newStreamResponse sometimes stops calling the callback

Hi,

I'm trying to use newStreamResponse and the header says:

> * @param callback function to retrieve the stream data (stream ends when a
> * zero size is returned) the callback will be called with
> * nullptr when the send is finished/interrupted so that it
> * cleans up its internals.

But sometimes the callback is not called and my program gets stuck.
Here is the simplest code I can write to reproduce :

```
#include

std::thread worker;

using namespace drogon;

struct {
std::atomic ready{ false }, done{ false };
std::function callback;
} stream;

static std::chrono::milliseconds pause;

void sendresponse() {
while (not stream.ready) std::this_thread::sleep_for(std::chrono::milliseconds(100));

printf("Handling request\n");
auto cb = [](char* buffer, std::size_t size) {
printf("size: %lld, buffer: %p\n", size, buffer);
if (size == 0) { stream.done = true; return (size_t) 0; };
std::this_thread::sleep_for(pause);
memset(buffer, 'A', size);
return size;
};

auto resp = drogon::HttpResponse::newStreamResponse(cb);
stream.callback(resp);
while (not stream.done) std::this_thread::sleep_for(std::chrono::milliseconds(100));
printf("Done\n");
app().quit();
}

int main(int arc, char* argv[]) {
pause = std::chrono::milliseconds(std::atoi(argv[1]));
worker = std::thread(sendresponse);
app().registerHandler(
"/",
[](const HttpRequestPtr &request,
std::function &&callback) {
stream.callback = callback;
stream.ready = true;
},
{Get});
app().addListener("0.0.0.0", 80).run();
worker.join();
return 0;
}
```

On an other computer I launch "curl -o dummy" and press Ctrl-C after a few seconds.

If launched with 1000 as argument I have these prints

> Handling request
> 20260105 15:42:39.646000 UTC 15692 DEBUG [makeHeaderString] send stream with transfer-encoding chunked - HttpResponseImpl.cc:547
> size: 16384, buffer: 000001D3B70D9FAE
> size: 16384, buffer: 000001D3B70D9FAE
> size: 0, buffer: 0000000000000000
> Done

If launched with 10 as argument I have these prints

> Handling request
> 20260105 15:44:04.411000 UTC 12272 DEBUG [makeHeaderString] send stream with transfer-encoding chunked - HttpResponseImpl.cc:547
> size: 16384, buffer: 000002A60B395FCE
> size: 16384, buffer: 000002A60B395FCE
> size: 16384, buffer: 000002A60B395FCE
> size: 16384, buffer: 000002A60B395FCE
> size: 16384, buffer: 000002A60B395FCE
> size: 16384, buffer: 000002A60B395FCE
> size: 16384, buffer: 000002A60B395FCE

--> Output hangs: the callback is not called anymore

If curl is launched from the same computer where drogon is running, everything works fine