Fix SSE client not clearing Last-Event-ID on an empty id field (Fix #2611)

An event with an empty id field must reset the last event ID, so that
no Last-Event-ID header is sent on reconnect. run_event_loop only
updated last_event_id_ when the id was non-empty, so it could not tell
an empty id field from an event with no id field, and kept sending the
stale ID.

Track whether an id field was seen with a has_id flag, as has_data
does for the data field, and add a regression test.

httplib.h

@@ -4377,7 +4377,7 @@ public:
 
 private:
   bool parse_sse_line(const std::string &line, SSEMessage &msg, int &retry_ms,
-                      bool &has_data);
+                      bool &has_data, bool &has_id);
   void run_event_loop();
   void dispatch_event(const SSEMessage &msg);
   bool should_reconnect(int count) const;
@@ -4887,7 +4887,8 @@ inline void SSEClient::stop() {
 }
 
 inline bool SSEClient::parse_sse_line(const std::string &line, SSEMessage &msg,
-                                      int &retry_ms, bool &has_data) {
+                                      int &retry_ms, bool &has_data,
+                                      bool &has_id) {
   // Blank line signals end of event
   if (line.empty()) { return true; }
 
@@ -4917,6 +4918,7 @@ inline bool SSEClient::parse_sse_line(const std::string &line, SSEMessage &msg,
   } else if (field == "id") {
     // Empty id is valid (clears the last event ID)
     msg.id = value;
+    has_id = true;
   } else if (field == "retry") {
     // Parse retry interval in milliseconds
     // Per the SSE spec, a value that is not all ASCII digits is ignored.
@@ -4987,7 +4989,8 @@ inline void SSEClient::run_event_loop() {
     // Event receiving loop
     std::string buffer;
     SSEMessage current_msg;
-    bool has_data = false;
+    auto has_data = false;
+    auto has_id = false;
 
     while (running_.load() && result.next()) {
       buffer.append(result.data(), result.size());
@@ -5006,13 +5009,13 @@ inline void SSEClient::run_event_loop() {
         if (!line.empty() && line.back() == '\r') { line.pop_back(); }
 
         // Parse the line and check if event is complete
-        auto event_complete =
-            parse_sse_line(line, current_msg, reconnect_interval_ms_, has_data);
+        auto event_complete = parse_sse_line(
+            line, current_msg, reconnect_interval_ms_, has_data, has_id);
 
         if (event_complete) {
           // Update last_event_id for reconnection, even for an event that
-          // has no data
-          if (!current_msg.id.empty()) { last_event_id_ = current_msg.id; }
+          // has no data. An empty id clears it.
+          if (has_id) { last_event_id_ = current_msg.id; }
 
           // An event without a data field is not dispatched
           if (has_data) { dispatch_event(current_msg); }
@@ -5020,6 +5023,7 @@ inline void SSEClient::run_event_loop() {
           // Reset the message for the next event either way
           current_msg.clear();
           has_data = false;
+          has_id = false;
         }
       }
 

test/test.cc

@@ -22470,6 +22470,30 @@ TEST_F(SSEParsingTest, EventWithoutDataUpdatesLastEventId) {
   EXPECT_EQ(msgs[0].data, "42");
 }
 
+TEST_F(SSEParsingTest, EmptyEventIdClearsLastEventId) {
+  // The first connection sets an id and then clears it with an empty one; the
+  // second reports whether it was reconnected with a Last-Event-ID
+  std::atomic<int> connection_count{0};
+  server_->Get("/id-clear", [&](const Request &req, Response &res) {
+    if (connection_count++ == 0) {
+      res.set_content("id: 1\ndata: first\n\nid:\ndata: second\n\n",
+                      "text/event-stream");
+    } else {
+      res.set_content(
+          std::string("data: ") +
+              (req.has_header("Last-Event-ID") ? "sent" : "not sent") +
+              "\n\nevent: end\ndata: end\n\n",
+          "text/event-stream");
+    }
+  });
+
+  auto msgs = collect("/id-clear");
+  ASSERT_EQ(msgs.size(), 3u);
+  EXPECT_EQ(msgs[0].id, "1");
+  EXPECT_EQ(msgs[1].id, "");
+  EXPECT_EQ(msgs[2].data, "not sent");
+}
+
 TEST_F(SSEParsingTest, CompleteEventParsing) {
   auto msgs = parse("event: notification\nid: evt-42\n"
                     "data: {\"type\":\"alert\"}\nretry: 1000\n\n");