perf(ext/http): optimize inferrable content-type responses

ext/fetch/23_response.js

@@ -122,6 +122,7 @@ function cloneInnerResponse(response) {
     body,
     headerList,
     urlList,
+    canInferContentType: response.canInferContentType,
     status: response.status,
     statusMessage: response.statusMessage,
     aborted: response.aborted,
@@ -142,6 +143,7 @@ function newInnerResponse(status = 200, statusMessage = "") {
     headerList: [],
     urlList: [],
     status,
+    canInferContentType: false,
     statusMessage,
     aborted: false,
     url() {
@@ -178,6 +180,8 @@ function abortedNetworkError() {
  * @param {{ body: fetchBody.InnerBody, contentType: string | null } | null} bodyWithType
  */
 function initializeAResponse(response, init, bodyWithType) {
+  let canInferContentType = false;
+
   // 1.
   if ((init.status < 200 || init.status > 599) && init.status != 101) {
     throw new RangeError(
@@ -228,10 +232,14 @@ function initializeAResponse(response, init, bodyWithType) {
         }
       }
       if (!hasContentType) {
+        // keep case in sync with `extractBody`
+        canInferContentType = contentType === "text/plain;charset=UTF-8";
         ArrayPrototypePush(list, ["Content-Type", contentType]);
       }
     }
   }
+
+  return canInferContentType;
 }
 
 class Response {
@@ -335,7 +343,11 @@ class Response {
     if (body !== null) {
       bodyWithType = extractBody(body);
     }
-    initializeAResponse(this, init, bodyWithType);
+    this[_response].canInferContentType = initializeAResponse(
+      this,
+      init,
+      bodyWithType,
+    );
     this[_brand] = _brand;
   }
 

ext/http/00_serve.ts

@@ -564,6 +564,12 @@ function mapToCallback(context, callback, onError) {
     const headers = inner.headerList;
     if (headers && headers.length > 0) {
       if (headers.length == 1) {
+        if (
+          inner.canInferContentType &&
+          headers[0][0] === "Content-Type"
+        ) {
+          continue;
+        }
         op_http_set_response_header(req, headers[0][0], headers[0][1]);
       } else {
         op_http_set_response_headers(req, headers);

ext/http/http_next.rs

@@ -739,7 +739,19 @@ pub fn op_http_set_response_body_text(
   let http =
     // SAFETY: external is deleted before calling this op.
     unsafe { take_external!(external, "op_http_set_response_body_text") };
+
   if !text.is_empty() {
+    {
+      let mut response_parts = http.response_parts();
+      if !response_parts.headers.contains_key(CONTENT_TYPE) {
+        response_parts.headers.append(
+          CONTENT_TYPE,
+          HeaderValue::from_static("text/plain; charset=utf-8"),
+        );
+      }
+    }
+
+    // set content-type to text/plain; charset=utf-8 if not set
     set_response(http, Some(text.len()), status, false, |compression| {
       ResponseBytesInner::from_vec(compression, text.into_bytes())
     });