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

1 files changed, 33 insertions, 0 deletions

diff --git a/std/io/readers.ts b/std/io/readers.ts
index 10069986c..201b87cd8 100644
--- a/std/io/readers.ts
+++ b/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;
+  }
+}