nghttpx: Log error code from getsockopt(SO_ERROR) on first write event

src/shrpx_http2_session.cc

@@ -1806,9 +1806,11 @@ int Http2Session::read_noop(const uint8_t *data, size_t datalen) { return 0; }
 int Http2Session::write_noop() { return 0; }
 
 int Http2Session::connected() {
-  if (!util::check_socket_connected(conn_.fd)) {
+  auto sock_error = util::get_socket_error(conn_.fd);
+  if (sock_error != 0) {
     SSLOG(WARN, this) << "Backend connect failed; addr="
-                      << util::to_numeric_addr(&addr_->addr);
+                      << util::to_numeric_addr(&addr_->addr)
+                      << ": errno=" << sock_error;
 
     downstream_failure(addr_);
 

src/shrpx_http_downstream_connection.cc

@@ -1143,11 +1143,13 @@ int HttpDownstreamConnection::process_input(const uint8_t *data,
 int HttpDownstreamConnection::connected() {
   auto &connect_blocker = addr_->connect_blocker;
 
-  if (!util::check_socket_connected(conn_.fd)) {
+  auto sock_error = util::get_socket_error(conn_.fd);
+  if (sock_error != 0) {
     conn_.wlimit.stopw();
 
     DCLOG(WARN, this) << "Backend connect failed; addr="
-                      << util::to_numeric_addr(&addr_->addr);
+                      << util::to_numeric_addr(&addr_->addr)
+                      << ": errno=" << sock_error;
 
     downstream_failure(addr_);
 

src/shrpx_live_check.cc

@@ -259,10 +259,12 @@ int LiveCheck::initiate_connection() {
 }
 
 int LiveCheck::connected() {
-  if (!util::check_socket_connected(conn_.fd)) {
+  auto sock_error = util::get_socket_error(conn_.fd);
+  if (sock_error != 0) {
     if (LOG_ENABLED(INFO)) {
       LOG(INFO) << "Backend connect failed; addr="
-                << util::to_numeric_addr(&addr_->addr);
+                << util::to_numeric_addr(&addr_->addr)
+                << ": errno=" << sock_error;
     }
 
     return -1;

src/shrpx_memcached_connection.cc

@@ -203,15 +203,16 @@ int MemcachedConnection::initiate_connection() {
 }
 
 int MemcachedConnection::connected() {
-  if (!util::check_socket_connected(conn_.fd)) {
+  auto sock_error = util::get_socket_error(conn_.fd);
+  if (sock_error != 0) {
+    MCLOG(WARN, this) << "memcached connect failed; addr="
+                      << util::to_numeric_addr(addr_)
+                      << ": errno=" << sock_error;
+
     connect_blocker_.on_failure();
 
     conn_.wlimit.stopw();
 
-    if (LOG_ENABLED(INFO)) {
-      MCLOG(INFO, this) << "memcached connect failed";
-    }
-
     return -1;
   }
 

src/util.cc

@@ -893,6 +893,16 @@ bool check_socket_connected(int fd) {
   return error == 0;
 }
 
+int get_socket_error(int fd) {
+  int error;
+  socklen_t len = sizeof(error);
+  if (getsockopt(fd, SOL_SOCKET, SO_ERROR, &error, &len) != 0) {
+    return -1;
+  }
+
+  return error;
+}
+
 bool ipv6_numeric_addr(const char *host) {
   uint8_t dst[16];
   return inet_pton(AF_INET6, host, dst) == 1;

src/util.h

@@ -574,6 +574,11 @@ int create_nonblock_socket(int family);
 
 bool check_socket_connected(int fd);
 
+// Returns the error code (errno) by inspecting SO_ERROR of given
+// |fd|.  This function returns the error code if it succeeds, or -1.
+// Returning 0 means no error.
+int get_socket_error(int fd);
+
 // Returns true if |host| is IPv6 numeric address (e.g., ::1)
 bool ipv6_numeric_addr(const char *host);