Preserve empty SSE data fields (#2594)

SSE events may contain an empty data field, and a data field without a colon also has an empty value. Track whether a data field was seen separately from the accumulated payload so empty events are dispatched and leading empty lines are preserved. Add an integration regression test for both forms.

httplib.h

@@ -4376,7 +4376,8 @@ public:
   void stop();
 
 private:
-  bool parse_sse_line(const std::string &line, SSEMessage &msg, int &retry_ms);
+  bool parse_sse_line(const std::string &line, SSEMessage &msg, int &retry_ms,
+                      bool &has_data);
   void run_event_loop();
   void dispatch_event(const SSEMessage &msg);
   bool should_reconnect(int count) const;
@@ -4886,7 +4887,7 @@ inline void SSEClient::stop() {
 }
 
 inline bool SSEClient::parse_sse_line(const std::string &line, SSEMessage &msg,
-                                      int &retry_ms) {
+                                      int &retry_ms, bool &has_data) {
   // Blank line signals end of event
   if (line.empty() || line == "\r") { return true; }
 
@@ -4895,16 +4896,11 @@ inline bool SSEClient::parse_sse_line(const std::string &line, SSEMessage &msg,
 
   // Find the colon separator
   auto colon_pos = line.find(':');
-  if (colon_pos == std::string::npos) {
-    // Line with no colon is treated as field name with empty value
-    return false;
-  }
-
   auto field = line.substr(0, colon_pos);
   std::string value;
 
   // Value starts after colon, skip optional single space
-  if (colon_pos + 1 < line.size()) {
+  if (colon_pos != std::string::npos && colon_pos + 1 < line.size()) {
     auto value_start = colon_pos + 1;
     if (line[value_start] == ' ') { value_start++; }
     value = line.substr(value_start);
@@ -4917,8 +4913,9 @@ inline bool SSEClient::parse_sse_line(const std::string &line, SSEMessage &msg,
     msg.event = value;
   } else if (field == "data") {
     // Multiple data lines are concatenated with newlines
-    if (!msg.data.empty()) { msg.data += "\n"; }
+    if (has_data) { msg.data += "\n"; }
     msg.data += value;
+    has_data = true;
   } else if (field == "id") {
     // Empty id is valid (clears the last event ID)
     msg.id = value;
@@ -4992,6 +4989,7 @@ inline void SSEClient::run_event_loop() {
     // Event receiving loop
     std::string buffer;
     SSEMessage current_msg;
+    bool has_data = false;
 
     while (running_.load() && result.next()) {
       buffer.append(result.data(), result.size());
@@ -5007,9 +5005,9 @@ inline void SSEClient::run_event_loop() {
 
         // Parse the line and check if event is complete
         auto event_complete =
-            parse_sse_line(line, current_msg, reconnect_interval_ms_);
+            parse_sse_line(line, current_msg, reconnect_interval_ms_, has_data);
 
-        if (event_complete && !current_msg.data.empty()) {
+        if (event_complete && has_data) {
           // Update last_event_id for reconnection
           if (!current_msg.id.empty()) { last_event_id_ = current_msg.id; }
 
@@ -5017,6 +5015,7 @@ inline void SSEClient::run_event_loop() {
           dispatch_event(current_msg);
 
           current_msg.clear();
+          has_data = false;
         }
       }
 

test/test.cc

@@ -22678,6 +22678,45 @@ TEST_F(SSEIntegrationTest, MultiLineDataIntegration) {
   EXPECT_EQ(received_data, "line1\nline2\nline3");
 }
 
+TEST_F(SSEIntegrationTest, EmptyDataLines) {
+  server_->Get("/empty-data-lines", [](const Request &, Response &res) {
+    res.set_chunked_content_provider("text/event-stream", [](size_t offset,
+                                                             DataSink &sink) {
+      if (offset == 0) {
+        const std::string events = "data:\n\ndata:\ndata: hello\n\ndata\n\n";
+        sink.write(events.data(), events.size());
+      }
+      return false;
+    });
+  });
+
+  Client client("localhost", get_port());
+  sse::SSEClient sse(client, "/empty-data-lines");
+  std::mutex mutex;
+  std::condition_variable cv;
+  std::vector<std::string> received;
+
+  sse.on_message([&](const sse::SSEMessage &msg) {
+    std::lock_guard<std::mutex> lock(mutex);
+    received.push_back(msg.data);
+    cv.notify_all();
+  });
+  sse.set_max_reconnect_attempts(1);
+  sse.start_async();
+
+  {
+    std::unique_lock<std::mutex> lock(mutex);
+    cv.wait_for(lock, std::chrono::seconds(2),
+                [&] { return received.size() >= 3; });
+  }
+  sse.stop();
+
+  ASSERT_GE(received.size(), 3u);
+  EXPECT_EQ(received[0], "");
+  EXPECT_EQ(received[1], "\nhello");
+  EXPECT_EQ(received[2], "");
+}
+
 // Test: Auto-reconnect after server disconnection
 TEST_F(SSEIntegrationTest, AutoReconnectAfterDisconnect) {
   std::atomic<int> connection_count{0};