refactor: cleanup error checks in Windows

README.md

@@ -3,7 +3,7 @@
 conncheck implements robust, fast, and reusable network connection liveness check in Go.
 Essentially, `conncheck.Do` implements the missing `net.Conn.IsOpen()` method that tells you
 if the connection has been closed on the client, was interrupted by network infrastructure, 
-or the server asked to close it, e.g. with an RST or FIN packet in case of TCP.
+or the server asked to close it e.g., with an RST or FIN packet in case of TCP.
 
 `conncheck` is useful in any network application with long-lived, sometimes idle, connections
 like SQL connection pools, Apache Thrift clients, etc.
@@ -30,9 +30,15 @@ the ping takes the time of a full round-trip and consumes precious server resour
 `conncheck` peeks without blocking from the connection at OS level. If the OS kernel is aware
 of the connection being closed or interrupted, by either peer, it will reject the peek.
 
-The Github team first documented the general approach in [a blog post](https://github.blog/engineering/three-bugs-in-the-go-mysql-driver/)
-and a fix in the MySQL Go SQL driver. The approach was later borrowed by other libraries that use
-long-lived connections like [Apache Thrift](https://github.com/apache/thrift/pull/2153).
+The Github team first documented the general approach in 
+[a blog post](https://github.blog/engineering/three-bugs-in-the-go-mysql-driver/)
+The approach was later borrowed by other libraries that use  long-lived connections 
+like [Apache Thrift](https://github.com/apache/thrift/pull/2153).
 
 Unlike existing solutions, `conncheck` is reusable and supports both Unix systems and Windows.
-PRs will be opened, using this code, for the OSS projects with existing limited solutions. 
+PRs will be opened, using this code, for the OSS projects with existing limited solutions.
+
+## Copyright and acknowledgements
+
+This library is licensed under the Apache 2 license. All code was written from scratch. The Unix code
+used the Apache Thrift implementation as a reference, while the Windows code is completely novel. 

conncheck.go

@@ -40,7 +40,7 @@ func Do(conn net.Conn) Status {
 	}
 
 	rawConn, err := sc.SyscallConn()
-	if err != nil {
+	if err != nil || rawConn == nil {
 		if errors.Is(err, syscall.EINVAL) || errors.Is(err, net.ErrClosed) {
 			return StatusNotOpen
 		}

conncheck_test.go

@@ -6,7 +6,10 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
+	"errors"
+	"fmt"
 	"net"
+	"syscall"
 	"testing"
 	"time"
 
@@ -26,8 +29,37 @@ func TestDo(t *testing.T) {
 	})
 	t.Run("clientClose", testClientClose)
 	t.Run("UDP", testUDP)
+	t.Run("unsupported", testUnsupported)
 }
 
+func testUnsupported(t *testing.T) {
+	for err, res := range map[error]conncheck.Status{
+		syscall.EINVAL:      conncheck.StatusNotOpen,
+		net.ErrClosed:       conncheck.StatusNotOpen,
+		errors.New("other"): conncheck.StatusUnknown,
+		nil:                 conncheck.StatusUnknown} {
+		t.Run("syscall.Conn error "+fmt.Sprint(err), func(t *testing.T) {
+			conn := unsupportedConn{syscallConnErr: err}
+			require.Equal(t, res, conncheck.Do(conn))
+		})
+	}
+}
+
+type unsupportedConn struct {
+	net.Conn
+
+	syscallConnErr error
+}
+
+func (u unsupportedConn) SyscallConn() (syscall.RawConn, error) {
+	return nil, u.syscallConnErr
+}
+
+var _ interface {
+	net.Conn
+	syscall.Conn
+} = unsupportedConn{}
+
 func testClientClose(t *testing.T) {
 	accepted, ln := createServer(t, nil)
 	defer func() {

peek_windows.go

@@ -55,15 +55,26 @@ func tryPeek(rawConn syscall.RawConn) Status {
 		return true // escape out of the RawConn.Read loop
 	})
 
+	if readErr != nil {
+		// The read was rejected
+		// The only possible reason was that the connection was just closed on this side.
+		return StatusNotOpen
+	}
+
 	if sockOptErr != nil {
 		// We couldn't set the timeout and do the check.
+		// TODO: Determine if the reason is that the socket is closed and return NotOpen in that case
 		return StatusUnknown
 	}
 
-	if readErr != nil {
+	if sockOptResetErr != nil {
+		// The turning the timeout back didn't work.
+		// The only possible reason was that the connection was just closed on this side.
 		return StatusNotOpen
 	}
 
+	// readErr, sockOptErr, sockOptResetErr are nil - test finished
+
 	if n > 0 || // we peeked something,
 		// or there was nothing in the buffer, which is indicated by n == 0 and either:
 		errors.Is(recvErr, windows.WSAETIMEDOUT) || // if the connection is in blocking mode (typical)
@@ -73,14 +84,8 @@ func tryPeek(rawConn syscall.RawConn) Status {
 		// connection is open and there is nothing in the buffer
 		// recvErr may not be nil even with n > 0. Still, if we read something, the connection is open.
 
-		if sockOptResetErr != nil {
-			// The socket was open, but turning the timeout back didn't work.
-			// The only possible reason was that the connection was just closed on this side.
-			return StatusNotOpen
-		}
-
 		return StatusOpen
 	}
 
-	return StatusNotOpen // recvErr is not nil or n == 0 with recvErr == nil which means EOF
+	return StatusNotOpen // recvErr is not a timeout, or n == 0 && recvErr == nil, which means EOF
 }