encapsulate deferred writing in WriteBuffer class

src/common/input/WriteBuffer.ts

@@ -0,0 +1,110 @@
+
+/**
+ * Copyright (c) 2019 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+declare const setTimeout: (handler: () => void, timeout?: number) => void;
+
+/**
+ * Safety watermark to avoid memory exhaustion and browser engine crash on fast data input.
+ * Enable flow control to avoid this limit and make sure that your backend correctly
+ * propagates this to the underlying pty. (see docs for further instructions)
+ * Since this limit is meant as a safety parachute to prevent browser crashs,
+ * it is set to a very high number. Typically xterm.js gets unresponsive with
+ * a 100 times lower number (>500 kB).
+ */
+const DISCARD_WATERMARK = 50000000; // ~50 MB
+
+/**
+ * The max number of ms to spend on writes before allowing the renderer to
+ * catch up with a 0ms setTimeout. A value of < 33 to keep us close to
+ * 30fps, and a value of < 16 to try to run at 60fps. Of course, the real FPS
+ * depends on the time it takes for the renderer to draw the frame.
+ */
+const WRITE_TIMEOUT_MS = 12;
+
+/**
+ * Threshold of max held chunks in the write buffer, that were already processed.
+ * This is a tradeoff between extensive write buffer shifts (bad runtime) and high
+ * memory consumption by data thats not used anymore.
+ */
+const WRITE_BUFFER_LENGTH_THRESHOLD = 50;
+
+export class WriteBuffer {
+  private _writeBuffer: (string | Uint8Array)[] = [];
+  private _callbacks: ((() => void) | undefined)[] = [];
+  private _pendingData = 0;
+  private _bufferOffset = 0;
+
+  constructor(private _action: (data: string | Uint8Array) => void) { }
+
+  public writeSync(data: string | Uint8Array): void {
+    // force sync processing on pending data chunks to avoid in-band data scrambling
+    // does the same as innerWrite but without event loop
+    if (this._writeBuffer.length) {
+      for (let i = this._bufferOffset; i < this._writeBuffer.length; ++i) {
+        const data = this._writeBuffer[i];
+        const cb = this._callbacks[i];
+        this._action(data);
+        if (cb) cb();
+      }
+      // reset all to avoid reprocessing of chunks with scheduled innerWrite call
+      this._writeBuffer = [];
+      this._callbacks = [];
+      this._pendingData = 0;
+      // stop scheduled innerWrite by offset > length condition
+      this._bufferOffset = 0x7FFFFFFF;
+    }
+    // handle current data chunk
+    this._action(data);
+  }
+
+  public write(data: string | Uint8Array, callback?: () => void): void {
+    if (this._pendingData > DISCARD_WATERMARK) {
+      throw new Error('write data discarded, use flow control to avoid losing data');
+    }
+
+    // schedule chunk processing for next event loop run
+    if (!this._writeBuffer.length) {
+      this._bufferOffset = 0;
+      setTimeout(() => this._innerWrite());
+    }
+
+    this._pendingData += data.length;
+    this._writeBuffer.push(data);
+    this._callbacks.push(callback);
+  }
+
+  protected _innerWrite(): void {
+    const startTime = Date.now();
+    while (this._writeBuffer.length > this._bufferOffset) {
+      const data = this._writeBuffer[this._bufferOffset];
+      const cb = this._callbacks[this._bufferOffset];
+      this._bufferOffset++;
+
+      this._action(data);
+      this._pendingData -= data.length;
+      if (cb) cb();
+
+      if (Date.now() - startTime >= WRITE_TIMEOUT_MS) {
+        break;
+      }
+    }
+    if (this._writeBuffer.length > this._bufferOffset) {
+      // Allow renderer to catch up before processing the next batch
+      // trim already processed chunks if we are above threshold
+      if (this._bufferOffset > WRITE_BUFFER_LENGTH_THRESHOLD) {
+        this._writeBuffer = this._writeBuffer.slice(this._bufferOffset);
+        this._callbacks = this._callbacks.slice(this._bufferOffset);
+        this._bufferOffset = 0;
+      }
+      setTimeout(() => this._innerWrite(), 0);
+    } else {
+      this._writeBuffer = [];
+      this._callbacks = [];
+      this._pendingData = 0;
+      this._bufferOffset = 0;
+    }
+  }
+}