fix: edge case in toAsyncIterator (#2335)

js/files_test.ts

@@ -29,6 +29,38 @@ testPerm({ read: true }, async function filesToAsyncIterator(): Promise<void> {
   assertEquals(totalSize, 12);
 });
 
+test(async function readerToAsyncIterator(): Promise<void> {
+  // ref: https://github.com/denoland/deno/issues/2330
+  const encoder = new TextEncoder();
+
+  class TestReader implements Deno.Reader {
+    private offset = 0;
+    private buf = new Uint8Array(encoder.encode(this.s));
+
+    constructor(private readonly s: string) {}
+
+    async read(p: Uint8Array): Promise<Deno.ReadResult> {
+      const n = Math.min(p.byteLength, this.buf.byteLength - this.offset);
+      p.set(this.buf.slice(this.offset, this.offset + n));
+      this.offset += n;
+
+      return {
+        nread: n,
+        eof: this.offset === this.buf.byteLength
+      };
+    }
+  }
+
+  const reader = new TestReader("hello world!");
+
+  let totalSize = 0;
+  for await (const buf of Deno.toAsyncIterator(reader)) {
+    totalSize += buf.byteLength;
+  }
+
+  assertEquals(totalSize, 12);
+});
+
 testPerm({ write: false }, async function writePermFailure(): Promise<void> {
   const filename = "tests/hello.txt";
   const writeModes: Deno.OpenMode[] = ["w", "a", "x"];

js/io.ts

@@ -144,6 +144,14 @@ export async function copy(dst: Writer, src: Reader): Promise<number> {
  */
 export function toAsyncIterator(r: Reader): AsyncIterableIterator<Uint8Array> {
   const b = new Uint8Array(1024);
+  // Keep track if end-of-file has been reached, then
+  // signal that iterator is done during subsequent next()
+  // call. This is required because `r` can return a `ReadResult`
+  // with data read and EOF reached. But if iterator returns
+  // `done` then `value` is discarded.
+  //
+  // See https://github.com/denoland/deno/issues/2330 for reference.
+  let sawEof = false;
 
   return {
     [Symbol.asyncIterator](): AsyncIterableIterator<Uint8Array> {
@@ -151,10 +159,16 @@ export function toAsyncIterator(r: Reader): AsyncIterableIterator<Uint8Array> {
     },
 
     async next(): Promise<IteratorResult<Uint8Array>> {
+      if (sawEof) {
+        return { value: new Uint8Array(), done: true };
+      }
+
       const result = await r.read(b);
+      sawEof = result.eof;
+
       return {
         value: b.subarray(0, result.nread),
-        done: result.eof
+        done: false
       };
     }
   };