fix(std/io): BufWriter/StringWriter bug (#6247)

4 files changed, 49 insertions, 2 deletions

diff --git a/std/io/bufio.ts b/std/io/bufio.ts
index 4ebec13e1..e4759dcb7 100644
--- a/std/io/bufio.ts
+++ b/std/io/bufio.ts
@@ -486,6 +486,7 @@ export class BufWriter extends AbstractBufBase implements Writer {
 
     this.checkBytesWritten(numBytesWritten);
 
+    this.buf = new Uint8Array(this.buf.length);
     this.usedBufferBytes = 0;
   }
 
@@ -580,6 +581,7 @@ export class BufWriterSync extends AbstractBufBase implements WriterSync {
 
     this.checkBytesWritten(numBytesWritten);
 
+    this.buf = new Uint8Array(this.buf.length);
     this.usedBufferBytes = 0;
   }
 
diff --git a/std/io/bufio_test.ts b/std/io/bufio_test.ts
index b03640d52..92119e4db 100644
--- a/std/io/bufio_test.ts
+++ b/std/io/bufio_test.ts
@@ -18,6 +18,7 @@ import {
 } from "./bufio.ts";
 import * as iotest from "./_iotest.ts";
 import { StringReader } from "./readers.ts";
+import { StringWriter } from "./writers.ts";
 import { charCode, copyBytes } from "./util.ts";
 
 const encoder = new TextEncoder();
@@ -469,3 +470,33 @@ Deno.test(
     );
   }
 );
+
+Deno.test({
+  name: "Reset buffer after flush",
+  async fn(): Promise<void> {
+    const stringWriter = new StringWriter();
+    const bufWriter = new BufWriter(stringWriter);
+    const encoder = new TextEncoder();
+    await bufWriter.write(encoder.encode("hello\nworld\nhow\nare\nyou?\n\n"));
+    await bufWriter.flush();
+    await bufWriter.write(encoder.encode("foobar\n\n"));
+    await bufWriter.flush();
+    const actual = stringWriter.toString();
+    assertEquals(actual, "hello\nworld\nhow\nare\nyou?\n\nfoobar\n\n");
+  },
+});
+
+Deno.test({
+  name: "Reset buffer after flush sync",
+  fn(): void {
+    const stringWriter = new StringWriter();
+    const bufWriter = new BufWriterSync(stringWriter);
+    const encoder = new TextEncoder();
+    bufWriter.writeSync(encoder.encode("hello\nworld\nhow\nare\nyou?\n\n"));
+    bufWriter.flush();
+    bufWriter.writeSync(encoder.encode("foobar\n\n"));
+    bufWriter.flush();
+    const actual = stringWriter.toString();
+    assertEquals(actual, "hello\nworld\nhow\nare\nyou?\n\nfoobar\n\n");
+  },
+});
diff --git a/std/io/writers.ts b/std/io/writers.ts
index 8e085c84b..d42f552e8 100644
--- a/std/io/writers.ts
+++ b/std/io/writers.ts
@@ -1,9 +1,10 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 type Writer = Deno.Writer;
+type WriterSync = Deno.WriterSync;
 import { decode, encode } from "../encoding/utf8.ts";
 
 /** Writer utility for buffering string chunks */
-export class StringWriter implements Writer {
+export class StringWriter implements Writer, WriterSync {
   private chunks: Uint8Array[] = [];
   private byteLength = 0;
   private cache: string | undefined;
@@ -15,10 +16,14 @@ export class StringWriter implements Writer {
   }
 
   write(p: Uint8Array): Promise<number> {
+    return Promise.resolve(this.writeSync(p));
+  }
+
+  writeSync(p: Uint8Array): number {
     this.chunks.push(p);
     this.byteLength += p.byteLength;
     this.cache = undefined;
-    return Promise.resolve(p.byteLength);
+    return p.byteLength;
   }
 
   toString(): string {
diff --git a/std/io/writers_test.ts b/std/io/writers_test.ts
index a50f8b3d1..f27885f81 100644
--- a/std/io/writers_test.ts
+++ b/std/io/writers_test.ts
@@ -12,3 +12,12 @@ test("ioStringWriter", async function (): Promise<void> {
   await copy(r, w);
   assertEquals(w.toString(), "base0123456789");
 });
+
+test("ioStringWriterSync", function (): void {
+  const encoder = new TextEncoder();
+  const w = new StringWriter("");
+  w.writeSync(encoder.encode("deno"));
+  assertEquals(w.toString(), "deno");
+  w.writeSync(encoder.encode("\nland"));
+  assertEquals(w.toString(), "deno\nland");
+});