fix: skip empty output buffers to avoid empty HTTP/2 DATA frames

take_output returns an empty buffer when there is nothing to send but
the output stream is not closed (e.g. while replaying already-completed
entries on resume-after-suspend). The SDK forwarded this empty buffer to
the response channel unconditionally, emitting one empty HTTP/2 DATA
frame per replayed await. Proxies such as Envoy terminate connections
with more than one consecutive empty-payload frame, so suspended
invocations never resumed.

Guard the three send sites (async_result_poll, select_poll,
consume_to_end) to skip empty buffers. The EOF arm is unchanged, so a
closed output still maps to UnexpectedOutputClosed.

Fixes #114

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tillrohrmann

src/endpoint/context.rs

@@ -757,6 +757,7 @@ impl ContextInternal {
 
         let out = inner_lock.vm.take_output();
         if let TakeOutputResult::Buffer(b) = out
+            && !b.is_empty()
             && !inner_lock.write.send(b)
         {
             // Nothing we can do anymore here

src/endpoint/futures/async_result_poll.rs

@@ -59,7 +59,11 @@ impl Future for VmAsyncResultPollFuture {
                     let out = inner_lock.vm.take_output();
                     match out {
                         TakeOutputResult::Buffer(b) => {
-                            if !inner_lock.write.send(b) {
+                            // Skip empty buffers: take_output returns an empty buffer when there's
+                            // nothing to send (e.g. while replaying completed entries). Sending it
+                            // would emit an empty HTTP/2 DATA frame per replayed await, which some
+                            // proxies (e.g. Envoy) reject when consecutive. See sdk-rust#114.
+                            if !b.is_empty() && !inner_lock.write.send(b) {
                                 return Poll::Ready(Err(ErrorInner::Suspended));
                             }
                         }

src/endpoint/futures/select_poll.rs

@@ -60,7 +60,11 @@ impl Future for VmSelectAsyncResultPollFuture {
                     let out = inner_lock.vm.take_output();
                     match out {
                         TakeOutputResult::Buffer(b) => {
-                            if !inner_lock.write.send(b) {
+                            // Skip empty buffers: take_output returns an empty buffer when there's
+                            // nothing to send (e.g. while replaying completed entries). Sending it
+                            // would emit an empty HTTP/2 DATA frame per replayed await, which some
+                            // proxies (e.g. Envoy) reject when consecutive. See sdk-rust#114.
+                            if !b.is_empty() && !inner_lock.write.send(b) {
                                 return Poll::Ready(Err(ErrorInner::Suspended));
                             }
                         }