Improve event stream resilience

packages/web/server/lib/opencode/proxy.js

@@ -6,6 +6,43 @@ import {
   shouldForwardProxyResponseHeader,
 } from '../../proxy-headers.js';
 
+export const waitForSseDrain = (res, signal) => new Promise((resolve) => {
+  if (signal?.aborted || res.writableEnded || res.destroyed) {
+    resolve();
+    return;
+  }
+
+  const cleanup = () => {
+    res.off?.('drain', onDone);
+    res.off?.('close', onDone);
+    res.off?.('error', onDone);
+    signal?.removeEventListener?.('abort', onDone);
+  };
+  const onDone = () => {
+    cleanup();
+    resolve();
+  };
+
+  res.once?.('drain', onDone);
+  res.once?.('close', onDone);
+  res.once?.('error', onDone);
+  signal?.addEventListener?.('abort', onDone, { once: true });
+});
+
+export const writeSseChunkWithBackpressure = async (res, value, signal) => {
+  if (!value || value.length === 0 || signal?.aborted || res.writableEnded || res.destroyed) {
+    return false;
+  }
+
+  const flushed = res.write(value);
+  if (flushed !== false) {
+    return true;
+  }
+
+  await waitForSseDrain(res, signal);
+  return !signal?.aborted && !res.writableEnded && !res.destroyed;
+};
+
 export const registerOpenCodeProxy = (app, deps) => {
   const {
     fs,
@@ -131,7 +168,10 @@ export const registerOpenCodeProxy = (app, deps) => {
           break;
         }
         if (value && value.length > 0) {
-          res.write(value);
+          const canContinue = await writeSseChunkWithBackpressure(res, value, abortController.signal);
+          if (!canContinue) {
+            break;
+          }
         }
       }