fix(checkConn): try to peek into the connection instead of consuming

Initially connCheck was reading a byte from the socket which won't allow
the code for processing push notification to work properly once a byte
is read. Try to peek instead of reading, so later the next operations
over the connection can work with the correct data.

Author: ndyakov

internal/pool/conn_check.go

@@ -8,10 +8,14 @@ import (
 	"net"
 	"syscall"
 	"time"
+	"unsafe"
 )
 
 var errUnexpectedRead = errors.New("unexpected read from socket")
 
+// connCheck checks if the connection is still alive and if there is data in the socket
+// it will try to peek at the next byte without consuming it since we may want to work with it
+// later on (e.g. push notifications)
 func connCheck(conn net.Conn) error {
 	// Reset previous timeout.
 	_ = conn.SetDeadline(time.Time{})
@@ -29,16 +33,25 @@ func connCheck(conn net.Conn) error {
 
 	if err := rawConn.Read(func(fd uintptr) bool {
 		var buf [1]byte
-		n, err := syscall.Read(int(fd), buf[:])
+		// Use MSG_PEEK to peek at data without consuming it
+		n, _, errno := syscall.Syscall6(
+			syscall.SYS_RECVFROM,
+			fd,
+			uintptr(unsafe.Pointer(&buf[0])),
+			1,
+			syscall.MSG_PEEK, // This ensures the byte stays in the socket buffer
+			0, 0,
+		)
+
 		switch {
-		case n == 0 && err == nil:
+		case n == 0 && errno == 0:
 			sysErr = io.EOF
 		case n > 0:
 			sysErr = errUnexpectedRead
-		case err == syscall.EAGAIN || err == syscall.EWOULDBLOCK:
+		case errno == syscall.EAGAIN || errno == syscall.EWOULDBLOCK:
 			sysErr = nil
 		default:
-			sysErr = err
+			sysErr = errno
 		}
 		return true
 	}); err != nil {