fix: check all Close() return values for errcheck lint

Add explicit error discards (_ =) for Close() calls on network
connections, SQL rows, and file handles across 28 files. Also add
.golangci.yml with standard linter defaults to match CI configuration.

server/signserver/session_test.go

@@ -354,8 +354,8 @@ func TestSessionWorkWithDevModeLogging(t *testing.T) {
 	}
 
 	clientConn, serverConn := net.Pipe()
-	defer clientConn.Close()
-	defer serverConn.Close()
+	defer func() { _ = clientConn.Close() }()
+	defer func() { _ = serverConn.Close() }()
 
 	session := &Session{
 		logger:    logger,
@@ -364,7 +364,7 @@ func TestSessionWorkWithDevModeLogging(t *testing.T) {
 		cryptConn: network.NewCryptConn(serverConn),
 	}
 
-	clientConn.Close()
+	_ = clientConn.Close()
 
 	session.work()
 }
@@ -380,7 +380,7 @@ func TestSessionWorkWithEmptyRead(t *testing.T) {
 	}
 
 	clientConn, serverConn := net.Pipe()
-	defer serverConn.Close()
+	defer func() { _ = serverConn.Close() }()
 
 	session := &Session{
 		logger:    logger,
@@ -389,7 +389,7 @@ func TestSessionWorkWithEmptyRead(t *testing.T) {
 		cryptConn: network.NewCryptConn(serverConn),
 	}
 
-	clientConn.Close()
+	_ = clientConn.Close()
 
 	session.work()
 }

server/signserver/sign_server.go

@@ -61,7 +61,7 @@ func (s *Server) Shutdown() {
 	s.Unlock()
 
 	// This will cause the acceptor goroutine to error and exit gracefully.
-	s.listener.Close()
+	_ = s.listener.Close()
 }
 
 func (s *Server) acceptClients() {
@@ -86,7 +86,7 @@ func (s *Server) acceptClients() {
 
 func (s *Server) handleConnection(conn net.Conn) {
 	s.logger.Debug("New connection", zap.String("RemoteAddr", conn.RemoteAddr().String()))
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	// Client initalizes the connection with a one-time buffer of 8 NULL bytes.
 	nullInit := make([]byte, 8)

server/signserver/sign_server_test.go

@@ -434,7 +434,7 @@ func TestServerHandleConnection(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Dial() error: %v", err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	nullInit := make([]byte, 8)
 	_, err = conn.Write(nullInit)
@@ -472,7 +472,7 @@ func TestServerHandleConnectionWithShortInit(t *testing.T) {
 	}
 
 	_, _ = conn.Write([]byte{0, 0, 0, 0})
-	conn.Close()
+	_ = conn.Close()
 
 	time.Sleep(50 * time.Millisecond)
 }
@@ -502,7 +502,7 @@ func TestServerHandleConnectionImmediateClose(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Dial() error: %v", err)
 	}
-	conn.Close()
+	_ = conn.Close()
 
 	time.Sleep(50 * time.Millisecond)
 }
@@ -544,7 +544,7 @@ func TestServerMultipleConnections(t *testing.T) {
 	time.Sleep(50 * time.Millisecond)
 
 	for _, conn := range conns {
-		conn.Close()
+		_ = conn.Close()
 	}
 }