refactor writeChunk to be more defensive and efficient

We now defend against overflows using the next views length instead of the current one. this protects us against a future where we use byobRequest and we get longer initial views than we might create after overflowing the first time. Additionally we add in an optimization when we have completely filled up the currentView where we avoid creating subarrays of the chunk to write since it lands exactly on a view boundary. Finally we move the view creation to beginWriting to avoid a runtime check on each write and because we want to reset the view on each beginWriting call in case a throw elsewhere in the program leaves the currentView in an unfinished state

packages/react-server/src/ReactServerStreamConfigBrowser.js

@@ -21,46 +21,66 @@ export function flushBuffered(destination: Destination) {
   // transform streams. https://github.com/whatwg/streams/issues/960
 }
 
+const VIEW_SIZE = 512;
 let currentView = null;
 let writtenBytes = 0;
 
-export function beginWriting(destination: Destination) {}
+export function beginWriting(destination: Destination) {
+  currentView = new Uint8Array(VIEW_SIZE);
+  writtenBytes = 0;
+}
 
 export function writeChunk(
   destination: Destination,
   chunk: PrecomputedChunk | Chunk,
 ): void {
-  if (currentView === null) {
-    currentView = new Uint8Array(512);
-    writtenBytes = 0;
+  if (chunk.length === 0) {
+    return;
   }
 
-  if (chunk.length > currentView.length) {
-    // this chunk is larger than our view which implies it was not
+  if (chunk.length > VIEW_SIZE) {
+    // this chunk may overflow a single view which implies it was not
     // one that is cached by the streaming renderer. We will enqueu
     // it directly and expect it is not re-used
     if (writtenBytes > 0) {
-      destination.enqueue(new Uint8Array(currentView.buffer, 0, writtenBytes));
-      currentView = null;
+      destination.enqueue(
+        new Uint8Array(
+          ((currentView: any): Uint8Array).buffer,
+          0,
+          writtenBytes,
+        ),
+      );
+      currentView = new Uint8Array(VIEW_SIZE);
       writtenBytes = 0;
     }
     destination.enqueue(chunk);
     return;
   }
 
-  const allowableBytes = currentView.length - writtenBytes;
-  if (allowableBytes < chunk.length) {
-    // this chunk would overflow the current view. We enqueu a full view
+  let bytesToWrite = chunk;
+  const allowableBytes = ((currentView: any): Uint8Array).length - writtenBytes;
+  if (allowableBytes < bytesToWrite.length) {
+    // this chunk would overflow the current view. We enqueue a full view
     // and start a new view with the remaining chunk
-    currentView.set(chunk.subarray(0, allowableBytes), writtenBytes);
-    destination.enqueue(currentView);
-    currentView = new Uint8Array(512);
-    currentView.set(chunk.subarray(allowableBytes));
-    writtenBytes = chunk.length - allowableBytes;
-  } else {
-    currentView.set(chunk, writtenBytes);
-    writtenBytes += chunk.length;
+    if (allowableBytes === 0) {
+      // the current view is already full, send it
+      destination.enqueue(currentView);
+    } else {
+      // fill up the current view and apply the remaining chunk bytes
+      // to a new view.
+      ((currentView: any): Uint8Array).set(
+        bytesToWrite.subarray(0, allowableBytes),
+        writtenBytes,
+      );
+      // writtenBytes += allowableBytes; // this can be skipped because we are going to immediately reset the view
+      destination.enqueue(currentView);
+      bytesToWrite = bytesToWrite.subarray(allowableBytes);
+    }
+    currentView = new Uint8Array(VIEW_SIZE);
+    writtenBytes = 0;
   }
+  ((currentView: any): Uint8Array).set(bytesToWrite, writtenBytes);
+  writtenBytes += bytesToWrite.length;
 }
 
 export function writeChunkAndReturn(