { status: 500 })
  }
}

/** SSE frame: complete the narrow RpcRequest<frame> into a ServerRequest full form (method = frame type). */
function fullFrame(narrow: RpcRequest<MuxFrame | HostFrame>): ServerRequest {
  return { type: 'server-request', rpcId: narrow.rpcId, method: narrow.payload.type, payload: narrow.payload }
}

/**
 * Wrap a frame stream as an SSE Response; stops when req.signal aborts. An
 * impl throw mid-stream emits one stream/error frame and then closes.
 */
function sseResponse(frames: AsyncIterable<RpcRequest<MuxFrame | HostFrame>>): Response {
  const encoder = new TextEncoder()
  const stream = new ReadableStream<Uint8Array>({
    async start(controller) {
      try {
        // Send an SSE comment line on open so clients/proxies see a live channel (the host
        // stream has no baseline frames and would otherwise emit zero bytes while idle;
        // a comment line is not a frame, so client frame parsing skips it naturally).
        controller.enqueue(encoder.encode(': connected\n\n'))
        for await (const narrow of frames) {
          controller.enqueue(encoder.encode(`data: ${JSON.stringify(fullFrame(narrow))}\n\n`))
        }
      } catch (error: unknown) {
        // Mid-stream impl failure → one stream/error frame, then close: the client must see
        // the failure instead of a silent end (which reads as a normal disconnect). A fresh
        // rpcId is minted — this is a server-initiated push like any other frame.
        const failure: MuxFrame | HostFrame = { type: 'stream/error', error: { code: 'internal', message: String(error), details: {} } }
        try {
          controller.enqueue(encoder.encode(`data: ${JSON.stringify(fullFrame({ rpcId: RpcId(randomUUID()), payload: failure }))}\n\n`))
        } catch {
          // Consumer already cancelled the stream: enqueue-after-cancel is the
          // only reachable error, and there is no one left to tell.
        }
      } finally {
        try {
          controller.close()
        } catch { /* already cancelled by the consumer: a double close is the only reachable error */ }
      }
    },
  })
  return new Response(stream, {
    headers: { 'content-type': 'text/event-stream', 'cache-control': 'no-cache' },
  })
}

/**
 * Wraps an ApiProxy into a pure fetch function (isomorphic point: feed the returned fetch straight to InProcessApiClient).
 * @param api - the host-side ApiProxy implementation.
 * @returns an object holding `fetch(Request)`; paths outside /api/ return 404.
 */
export function toFetchHandler(api: ApiProxy): { fetch: typeof fetch } {
  return {
    // Signature matches global fetch: the isomorphic point hands this function to InProcessApiClient as its transport aspect,
    // Clients call in (url, init) form — normalize to Request before handling.
    async fetch(input: RequestInfo | URL, init?: RequestInit): Promise<Response> {
      const req = input instanceof Request ? input : new Request(input, init)
      const url = new URL(req.url)
      const path = url.pathname

      if (path === '/api/events.mux' && req.method === 'GET') {
        return sseResponse(api.events.mux({ rpcId: RpcId(randomUUID()), payload: {} }, req.signal))
      }
      if (path === '/api/events.host' && req.method === 'GET') {
        return sseResponse(api.events.host({ rpcId: RpcId(randomUUID()), payload: {} }, req.signal))
      }

      if (req.method !== 'POST' || !path.startsWith('/api/')) {
        return new Response('not found', { status: 404 })
      }

      let body: unknown
      try {
        body = await req.json()
      } catch {
        // 400 = carrier layer (body is not even JSON); valid JSON with a bad shape goes 200 + bad-request.
        return new Response('body is not JSON', { status: 400 })
      }

      if (path === '/api/respond') {
        const parsed = clientResponseSchema.safeParse(body)
        if (!parsed.success) return Response.json({ accepted: false, reason: 'bad-response' })
        return Response.json(await api.respond(parsed.data))
      }

      const method = methodFor(path.slice('/api/'.length))
      if (method === undefined) return new Response('not found', { status: 404 })

      const envelope = clientRequestSchema.safeParse(body)
      if (!envelope.success) {
        // Best effort at correlation: salvage a string rpcId from the raw body;
        // otherwise the fixed sentinel keeps the response a valid ServerResponse.
        const rawId = (body as { rpcId?: unknown } | null)?.rpcId
        const rpcId = typeof rawId === 'string' ? RpcId(rawId) : INVALID_REQUEST_RPC_ID
        return errorResponse(rpcId, { code: 'bad-request', message: 'invalid client-request message', details: { issues: envelope.error.issues } })
      }
      const message: ClientRequest = envelope.data
      if (message.method !== method) {
        return errorResponse(message.rpcId, { code: 'bad-request', message: `method "${message.method}" does not match path "${method}"`, details: { issues: [] } })
      }
      return handleUnary(api, method, message)
    },
  }
}