3 files changed, 30 insertions, 4 deletions
diff --git a/ext/fetch/22_body.js b/ext/fetch/22_body.js
index a51cdc184..97a8a8db1 100644
--- a/ext/fetch/22_body.js
+++ b/ext/fetch/22_body.js
@@ -64,10 +64,12 @@
   }
 
   class InnerBody {
+    #knownExactLength = null;
+
     /**
      * @param {ReadableStream<Uint8Array> | { body: Uint8Array | string, consumed: boolean }} stream
      */
-    constructor(stream) {
+    constructor(stream, knownExactLength) {
       /** @type {ReadableStream<Uint8Array> | { body: Uint8Array | string, consumed: boolean }} */
       this.streamOrStatic = stream ??
         { body: new Uint8Array(), consumed: false };
@@ -75,6 +77,8 @@
       this.source = null;
       /** @type {null | number} */
       this.length = null;
+
+      this.#knownExactLength = knownExactLength;
     }
 
     get stream() {
@@ -147,14 +151,31 @@
         const reader = this.stream.getReader();
         /** @type {Uint8Array[]} */
         const chunks = [];
+
+        let finalBuffer = this.#knownExactLength
+          ? new Uint8Array(this.#knownExactLength)
+          : null;
+
         let totalLength = 0;
         while (true) {
           const { value: chunk, done } = await reader.read();
           if (done) break;
-          ArrayPrototypePush(chunks, chunk);
+
+          if (finalBuffer) {
+            // fast path, content-length is present
+            TypedArrayPrototypeSet(finalBuffer, chunk, totalLength);
+          } else {
+            // slow path, content-length is not present
+            ArrayPrototypePush(chunks, chunk);
+          }
           totalLength += chunk.byteLength;
         }
-        const finalBuffer = new Uint8Array(totalLength);
+
+        if (finalBuffer) {
+          return finalBuffer;
+        }
+
+        finalBuffer = new Uint8Array(totalLength);
         let i = 0;
         for (const chunk of chunks) {
           TypedArrayPrototypeSet(finalBuffer, chunk, i);
@@ -199,7 +220,7 @@
     clone() {
       const [out1, out2] = this.stream.tee();
       this.streamOrStatic = out1;
-      const second = new InnerBody(out2);
+      const second = new InnerBody(out2, this.#knownExactLength);
       second.source = core.deserialize(core.serialize(this.source));
       second.length = this.length;
       return second;
diff --git a/ext/fetch/26_fetch.js b/ext/fetch/26_fetch.js
index c980bc9b8..13c34f534 100644
--- a/ext/fetch/26_fetch.js
+++ b/ext/fetch/26_fetch.js
@@ -335,6 +335,7 @@
       } else {
         response.body = new InnerBody(
           createResponseBodyStream(resp.responseRid, terminator),
+          resp.contentLength,
         );
       }
     }
diff --git a/ext/fetch/lib.rs b/ext/fetch/lib.rs
index 20db7abbc..3988acf9e 100644
--- a/ext/fetch/lib.rs
+++ b/ext/fetch/lib.rs
@@ -361,6 +361,7 @@ pub struct FetchResponse {
   headers: Vec<(ByteString, ByteString)>,
   url: String,
   response_rid: ResourceId,
+  content_length: Option<u64>,
 }
 
 #[op]
@@ -391,6 +392,8 @@ pub async fn op_fetch_send(
     res_headers.push((key.as_str().into(), val.as_bytes().into()));
   }
 
+  let content_length = res.content_length();
+
   let stream: BytesStream = Box::pin(res.bytes_stream().map(|r| {
     r.map_err(|err| std::io::Error::new(std::io::ErrorKind::Other, err))
   }));
@@ -409,6 +412,7 @@ pub async fn op_fetch_send(
     headers: res_headers,
     url,
     response_rid: rid,
+    content_length,
   })
 }