3 files changed, 38 insertions, 1 deletions
diff --git a/std/io/bufio.ts b/std/io/bufio.ts
index aa74809fe..5c005672a 100644
--- a/std/io/bufio.ts
+++ b/std/io/bufio.ts
@@ -338,7 +338,11 @@ export class BufReader implements Reader {
       // Buffer full?
       if (this.buffered() >= this.buf.byteLength) {
         this.r = this.w;
-        throw new BufferFullError(this.buf);
+        // #4521 The internal buffer should not be reused across reads because it causes corruption of data.
+        const oldbuf = this.buf;
+        const newbuf = this.buf.slice(0);
+        this.buf = newbuf;
+        throw new BufferFullError(oldbuf);
       }
 
       s = this.w - this.r; // do not rescan area we scanned before
diff --git a/std/io/bufio_test.ts b/std/io/bufio_test.ts
index c49023814..d3e39bff6 100644
--- a/std/io/bufio_test.ts
+++ b/std/io/bufio_test.ts
@@ -12,6 +12,7 @@ import {
   BufWriterSync,
   BufferFullError,
   PartialReadError,
+  ReadLineResult,
   readStringDelim,
   readLines,
 } from "./bufio.ts";
@@ -445,3 +446,22 @@ Deno.test("readStringDelimAndLines", async function (): Promise<void> {
   assertEquals(lines_.length, 10);
   assertEquals(lines_, ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"]);
 });
+
+Deno.test(async function bufReaderShouldNotShareArrayBufferAcrossReads() {
+  const decoder = new TextDecoder();
+  const data = "abcdefghijklmnopqrstuvwxyz";
+  const bufSize = 25;
+  const b = new BufReader(stringsReader(data), bufSize);
+
+  const r1 = (await b.readLine()) as ReadLineResult;
+  assertNotEOF(r1);
+  assertEquals(decoder.decode(r1.line), "abcdefghijklmnopqrstuvwxy");
+
+  const r2 = (await b.readLine()) as ReadLineResult;
+  assertNotEOF(r2);
+  assertEquals(decoder.decode(r2.line), "z");
+  assert(
+    r1.line.buffer !== r2.line.buffer,
+    "array buffer should not be shared across reads"
+  );
+});
diff --git a/std/textproto/test.ts b/std/textproto/test.ts
index 3b71bc08c..ec66bfd8c 100644
--- a/std/textproto/test.ts
+++ b/std/textproto/test.ts
@@ -180,3 +180,16 @@ test({
     assertEquals(m.get("Content-Disposition"), 'form-data; name="test"');
   },
 });
+
+test({
+  name: "[textproto] #4521 issue",
+  async fn() {
+    const input = "abcdefghijklmnopqrstuvwxyz";
+    const bufSize = 25;
+    const tp = new TextProtoReader(
+      new BufReader(stringsReader(input), bufSize)
+    );
+    const line = await tp.readLine();
+    assertEquals(line, input);
+  },
+});