Add fix for deserializing XSUB/XPUB subscription message (sonic-net#1…

…6598)

### Why I did it

##### Work item tracking
- Microsoft ADO **(number only)**:24851367

#### How I did it

Read subscription message when capture service starts, before reading cached events.

#### How to verify it

UT/Manual testing

src/sonic-eventd/src/eventd.cpp

@@ -1,6 +1,7 @@
 #include <thread>
 #include "eventd.h"
 #include "dbconnector.h"
+#include "zmq.h"
 
 /*
  * There are 5 threads, including the main
@@ -355,6 +356,7 @@ capture_service::do_capture()
     int init_cnt;
     void *cap_sub_sock = NULL;
     counters_t total_overflow = 0;
+    static bool init_done = false;
 
     typedef enum {
         /*
@@ -391,6 +393,25 @@ capture_service::do_capture()
 
     m_cap_run = true;
 
+    if(!init_done) {
+        zmq_msg_t msg;
+        zmq_msg_init(&msg);
+        int rc = zmq_msg_recv(&msg, cap_sub_sock, 0);
+        RET_ON_ERR(rc == 1, "Failed to read subscription message when XSUB connects to XPUB");
+        /*
+         * When XSUB socket connects to XPUB, a subscription message is sent as a single byte 1.
+         * When capture service begins to read, the very first message that it will read is this
+         * control character.
+         *
+         * We will handle by reading this message and dropping it before we begin reading for
+         * cached events.
+         *
+         * This behavior will only happen once when XSUB connects to XPUB not everytime cache is started.
+         *
+         */
+         init_done = true;
+    }
+
     while (m_ctrl != START_CAPTURE) {
         /* Wait for capture start */
         this_thread::sleep_for(chrono::milliseconds(10));

src/sonic-eventd/tests/eventd_ut.cpp

@@ -159,12 +159,20 @@ void run_cap(void *zctx, bool &term, string &read_source,
     internal_event_t ev_int;
     int block_ms = 200;
     int i=0;
+    static int proxy_finished_init = false;
 
     EXPECT_TRUE(NULL != mock_cap);
     EXPECT_EQ(0, zmq_connect(mock_cap, get_config(CAPTURE_END_KEY).c_str()));
     EXPECT_EQ(0, zmq_setsockopt(mock_cap, ZMQ_SUBSCRIBE, "", 0));
     EXPECT_EQ(0, zmq_setsockopt(mock_cap, ZMQ_RCVTIMEO, &block_ms, sizeof (block_ms)));
 
+    if(!proxy_finished_init) {
+        zmq_msg_t msg;
+        zmq_msg_init(&msg);
+        EXPECT_EQ(1, zmq_msg_recv(&msg, mock_cap, 0)); // Subscription message
+        proxy_finished_init = true;
+    }
+
     while(!term) {
         string source;
         internal_event_t ev_int;