feat(std/io): add LimitedReader (#6026)

std/io/readers.ts

@@ -1,4 +1,10 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+// Based on https://github.com/golang/go/blob/0452f9460f50f0f0aba18df43dc2b31906fb66cc/src/io/io.go
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 type Reader = Deno.Reader;
 import { encode } from "../encoding/utf8.ts";
 
@@ -40,3 +46,30 @@ export class MultiReader implements Reader {
     return result;
   }
 }
+
+/**
+ * A `LimitedReader` reads from `reader` but limits the amount of data returned to just `limit` bytes.
+ * Each call to `read` updates `limit` to reflect the new amount remaining.
+ * `read` returns `null` when `limit` <= `0` or
+ * when the underlying `reader` returns `null`.
+ */
+export class LimitedReader implements Deno.Reader {
+  constructor(public reader: Deno.Reader, public limit: number) {}
+
+  async read(p: Uint8Array): Promise<number | null> {
+    if (this.limit <= 0) {
+      return null;
+    }
+
+    if (p.length > this.limit) {
+      p = p.subarray(0, this.limit);
+    }
+    const n = await this.reader.read(p);
+    if (n == null) {
+      return null;
+    }
+
+    this.limit -= n;
+    return n;
+  }
+}

std/io/readers_test.ts

@@ -1,6 +1,6 @@
 const { copy, test } = Deno;
 import { assertEquals } from "../testing/asserts.ts";
-import { MultiReader, StringReader } from "./readers.ts";
+import { LimitedReader, MultiReader, StringReader } from "./readers.ts";
 import { StringWriter } from "./writers.ts";
 import { copyN } from "./ioutil.ts";
 import { decode } from "../encoding/utf8.ts";
@@ -36,3 +36,28 @@ test("ioMultiReader", async function (): Promise<void> {
   await copy(r, w);
   assertEquals(w.toString(), "abcdef");
 });
+
+test("ioLimitedReader", async function (): Promise<void> {
+  let sr = new StringReader("abc");
+  let r = new LimitedReader(sr, 2);
+  let buffer = await Deno.readAll(r);
+  assertEquals(decode(buffer), "ab");
+  assertEquals(decode(await Deno.readAll(sr)), "c");
+  sr = new StringReader("abc");
+  r = new LimitedReader(sr, 3);
+  buffer = await Deno.readAll(r);
+  assertEquals(decode(buffer), "abc");
+  assertEquals((await Deno.readAll(r)).length, 0);
+  sr = new StringReader("abc");
+  r = new LimitedReader(sr, 4);
+  buffer = await Deno.readAll(r);
+  assertEquals(decode(buffer), "abc");
+  assertEquals((await Deno.readAll(r)).length, 0);
+});
+
+test("ioLimitedReader", async function (): Promise<void> {
+  const rb = new StringReader("abc");
+  const wb = new StringWriter();
+  await Deno.copy(new LimitedReader(rb, -1), wb);
+  assertEquals(wb.toString(), "");
+});