@langchain/core
Version:
Core LangChain.js abstractions and schemas
235 lines (234 loc) • 9.76 kB
JavaScript
/* eslint-disable prefer-template */
/* eslint-disable default-case */
/* eslint-disable no-plusplus */
// Adapted from https://github.com/gfortaine/fetch-event-source/blob/main/src/parse.ts
// due to a packaging issue in the original.
// MIT License
import { IterableReadableStream } from "./stream.js";
export const EventStreamContentType = "text/event-stream";
/**
* Converts a ReadableStream into a callback pattern.
* @param stream The input ReadableStream.
* @param onChunk A function that will be called on each new byte chunk in the stream.
* @returns {Promise<void>} A promise that will be resolved when the stream closes.
*/
export async function getBytes(
// eslint-disable-next-line @typescript-eslint/no-explicit-any
stream, onChunk) {
// TODO: Use Async iteration for both cases?
// eslint-disable-next-line no-instanceof/no-instanceof
if (stream instanceof ReadableStream) {
const reader = stream.getReader();
// CHANGED: Introduced a "flush" mechanism to process potential pending messages when the stream ends.
// This change is essential to ensure that we capture every last piece of information from streams,
// such as those from Azure OpenAI, which may not terminate with a blank line. Without this
// mechanism, we risk ignoring a possibly significant last message.
// See https://github.com/langchain-ai/langchainjs/issues/1299 for details.
// eslint-disable-next-line no-constant-condition
while (true) {
const result = await reader.read();
if (result.done) {
onChunk(new Uint8Array(), true);
break;
}
onChunk(result.value);
}
}
else {
try {
// Handle Node.js Readable streams with async iteration
for await (const chunk of stream) {
onChunk(new Uint8Array(chunk));
}
onChunk(new Uint8Array(), true);
// eslint-disable-next-line @typescript-eslint/no-explicit-any
}
catch (e) {
throw new Error([
"Parsing event source stream failed.",
"Ensure your implementation of fetch returns a web or Node readable stream.",
`Error: ${e.message}`,
].join("\n"));
}
}
}
/**
* Parses arbitary byte chunks into EventSource line buffers.
* Each line should be of the format "field: value" and ends with \r, \n, or \r\n.
* @param onLine A function that will be called on each new EventSource line.
* @returns A function that should be called for each incoming byte chunk.
*/
export function getLines(onLine) {
let buffer;
let position; // current read position
let fieldLength; // length of the `field` portion of the line
let discardTrailingNewline = false;
// return a function that can process each incoming byte chunk:
return function onChunk(arr, flush) {
if (flush) {
onLine(arr, 0, true);
return;
}
if (buffer === undefined) {
buffer = arr;
position = 0;
fieldLength = -1;
}
else {
// we're still parsing the old line. Append the new bytes into buffer:
buffer = concat(buffer, arr);
}
const bufLength = buffer.length;
let lineStart = 0; // index where the current line starts
while (position < bufLength) {
if (discardTrailingNewline) {
if (buffer[position] === 10 /* ControlChars.NewLine */) {
lineStart = ++position; // skip to next char
}
discardTrailingNewline = false;
}
// start looking forward till the end of line:
let lineEnd = -1; // index of the \r or \n char
for (; position < bufLength && lineEnd === -1; ++position) {
switch (buffer[position]) {
case 58 /* ControlChars.Colon */:
if (fieldLength === -1) {
// first colon in line
fieldLength = position - lineStart;
}
break;
// eslint-disable-next-line @typescript-eslint/ban-ts-comment
// @ts-ignore:7029 \r case below should fallthrough to \n:
case 13 /* ControlChars.CarriageReturn */:
discardTrailingNewline = true;
// eslint-disable-next-line no-fallthrough
case 10 /* ControlChars.NewLine */:
lineEnd = position;
break;
}
}
if (lineEnd === -1) {
// We reached the end of the buffer but the line hasn't ended.
// Wait for the next arr and then continue parsing:
break;
}
// we've reached the line end, send it out:
onLine(buffer.subarray(lineStart, lineEnd), fieldLength);
lineStart = position; // we're now on the next line
fieldLength = -1;
}
if (lineStart === bufLength) {
buffer = undefined; // we've finished reading it
}
else if (lineStart !== 0) {
// Create a new view into buffer beginning at lineStart so we don't
// need to copy over the previous lines when we get the new arr:
buffer = buffer.subarray(lineStart);
position -= lineStart;
}
};
}
/**
* Parses line buffers into EventSourceMessages.
* @param onId A function that will be called on each `id` field.
* @param onRetry A function that will be called on each `retry` field.
* @param onMessage A function that will be called on each message.
* @returns A function that should be called for each incoming line buffer.
*/
export function getMessages(onMessage, onId, onRetry) {
let message = newMessage();
const decoder = new TextDecoder();
// return a function that can process each incoming line buffer:
return function onLine(line, fieldLength, flush) {
if (flush) {
if (!isEmpty(message)) {
onMessage?.(message);
message = newMessage();
}
return;
}
if (line.length === 0) {
// empty line denotes end of message. Trigger the callback and start a new message:
onMessage?.(message);
message = newMessage();
}
else if (fieldLength > 0) {
// exclude comments and lines with no values
// line is of format "<field>:<value>" or "<field>: <value>"
// https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation
const field = decoder.decode(line.subarray(0, fieldLength));
const valueOffset = fieldLength + (line[fieldLength + 1] === 32 /* ControlChars.Space */ ? 2 : 1);
const value = decoder.decode(line.subarray(valueOffset));
switch (field) {
case "data":
// if this message already has data, append the new value to the old.
// otherwise, just set to the new value:
message.data = message.data ? message.data + "\n" + value : value; // otherwise,
break;
case "event":
message.event = value;
break;
case "id":
onId?.((message.id = value));
break;
case "retry": {
const retry = parseInt(value, 10);
if (!Number.isNaN(retry)) {
// per spec, ignore non-integers
onRetry?.((message.retry = retry));
}
break;
}
}
}
};
}
function concat(a, b) {
const res = new Uint8Array(a.length + b.length);
res.set(a);
res.set(b, a.length);
return res;
}
function newMessage() {
// data, event, and id must be initialized to empty strings:
// https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation
// retry should be initialized to undefined so we return a consistent shape
// to the js engine all the time: https://mathiasbynens.be/notes/shapes-ics#takeaways
return {
data: "",
event: "",
id: "",
retry: undefined,
};
}
export function convertEventStreamToIterableReadableDataStream(stream, onMetadataEvent) {
const dataStream = new ReadableStream({
async start(controller) {
const enqueueLine = getMessages((msg) => {
if (msg.event === "error") {
throw new Error(msg.data ?? "Unspecified event streaming error.");
}
else if (msg.event === "metadata") {
onMetadataEvent?.(msg);
}
else {
if (msg.data)
controller.enqueue(msg.data);
}
});
const onLine = (line, fieldLength, flush) => {
enqueueLine(line, fieldLength, flush);
if (flush)
controller.close();
};
await getBytes(stream, getLines(onLine));
},
});
return IterableReadableStream.fromReadableStream(dataStream);
}
function isEmpty(message) {
return (message.data === "" &&
message.event === "" &&
message.id === "" &&
message.retry === undefined);
}