fix(mysqlx): harden check_connect() poll and getsockopt handling

Addresses the coderabbitai outside-diff finding originally surfaced on
PR #5641: the previous check_connect() conflated poll() errors with
timeouts, never retried on EINTR, did not check getsockopt()'s return,
and accepted POLLNVAL / POLLERR / POLLHUP as though they were normal
"not ready" states.

The practical failure mode was slow and misleading: a fatal poll error
or a bad descriptor was masked as "not ready yet" until the outer
connect_timeout_ms_ (default 10s) elapsed, at which point the session
reported a generic "connect failed" reason rather than the actual
cause. A concurrent signal (EINTR) would extend that masked window
further. Meanwhile, a failing getsockopt() with an uninspected return
would leave `err` at its initializer value of 0 and optimistically
transition the state machine to AUTHENTICATING, so the very next read
or write on the invalid fd would fail in a less diagnostic path.

What changed:
- poll() is now retried on EINTR.
- poll() returning -1 sets ERROR_STATE and returns -1 immediately.
- POLLNVAL / POLLERR / POLLHUP in revents are treated as hard errors
  even if POLLOUT is also set, since the underlying socket is already
  in a terminal async condition.
- getsockopt() return is checked; a nonzero return is now a hard error
  rather than a silent state advance.

Testing: three new assertions in mysqlx_robustness_unit-t.cpp exercise
the bad-fd error path, the happy-path connect, and the not-ready
timeout path. Plan count 33 → 42.

Out of scope: other poll() sites in the mysqlx plugin (the unit test
helpers at the top of mysqlx_robustness_unit-t.cpp have their own
poll() loops but they aren't on the backend-connect critical path);
the connect_timeout_ms_ check itself, which is unchanged.

Pre-existing ripple: same Makefile link-gap fix as earlier PRs — adds
mysqlx_config_store.cpp to the mysqlx_robustness_unit-t link line so
MysqlxConfigStore::resolve_identity() resolves when the binary links.

plugins/mysqlx/src/mysqlx_connection.cpp

@@ -72,11 +72,25 @@ int MysqlxConnection::check_connect() {
 	pfd.fd = fd_;
 	pfd.events = POLLOUT;
 	pfd.revents = 0;
-	int pr = poll(&pfd, 1, 0);
-	if (pr <= 0) return 1;  // not ready yet
+	int pr;
+	do {
+		pr = poll(&pfd, 1, 0);
+	} while (pr < 0 && errno == EINTR);
+	if (pr < 0) {
+		state_ = ERROR_STATE;
+		return -1;
+	}
+	if (pr == 0) return 1;  // not ready yet
+	if (pfd.revents & (POLLNVAL | POLLERR | POLLHUP)) {
+		state_ = ERROR_STATE;
+		return -1;
+	}
 	int err = 0;
 	socklen_t len = sizeof(err);
-	getsockopt(fd_, SOL_SOCKET, SO_ERROR, &err, &len);
+	if (getsockopt(fd_, SOL_SOCKET, SO_ERROR, &err, &len) != 0) {
+		state_ = ERROR_STATE;
+		return -1;
+	}
 	if (err == 0) { state_ = AUTHENTICATING; return 0; }
 	state_ = ERROR_STATE; return -1;
 }

test/tap/tests/unit/Makefile

@@ -524,8 +524,8 @@ mysqlx_credential_verify_unit-t: mysqlx_credential_verify_unit-t.cpp $(PROXYSQL_
 		$(IDIRS) $(LDIRS) $(OPT) $(LIBPROXYSQLAR_FULL) $(STATIC_LIBS) \
 		$(MYLIBS) -lprotobuf -lssl -lcrypto $(ALLOW_MULTI_DEF) -o $@
 
-mysqlx_robustness_unit-t: mysqlx_robustness_unit-t.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_session.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_data_stream.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_connection.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_thread.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_protocol.cpp $(MYSQLX_PROTO_OBJS) $(TEST_HELPERS_OBJ) $(LIBPROXYSQLAR)
-	$(CXX) $< $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_session.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_data_stream.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_connection.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_thread.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_protocol.cpp $(MYSQLX_PROTO_OBJS) $(TEST_HELPERS_OBJ) \
+mysqlx_robustness_unit-t: mysqlx_robustness_unit-t.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_session.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_data_stream.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_connection.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_thread.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_protocol.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_config_store.cpp $(MYSQLX_PROTO_OBJS) $(TEST_HELPERS_OBJ) $(LIBPROXYSQLAR)
+	$(CXX) $< $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_session.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_data_stream.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_connection.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_thread.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_protocol.cpp $(PROXYSQL_PATH)/plugins/mysqlx/src/mysqlx_config_store.cpp $(MYSQLX_PROTO_OBJS) $(TEST_HELPERS_OBJ) \
 		-I$(PROXYSQL_PATH)/plugins/mysqlx/include -I$(MYSQLX_PROTO_DIR) \
 		$(IDIRS) $(LDIRS) $(OPT) $(LIBPROXYSQLAR_FULL) $(STATIC_LIBS) \
 		$(MYLIBS) -lprotobuf -lssl -lcrypto $(ALLOW_MULTI_DEF) -lpthread -o $@

