Better error handling

err.go

@@ -2,6 +2,7 @@ package tcp
 
 import (
 	"errors"
+	"syscall"
 )
 
 // ErrTimeout indicates I/O timeout
@@ -15,3 +16,13 @@ type timeoutError struct{}
 func (e *timeoutError) Error() string   { return "I/O timeout" }
 func (e *timeoutError) Timeout() bool   { return true }
 func (e *timeoutError) Temporary() bool { return true }
+
+// ErrConnect is an error occurs while connecting to the host
+// To get the detail of underlying error, lookup ErrorCode() in 'man 2 connect'
+type ErrConnect struct {
+	syscall.Errno
+}
+
+func newErrConnect(errCode int) *ErrConnect {
+	return &ErrConnect{syscall.Errno(errCode)}
+}

shaker.go

@@ -31,7 +31,6 @@
 package tcp
 
 import (
-	"fmt"
 	"os"
 	"runtime"
 	"sync"
@@ -97,12 +96,12 @@ func (s *Shaker) Test(addr string, timeout time.Duration) error {
 	// check for connect error
 	for {
 		succeed, err := s.wait(fd, timeoutMS)
-		if err != nil {
-			return fmt.Errorf("connect error: %s", err)
-		}
 		if reached(deadline) {
 			return ErrTimeout
 		}
+		if err != nil {
+			return err
+		}
 		if succeed {
 			return nil
 		}
@@ -170,7 +169,7 @@ func (s *Shaker) wait(fd int, timeoutMS int) (bool, error) {
 				return false, os.NewSyscallError("getsockopt", err)
 			}
 			if errCode != 0 {
-				return false, fmt.Errorf("getsockopt[%d]", errCode)
+				return false, newErrConnect(errCode)
 			}
 			return true, nil
 		}

shaker_test.go

@@ -20,6 +20,10 @@ func ExampleShaker() {
 	case nil:
 		fmt.Println("Connect to Google succeded")
 	default:
-		fmt.Println("Connect to Google failed:", err)
+		if e, ok := err.(*ErrConnect); ok {
+			fmt.Println("Connect to Google failed:", e)
+		} else {
+			fmt.Println("Error occurred while connecting:", err)
+		}
 	}
 }