fix(std/http): avoid directly modifying the headers object (#5024)

std/http/file_server.ts

@@ -11,7 +11,6 @@ import { posix, extname } from "../path/mod.ts";
 import { listenAndServe, ServerRequest, Response } from "./server.ts";
 import { parse } from "../flags/mod.ts";
 import { assert } from "../testing/asserts.ts";
-import { setContentLength } from "./io.ts";
 
 interface EntryInfo {
   mode: string;
@@ -173,7 +172,6 @@ async function serveDir(
     body: page,
     headers,
   };
-  setContentLength(res);
   return res;
 }
 

std/http/io.ts

@@ -214,24 +214,6 @@ export async function writeTrailers(
   await writer.flush();
 }
 
-export function setContentLength(r: Response): void {
-  if (!r.headers) {
-    r.headers = new Headers();
-  }
-
-  if (r.body) {
-    if (!r.headers.has("content-length")) {
-      // typeof r.body === "string" handled in writeResponse.
-      if (r.body instanceof Uint8Array) {
-        const bodyLength = r.body.byteLength;
-        r.headers.set("content-length", bodyLength.toString());
-      } else {
-        r.headers.set("transfer-encoding", "chunked");
-      }
-    }
-  }
-}
-
 export async function writeResponse(
   w: Deno.Writer,
   r: Response
@@ -253,14 +235,21 @@ export async function writeResponse(
 
   let out = `HTTP/${protoMajor}.${protoMinor} ${statusCode} ${statusText}\r\n`;
 
-  setContentLength(r);
-  assert(r.headers != null);
-  const headers = r.headers;
+  const headers = r.headers ?? new Headers();
+
+  if (r.body && !headers.get("content-length")) {
+    if (r.body instanceof Uint8Array) {
+      out += `content-length: ${r.body.byteLength}\r\n`;
+    } else if (!headers.get("transfer-encoding")) {
+      out += "transfer-encoding: chunked\r\n";
+    }
+  }
 
   for (const [key, value] of headers) {
     out += `${key}: ${value}\r\n`;
   }
-  out += "\r\n";
+
+  out += `\r\n`;
 
   const header = encoder.encode(out);
   const n = await writer.write(header);

std/http/io_test.ts

@@ -19,7 +19,7 @@ import { chunkedBodyReader } from "./io.ts";
 import { ServerRequest, Response } from "./server.ts";
 import { StringReader } from "../io/readers.ts";
 import { mockConn } from "./mock.ts";
-const { Buffer, test } = Deno;
+const { Buffer, test, readAll } = Deno;
 
 test("bodyReader", async () => {
   const text = "Hello, Deno";
@@ -357,6 +357,17 @@ test("writeResponse with trailer", async () => {
   assertEquals(ret, exp);
 });
 
+test("writeResponseShouldNotModifyOriginHeaders", async () => {
+  const headers = new Headers();
+  const buf = new Deno.Buffer();
+
+  await writeResponse(buf, { body: "foo", headers });
+  assert(decode(await readAll(buf)).includes("content-length: 3"));
+
+  await writeResponse(buf, { body: "hello", headers });
+  assert(decode(await readAll(buf)).includes("content-length: 5"));
+});
+
 test("readRequestError", async function (): Promise<void> {
   const input = `GET / HTTP/1.1
 malformedHeader