Add custom error type for ECONNREFUSED

internal/socket_unix.go

@@ -156,7 +156,13 @@ func connect(fd int, remoteAddr net.Addr, timeout time.Duration, opts ...sonicop
 			return os.NewSyscallError("getsockopt", err)
 		}
 		if socketErr != 0 {
-			return syscall.Errno(socketErr)
+			var err error = syscall.Errno(socketErr)
+			if errors.Is(err, syscall.ECONNREFUSED) {
+				// This is the most likely to happen and on which callers will have custom logic. The rest we can just
+				// propagate.
+				return sonicerrors.ErrConnRefused
+			}
+			return err
 		}
 	}
 

sonicerrors/errors.go

@@ -8,4 +8,5 @@ var (
 	ErrTimeout                = errors.New("operation timed out")
 	ErrNeedMore               = errors.New("need to read/write more bytes")
 	ErrNoBufferSpaceAvailable = errors.New("no buffer space available")
+	ErrConnRefused            = errors.New("connection refused") // a connect() on a stream socket found no one listening on the remote address
 )