Fix #1724

httplib.h

@@ -382,6 +382,7 @@ public:
   DataSink &operator=(DataSink &&) = delete;
 
   std::function<bool(const char *data, size_t data_len)> write;
+  std::function<bool()> is_writable;
   std::function<void()> done;
   std::function<void(const Headers &trailer)> done_with_trailer;
   std::ostream os;
@@ -3959,6 +3960,8 @@ inline bool write_content(Stream &strm, const ContentProvider &content_provider,
     return ok;
   };
 
+  data_sink.is_writable = [&]() -> bool { return strm.is_writable(); };
+
   while (offset < end_offset && !is_shutting_down()) {
     if (!strm.is_writable()) {
       error = Error::Write;
@@ -4003,6 +4006,8 @@ write_content_without_length(Stream &strm,
     return ok;
   };
 
+  data_sink.is_writable = [&]() -> bool { return strm.is_writable(); };
+
   data_sink.done = [&](void) { data_available = false; };
 
   while (data_available && !is_shutting_down()) {
@@ -4053,6 +4058,8 @@ write_content_chunked(Stream &strm, const ContentProvider &content_provider,
     return ok;
   };
 
+  data_sink.is_writable = [&]() -> bool { return strm.is_writable(); };
+
   auto done_with_trailer = [&](const Headers *trailer) {
     if (!ok) { return; }
 

test/test.cc

@@ -4464,6 +4464,43 @@ TEST(ErrorHandlerWithContentProviderTest, ErrorHandler) {
   EXPECT_EQ("helloworld", res->body);
 }
 
+TEST(LongPollingTest, ClientCloseDetection) {
+  Server svr;
+
+  svr.Get("/events", [&](const Request & /*req*/, Response &res) {
+    res.set_chunked_content_provider(
+        "text/plain", [](std::size_t const, DataSink &sink) -> bool {
+          EXPECT_TRUE(sink.is_writable()); // the socket is alive
+          sink.os << "hello";
+
+          auto count = 10;
+          while (count > 0 && sink.is_writable()) {
+            this_thread::sleep_for(chrono::milliseconds(10));
+          }
+          EXPECT_FALSE(sink.is_writable()); // the socket is closed
+          return true;
+        });
+  });
+
+  auto listen_thread = std::thread([&svr]() { svr.listen("localhost", PORT); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    listen_thread.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+
+  svr.wait_until_ready();
+
+  Client cli("localhost", PORT);
+
+  auto res = cli.Get("/events", [&](const char *data, size_t data_length) {
+    EXPECT_EQ("hello", string(data, data_length));
+    return false; // close the socket immediately.
+  });
+
+  ASSERT_FALSE(res);
+}
+
 TEST(GetWithParametersTest, GetWithParameters) {
   Server svr;