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//
// Why not native EventSource?
// 1. EventSource only supports GET. Compose bodies (briefs) must go in a
// request body — never a query string — because Cloudflare Worker access
// logs capture query strings, violating the platform invariant of never
// logging message bodies or raw recipient identifiers.
// 2. EventSource hides HTTP status codes. A 401 on the stream is
// indistinguishable from a network drop. Using fetch() + ReadableStream
// lets us inspect status BEFORE consuming the stream and differentiate
// auth failure, rate limit, client error, and server error.
//
// Track C plan §6 — this is the primitive for future streaming consumers.
export type SSEEvent = {
event: string;
data: string;
id?: string;
retry?: number;
};
export type SSEErrorKind =
| "auth_expired"
| "rate_limit"
| "forbidden"
| "client_error"
| "server_error"
| "network"
| "aborted";
export class SSEError extends Error {
constructor(
public kind: SSEErrorKind,
message: string,
public status?: number,
public retryAfterSeconds?: number,
) {
super(message);
this.name = "SSEError";
}
}
export type StreamFetchOptions = {
method?: "GET" | "POST";
body?: unknown; // serialized as JSON if not undefined
headers?: Record<string, string>;
signal?: AbortSignal;
};
/**
* Open a streaming fetch and return an async iterable of parsed SSE events.
*
* Throws SSEError synchronously (inside the returned AsyncIterable) on any
* non-2xx HTTP status, network error, or abort. Consumers should iterate
* inside a try/catch.
*
* Parses standard SSE format: blocks separated by blank lines, each block
* containing `event: <name>` and/or `data: <payload>` lines. `data:` can span
* multiple lines (concatenated with newline per the SSE spec).
*/
export async function* streamSSE(
url: string,
options: StreamFetchOptions = {},
): AsyncIterable<SSEEvent> {
const { method = "POST", body, headers = {}, signal } = options;
let response: Response;
try {
response = await fetch(url, {
method,
headers: {
"Content-Type": "application/json",
Accept: "text/event-stream",
...headers,
},
body: body === undefined ? undefined : JSON.stringify(body),
credentials: "include",
signal,
});
} catch (err) {
if (err instanceof DOMException && err.name === "AbortError") {
throw new SSEError("aborted", "Stream aborted before start");
}
throw new SSEError(
"network",
err instanceof Error ? err.message : "Network error",
);
}
if (!response.ok) {
throw mapHttpErrorToSSEError(response);
}
if (!response.body) {
throw new SSEError("server_error", "Response has no body", response.status);
}
const reader = response.body
.pipeThrough(new TextDecoderStream())
.getReader();
let buffer = "";
try {
while (true) {
const { done, value } = await reader.read();
if (done) {
// Flush any trailing event if buffer isn't empty. Spec says events
// are terminated by a blank line, but tolerate missing trailer.
if (buffer.trim().length > 0) {
const evt = parseSSEBlock(buffer);
Eif (evt) yield evt;
}
return;
}
buffer += value;
// Process complete events (delimited by blank line: \n\n or \r\n\r\n).
let sepIdx = findEventSeparator(buffer);
while (sepIdx !== -1) {
const block = buffer.slice(0, sepIdx);
buffer = buffer.slice(sepIdx + separatorLength(buffer, sepIdx));
const evt = parseSSEBlock(block);
if (evt) yield evt;
sepIdx = findEventSeparator(buffer);
}
}
} catch (err) {
if (err instanceof DOMException && err.name === "AbortError") {
throw new SSEError("aborted", "Stream aborted");
}
Iif (err instanceof SSEError) throw err;
throw new SSEError(
"network",
err instanceof Error ? err.message : "Stream read error",
);
} finally {
try {
reader.releaseLock();
} catch {
// ignore
}
}
}
function mapHttpErrorToSSEError(response: Response): SSEError {
const { status } = response;
if (status === 401) {
return new SSEError("auth_expired", "Session expired", status);
}
if (status === 403) {
return new SSEError("forbidden", "Not authorized", status);
}
if (status === 429) {
const retryAfter = response.headers.get("Retry-After");
const seconds = retryAfter ? parseInt(retryAfter, 10) : undefined;
return new SSEError(
"rate_limit",
"Rate limit exceeded",
status,
Number.isFinite(seconds) ? seconds : undefined,
);
}
if (status >= 400 && status < 500) {
return new SSEError("client_error", `Client error ${status}`, status);
}
return new SSEError("server_error", `Server error ${status}`, status);
}
function findEventSeparator(buf: string): number {
// Find the earliest of "\n\n" or "\r\n\r\n".
const lf = buf.indexOf("\n\n");
const crlf = buf.indexOf("\r\n\r\n");
if (lf === -1) return crlf;
Eif (crlf === -1) return lf;
return Math.min(lf, crlf);
}
function separatorLength(buf: string, sepIdx: number): number {
return buf.startsWith("\r\n\r\n", sepIdx) ? 4 : 2;
}
function parseSSEBlock(block: string): SSEEvent | null {
const lines = block.split(/\r?\n/);
let event = "message";
const dataParts: string[] = [];
let id: string | undefined;
let retry: number | undefined;
for (const line of lines) {
if (!line || line.startsWith(":")) continue; // comment or empty
const colonIdx = line.indexOf(":");
if (colonIdx === -1) continue;
const field = line.slice(0, colonIdx);
// Strip at most one leading space from value per the SSE spec.
let value = line.slice(colonIdx + 1);
Eif (value.startsWith(" ")) value = value.slice(1);
switch (field) {
case "event":
event = value;
break;
case "data":
dataParts.push(value);
break;
case "id":
id = value;
break;
case "retry": {
const n = parseInt(value, 10);
if (Number.isFinite(n)) retry = n;
break;
}
default:
// Unknown fields are ignored per spec.
break;
}
}
if (dataParts.length === 0) return null;
return {
event,
data: dataParts.join("\n"),
id,
retry,
};
}
|