front_end/core/sdk/ServerSentEventsProtocol.ts

// Copyright 2023 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import type * as Protocol from '../../generated/protocol.js';

/**
 * Implements Server-Sent-Events protocl parsing as described by
 * https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream
 *
 * Webpages can use SSE over fetch/XHR and not go through EventSource. DevTools
 * only receives the raw binary data in this case, which means we have to decode
 * and parse the event stream ourselves here.
 *
 * Implementation mostly ported over from blink
 * third_party/blink/renderer/modules/eventsource/event_source_parser.cc.
 */
export class ServerSentEventsParser {
  #onEventCallback: (eventType: string, data: string, eventId: string) => void;
  #decoder: Base64TextDecoder;

  // Parser state.
  #isRecognizingCrLf = false;
  #line = '';
  #id = '';
  #data = '';
  #eventType = '';

  constructor(callback: (eventType: string, data: string, eventId: string) => void, encodingLabel?: string) {
    this.#onEventCallback = callback;
    this.#decoder = new Base64TextDecoder(this.#onTextChunk.bind(this), encodingLabel);
  }

  async addBase64Chunk(raw: Protocol.binary): Promise<void> {
    await this.#decoder.addBase64Chunk(raw);
  }

  addTextChunk(chunk: string): void {
    this.#onTextChunk(chunk);
  }

  #onTextChunk(chunk: string): void {
    // A line consists of "this.#line" plus a slice of "chunk[start:<next new cr/lf>]".
    let start = 0;
    for (let i = 0; i < chunk.length; ++i) {
      if (this.#isRecognizingCrLf && chunk[i] === '\n') {
        // We found the latter part of "\r\n".
        this.#isRecognizingCrLf = false;
        ++start;
        continue;
      }
      this.#isRecognizingCrLf = false;
      if (chunk[i] === '\r' || chunk[i] === '\n') {
        this.#line += chunk.substring(start, i);
        this.#parseLine();
        this.#line = '';
        start = i + 1;
        this.#isRecognizingCrLf = chunk[i] === '\r';
      }
    }
    this.#line += chunk.substring(start);
  }

  #parseLine(): void {
    if (this.#line.length === 0) {
      // We dispatch an event when seeing an empty line.
      if (this.#data.length > 0) {
        const data = this.#data.slice(0, -1);  // Remove the last newline.
        this.#onEventCallback(this.#eventType || 'message', data, this.#id);
        this.#data = '';
      }
      this.#eventType = '';
      return;
    }

    let fieldNameEnd = this.#line.indexOf(':');
    let fieldValueStart;
    if (fieldNameEnd < 0) {
      fieldNameEnd = this.#line.length;
      fieldValueStart = fieldNameEnd;
    } else {
      fieldValueStart = fieldNameEnd + 1;
      if (fieldValueStart < this.#line.length && this.#line[fieldValueStart] === ' ') {
        // Skip a single space preceeding the value.
        ++fieldValueStart;
      }
    }
    const fieldName = this.#line.substring(0, fieldNameEnd);
    if (fieldName === 'event') {
      this.#eventType = this.#line.substring(fieldValueStart);
      return;
    }
    if (fieldName === 'data') {
      this.#data += this.#line.substring(fieldValueStart);
      this.#data += '\n';
    }
    if (fieldName === 'id') {
      // We should do a check here whether the id field contains "\0" and ignore it.
      this.#id = this.#line.substring(fieldValueStart);
    }
    // Ignore all other fields. Also ignore "retry", we won't forward that to the backend.
  }
}

/**
 * Small helper class that can decode a stream of base64 encoded bytes. Specify the
 * text encoding for the raw bytes via constructor. Default is utf-8.
 */
class Base64TextDecoder {
  #decoder: TextDecoderStream;
  #writer: WritableStreamDefaultWriter;

  constructor(onTextChunk: (chunk: string) => void, encodingLabel?: string) {
    this.#decoder = new TextDecoderStream(encodingLabel);
    this.#writer = this.#decoder.writable.getWriter();
    void this.#decoder.readable.pipeTo(new WritableStream({write: onTextChunk}));
  }

  async addBase64Chunk(chunk: Protocol.binary): Promise<void> {
    const binString = window.atob(chunk);
    const bytes = Uint8Array.from(binString, m => m.codePointAt(0) as number);

    await this.#writer.ready;
    await this.#writer.write(bytes);
  }
}