Handle synchronous JS exceptions in PumpToReader

src/workerd/api/streams/standard.c++

@@ -2869,8 +2869,19 @@ public:
         // that the PumpToReader, and the sink it owns, are always accessed from the right
         // IoContext. Thw WeakRef ensures that if the PumpToReader is freed while
         // the JS continuation is pending, there won't be a dangling reference.
-        return ioContext.awaitJs(
-            pumpLoop(js, ioContext, kj::mv(readable), ioContext.addObject(addWeakRef())));
+        //
+        // The call to awaitJs here can throw a JsExceptionThrown error synchronously
+        // if, for instance, the isolate is being terminated due to an exception.
+        // We wrap this in a js.tryCatch to catch that case and return a rejected
+        // kj::Promise instead.
+        return js.tryCatch([&] {
+          return ioContext.awaitJs(
+              pumpLoop(js, ioContext, kj::mv(readable), ioContext.addObject(addWeakRef())));
+        }, [&](jsg::Value exception) {
+          auto handle = exception.getHandle(js);
+          KJ_ASSERT(!handle.IsEmpty());
+          return js.exceptionToKj(kj::mv(exception));
+        });
       }
       KJ_CASE_ONEOF(pumping, Pumping) {
         return KJ_EXCEPTION(FAILED, "pumping is already in progress");
@@ -3110,10 +3121,8 @@ private:
         }, [&](jsg::Value exception) {
           // Exceptions here should be rare, and fairly odd. We would get here,
           // for instance, if the call to .Then inside read().then(...) failed.
-          // We need to understand more about this case so we're going to temporarily
-          // add logging to see if we can catch it in the act.
-          KJ_LOG(ERROR, "Unexpected exception in PumpToReader pump loop: ",
-                 exception.getHandle(js));
+          auto handle = exception.getHandle(js);
+          KJ_ASSERT(!handle.IsEmpty());
           return js.rejectedPromise<void>(kj::mv(exception));
         });
       }