test/tap/tests/unit/mysqlx_robustness_unit-t.cpp

@@ -15,6 +15,7 @@
 #include <cerrno>
 #include <cstdio>
 #include <cstring>
+#include <fcntl.h>
 #include <poll.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
@@ -525,6 +526,75 @@ static void test_client_disconnect_detected() {
 	close(fds[0]);
 }
 
+static void test_check_connect_bad_fd() {
+	// Bug fix (PR #5641 review): check_connect() must report a hard error
+	// when poll() sees POLLNVAL on a closed/invalid fd, instead of waiting
+	// out the connect timeout. Create an fd, close it so it's invalid, then
+	// hand it to the connection and call check_connect().
+	int s = socket(AF_INET, SOCK_STREAM | SOCK_NONBLOCK, 0);
+	ok(s >= 0, "socket() created for bad-fd test");
+	close(s);
+
+	MysqlxConnection conn;
+	conn.set_fd(s);
+	conn.set_state(MysqlxConnection::CONNECTING);
+	conn.set_connect_timeout(10000);
+
+	int rc = conn.check_connect();
+	ok(rc == -1, "check_connect() returns -1 on invalid fd");
+	ok(conn.get_state() == MysqlxConnection::ERROR_STATE,
+	   "check_connect() transitions to ERROR_STATE on invalid fd");
+
+	conn.set_fd(-1);  // prevent dtor from double-closing
+}
+
+static void test_check_connect_success_path() {
+	// Positive path: an already-connected socketpair endpoint reports POLLOUT
+	// ready immediately, SO_ERROR is 0, so check_connect() must transition to
+	// AUTHENTICATING and return 0.
+	int fds[2];
+	int sp = socketpair(AF_UNIX, SOCK_STREAM, 0, fds);
+	ok(sp == 0, "socketpair() created for success-path test");
+
+	// Make non-blocking to mimic start_connect() fd flags.
+	int flags = fcntl(fds[0], F_GETFL, 0);
+	fcntl(fds[0], F_SETFL, flags | O_NONBLOCK);
+
+	MysqlxConnection conn;
+	conn.set_fd(fds[0]);
+	conn.set_state(MysqlxConnection::CONNECTING);
+	conn.set_connect_timeout(10000);
+
+	int rc = conn.check_connect();
+	ok(rc == 0, "check_connect() returns 0 when socket is writable and SO_ERROR==0");
+	ok(conn.get_state() == MysqlxConnection::AUTHENTICATING,
+	   "check_connect() transitions to AUTHENTICATING on successful connect");
+
+	conn.set_fd(-1);  // detach before manual close
+	close(fds[0]);
+	close(fds[1]);
+}
+
+static void test_check_connect_not_ready() {
+	// Socket that has NOT connected yet: poll() returns 0 (no POLLOUT),
+	// so check_connect() must return 1 and leave state unchanged.
+	int s = socket(AF_INET, SOCK_STREAM | SOCK_NONBLOCK, 0);
+	ok(s >= 0, "socket() created for not-ready test");
+
+	MysqlxConnection conn;
+	conn.set_fd(s);
+	conn.set_state(MysqlxConnection::CONNECTING);
+	conn.set_connect_timeout(10000);
+
+	int rc = conn.check_connect();
+	ok(rc == 1, "check_connect() returns 1 (not ready) on unconnected socket");
+	ok(conn.get_state() == MysqlxConnection::CONNECTING,
+	   "check_connect() leaves state as CONNECTING when not ready");
+
+	conn.set_fd(-1);
+	close(s);
+}
+
 static void test_forward_empty_frame() {
 	int client_fds[2], backend_fds[2];
 	socketpair(AF_UNIX, SOCK_STREAM, 0, client_fds);
@@ -567,7 +637,7 @@ static void test_forward_empty_frame() {
 }
 
 int main() {
-	plan(33);
+	plan(42);
 
 	test_server_response_terminal_frame();
 	test_server_response_non_terminal_keeps_waiting();
@@ -581,6 +651,9 @@ int main() {
 	test_return_backend_no_thread();
 	test_connection_limit_config();
 	test_client_disconnect_detected();
+	test_check_connect_bad_fd();
+	test_check_connect_success_path();
+	test_check_connect_not_ready();
 	test_forward_empty_frame();
 
 	return exit_status();