Encode binary streams as a single collapsed Blob

Since we buffer anyway, we might as well combine the chunks of binary
streams.

Ideally, we'd be able to just use the stream from the Blob but Node.js
doesn't return byob streams from Blob. Additionally, we don't actually
stream the content of Blobs due to the layering with busboy atm. We could
do that for binary streams in particular by replacing the File layering
with a stream.

packages/react-client/src/ReactFlightReplyClient.js

@@ -208,7 +208,7 @@ export function processReply(
     return '$' + tag + blobId.toString(16);
   }
 
-  function serializeReadableStream(stream: ReadableStream): string {
+  function serializeBinaryReader(reader: any): string {
     if (formData === null) {
       // Upgrade to use FormData to allow us to stream this value.
       formData = new FormData();
@@ -218,23 +218,43 @@ export function processReply(
     pendingParts++;
     const streamId = nextPartId++;
 
-    // Detect if this is a BYOB stream. BYOB streams should be able to be read as bytes on the
-    // receiving side. It also implies that different chunks can be split up or merged as opposed
-    // to a readable stream that happens to have Uint8Array as the type which might expect it to be
-    // received in the same slices.
-    // $FlowFixMe: This is a Node.js extension.
-    let supportsBYOB: void | boolean = stream.supportsBYOB;
-    if (supportsBYOB === undefined) {
-      try {
-        // $FlowFixMe[extra-arg]: This argument is accepted.
-        stream.getReader({mode: 'byob'}).releaseLock();
-        supportsBYOB = true;
-      } catch (x) {
-        supportsBYOB = false;
+    const buffer = [];
+
+    function progress(entry: {done: boolean, value: ReactServerValue, ...}) {
+      if (entry.done) {
+        const blobId = nextPartId++;
+        // eslint-disable-next-line react-internal/safe-string-coercion
+        data.append(formFieldPrefix + blobId, new Blob(buffer));
+        // eslint-disable-next-line react-internal/safe-string-coercion
+        data.append(
+          formFieldPrefix + streamId,
+          '"$o' + blobId.toString(16) + '"',
+        );
+        // eslint-disable-next-line react-internal/safe-string-coercion
+        data.append(formFieldPrefix + streamId, 'C'); // Close signal
+        pendingParts--;
+        if (pendingParts === 0) {
+          resolve(data);
+        }
+      } else {
+        buffer.push(entry.value);
+        reader.read(new Uint8Array(1024)).then(progress, reject);
       }
     }
+    reader.read(new Uint8Array(1024)).then(progress, reject);
 
-    const reader = stream.getReader();
+    return '$r' + streamId.toString(16);
+  }
+
+  function serializeReader(reader: ReadableStreamReader): string {
+    if (formData === null) {
+      // Upgrade to use FormData to allow us to stream this value.
+      formData = new FormData();
+    }
+    const data = formData;
+
+    pendingParts++;
+    const streamId = nextPartId++;
 
     function progress(entry: {done: boolean, value: ReactServerValue, ...}) {
       if (entry.done) {
@@ -258,7 +278,20 @@ export function processReply(
     }
     reader.read().then(progress, reject);
 
-    return '$' + (supportsBYOB ? 'r' : 'R') + streamId.toString(16);
+    return '$R' + streamId.toString(16);
+  }
+
+  function serializeReadableStream(stream: ReadableStream): string {
+    // Detect if this is a BYOB stream. BYOB streams should be able to be read as bytes on the
+    // receiving side. For binary streams, we serialize them as plain Blobs.
+    let binaryReader;
+    try {
+      // $FlowFixMe[extra-arg]: This argument is accepted.
+      binaryReader = stream.getReader({mode: 'byob'});
+    } catch (x) {
+      return serializeReader(stream.getReader());
+    }
+    return serializeBinaryReader(binaryReader);
   }
 
   function serializeAsyncIterable(