Add preliminary message backup harness

ts/util/DelimitedStream.ts

@@ -0,0 +1,91 @@
+// Copyright 2023 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { Transform } from 'stream';
+
+import { missingCaseError } from './missingCaseError';
+
+enum State {
+  Prefix = 'Prefix',
+  Data = 'Data',
+}
+
+export class DelimitedStream extends Transform {
+  private state = State.Prefix;
+  private prefixValue = 0;
+  private prefixSize = 0;
+  private parts = new Array<Buffer>();
+
+  constructor() {
+    super({ readableObjectMode: true });
+  }
+
+  override _transform(
+    chunk: Buffer,
+    _encoding: BufferEncoding,
+    done: (error?: Error) => void
+  ): void {
+    let offset = 0;
+    while (offset < chunk.length) {
+      if (this.state === State.Prefix) {
+        const b = chunk[offset];
+        offset += 1;
+
+        // See: https://protobuf.dev/programming-guides/encoding/
+        // eslint-disable-next-line no-bitwise
+        const isLast = (b & 0x80) === 0;
+        // eslint-disable-next-line no-bitwise
+        const value = b & 0x7f;
+
+        // eslint-disable-next-line no-bitwise
+        this.prefixValue |= value << (7 * this.prefixSize);
+        this.prefixSize += 1;
+
+        // Check that we didn't go over 32bits. Node.js buffers can never
+        // be larger than 2gb anyway!
+        if (this.prefixSize > 4) {
+          done(new Error('Delimiter encoding overflow'));
+          return;
+        }
+
+        if (isLast) {
+          this.state = State.Data;
+        }
+      } else if (this.state === State.Data) {
+        const toTake = Math.min(this.prefixValue, chunk.length - offset);
+        const part = chunk.slice(offset, offset + toTake);
+        offset += toTake;
+        this.prefixValue -= toTake;
+
+        this.parts.push(part);
+
+        if (this.prefixValue <= 0) {
+          this.state = State.Prefix;
+          this.prefixSize = 0;
+          this.prefixValue = 0;
+
+          const whole = Buffer.concat(this.parts);
+          this.parts = [];
+          this.push(whole);
+        }
+      } else {
+        throw missingCaseError(this.state);
+      }
+    }
+    done();
+  }
+
+  override _flush(done: (error?: Error) => void): void {
+    if (this.state !== State.Prefix) {
+      done(new Error('Unfinished data'));
+      return;
+    }
+
+    if (this.prefixSize !== 0) {
+      done(new Error('Unfinished prefix'));
+      return;
+    }
+
+    done();
+  }
+}