Move collectUint8Arrays() to util/async.ts, clean up, fix bugs, add tests

Original: dcad420b92

util/async.ts

@@ -83,3 +83,28 @@ export class MuxAsyncIterator<T> implements AsyncIterable<T> {
     return this.iterate();
   }
 }
+
+/** Collects all Uint8Arrays from an AsyncIterable and retuns a single
+ * Uint8Array with the concatenated contents of all the collected arrays.
+ */
+export async function collectUint8Arrays(
+  it: AsyncIterable<Uint8Array>
+): Promise<Uint8Array> {
+  const chunks = [];
+  let length = 0;
+  for await (const chunk of it) {
+    chunks.push(chunk);
+    length += chunk.length;
+  }
+  if (chunks.length === 1) {
+    // No need to copy.
+    return chunks[0];
+  }
+  const collected = new Uint8Array(length);
+  let offset = 0;
+  for (let chunk of chunks) {
+    collected.set(chunk, offset);
+    offset += chunk.length;
+  }
+  return collected;
+}