[4.3.4] SSE bug: Http1ServerResponse.sink() does not mark response as having an entity, causing false "A route MUST call either send, reroute, or next" error

[hrstoyanov]

Environment Details

• Helidon Version: 4.3.4
• Helidon SE or Helidon MP: SE
• JDK version: 25.0.2
• OS: MacOs Tahoe

---

Another show-stopper for me:

Bug description

Http1ServerResponse.sink() creates a sink that writes directly to the low-level dataWriter, but never marks the response as having an entity. This causes Helidon's routing to think no response was produced.

The check (HttpRoutingImpl.java, lines 196-211)

After every handler returns, the router checks in order:

1. response.shouldReroute() → false
2. response.isNexted() → false
3. response.hasEntity() → false ← BUG: should be true
4. → throws "A route MUST call either send, reroute, or next"

Why hasEntity() returns false?

Http1ServerResponse.hasEntity() (line 274):

public boolean hasEntity() {
    return isSent || streamingEntity;
}

• isSent — only set to true in the closeRunnable (line 326), which runs when the sink is closed, not when it's created. An SSE sink is long-lived — it may not close until much later.
• streamingEntity — only set to true by outputStream() (line 451). The DataWriterSseSink bypasses outputStream() entirely — it writes directly to ctx.dataWriter().writeNow() (DataWriterSseSink line 124).

So, after the handler calls response.sink(SseSink.TYPE), writes status+headers, and emits SSE events, neither flag is set.

Suggested fix

In Http1ServerResponse.sink() (line 308), after the sink is successfully created, mark the response:

public <X extends Sink<?>> X sink(GenericType<X> sinkType) {
    for (SinkProvider<?> p : SINK_PROVIDERS) {
        if (p.supports(sinkType, request)) {
            X sink = (X) p.create(...);
            this.isSent = true;   // ← the fix: sink took over the response
            return sink;
        }
    }
    ...
}

Setting isSent = true immediately is correct because the sink's constructor (DataWriterSseSink line 76) already calls writeStatusAndHeaders() — the HTTP status and headers have been flushed to the wire. The response is sent at that point.

Setting isSent = true in sink() is the right approach because:
1. Headers are already written - DataWriterSseSink constructor calls writeStatusAndHeaders() immediately
2. Sink owns the response - Once created, only the sink should write to the response
3. Consistent with send() - Calling res.send() also sets isSent = true

Minimal reproducer:

     routing(HttpRouting.builder()
          .any("/sse", (req, res) -> {
              var sink = res.sink(SseSink.TYPE);
              sink.emit(SseEvent.builder().data("test").build());
              // BUG: No res.send() called, throws error even though SSE was sent
     }))

[hrstoyanov]

Ideally this fix should be combined with the suggested fix for #11053