TCPImagePuller: add OS-level TCP keep-alive

image_puller/TCPImagePuller.cpp

@@ -6,6 +6,7 @@
 
 #include <sys/socket.h>
 #include <arpa/inet.h>
+#include <netinet/tcp.h>
 #include <unistd.h>
 #include <cerrno>
 #include <limits>
@@ -227,6 +228,19 @@ bool TCPImagePuller::EnsureConnected() {
         if (receive_buffer_size)
             setsockopt(new_fd, SOL_SOCKET, SO_RCVBUF, &receive_buffer_size.value(), sizeof(int32_t));
 
+        // OS-level TCP keep-alive: detect a silently-dead pusher (no RST, e.g. crash or
+        // network partition) even while recv() is blocked in a long inter-image gap. Mirror
+        // the pusher's settings so detection is symmetric (~60s) and stays well clear of the
+        // 250 ms app-level BUSY heartbeat.
+        int one = 1;
+        setsockopt(new_fd, SOL_SOCKET, SO_KEEPALIVE, &one, sizeof(one));
+        int idle = 30;
+        int intvl = 10;
+        int cnt = 3;
+        setsockopt(new_fd, IPPROTO_TCP, TCP_KEEPIDLE, &idle, sizeof(idle));
+        setsockopt(new_fd, IPPROTO_TCP, TCP_KEEPINTVL, &intvl, sizeof(intvl));
+        setsockopt(new_fd, IPPROTO_TCP, TCP_KEEPCNT, &cnt, sizeof(cnt));
+
         if (connect(new_fd, ai->ai_addr, ai->ai_addrlen) == 0)
             break;