net: fix UDPConn readers to return truncated payload size instead of 0

Calling UDPConn readers (Read, ReadFrom, ReadMsgUDP) to read part of
datagram returns error (in Windows), mentioning there is more data
available, and 0 as size of read data, even though part of data is
already read.

This fix makes UDPConn readers to return truncated payload size,
even there is error due more data available to read.

Fixes #14074
Updates #18056

Change-Id: Id7eec7f544dd759b2d970fa2561eef2937ec4662
Reviewed-on: https://go-review.googlesource.com/92475
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/internal/poll/fd_windows.go

@@ -225,6 +225,10 @@ func (s *ioSrv) ExecIO(o *operation, submit func(o *operation) error) (int, erro
 		// All is good. Extract our IO results and return.
 		if o.errno != 0 {
 			err = syscall.Errno(o.errno)
+			// More data available. Return back the size of received data.
+			if err == syscall.ERROR_MORE_DATA || err == syscall.WSAEMSGSIZE {
+				return int(o.qty), err
+			}
 			return 0, err
 		}
 		return int(o.qty), nil

src/net/net.go

@@ -303,20 +303,23 @@ func (c *conn) File() (f *os.File, err error) {
 // Multiple goroutines may invoke methods on a PacketConn simultaneously.
 type PacketConn interface {
 	// ReadFrom reads a packet from the connection,
-	// copying the payload into b. It returns the number of
-	// bytes copied into b and the return address that
+	// copying the payload into p. It returns the number of
+	// bytes copied into p and the return address that
 	// was on the packet.
+	// It returns the number of bytes read (0 <= n <= len(p))
+	// and any error encountered. Callers should always process
+	// the n > 0 bytes returned before considering the error err.
 	// ReadFrom can be made to time out and return
 	// an Error with Timeout() == true after a fixed time limit;
 	// see SetDeadline and SetReadDeadline.
-	ReadFrom(b []byte) (n int, addr Addr, err error)
+	ReadFrom(p []byte) (n int, addr Addr, err error)
 
-	// WriteTo writes a packet with payload b to addr.
+	// WriteTo writes a packet with payload p to addr.
 	// WriteTo can be made to time out and return
 	// an Error with Timeout() == true after a fixed time limit;
 	// see SetDeadline and SetWriteDeadline.
 	// On packet-oriented connections, write timeouts are rare.
-	WriteTo(b []byte, addr Addr) (n int, err error)
+	WriteTo(p []byte, addr Addr) (n int, err error)
 
 	// Close closes the connection.
 	// Any blocked ReadFrom or WriteTo operations will be unblocked and return errors.

src/net/timeout_test.go

@@ -482,7 +482,7 @@ func TestReadFromTimeout(t *testing.T) {
 					time.Sleep(tt.timeout / 3)
 					continue
 				}
-				if n != 0 {
+				if nerr, ok := err.(Error); ok && nerr.Timeout() && n != 0 {
 					t.Fatalf("#%d/%d: read %d; want 0", i, j, n)
 				}
 				break

src/net/udpsock_test.go

@@ -398,9 +398,56 @@ func TestUDPZeroByteBuffer(t *testing.T) {
 		switch err {
 		case nil: // ReadFrom succeeds
 		default: // Read may timeout, it depends on the platform
-			if nerr, ok := err.(Error); (!ok || !nerr.Timeout()) && runtime.GOOS != "windows" { // Windows returns WSAEMSGSIZ
+			if nerr, ok := err.(Error); (!ok || !nerr.Timeout()) && runtime.GOOS != "windows" { // Windows returns WSAEMSGSIZE
 				t.Fatal(err)
 			}
 		}
 	}
 }
+
+func TestUDPReadSizeError(t *testing.T) {
+	switch runtime.GOOS {
+	case "nacl", "plan9":
+		t.Skipf("not supported on %s", runtime.GOOS)
+	}
+
+	c1, err := newLocalPacketListener("udp")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c1.Close()
+
+	c2, err := Dial("udp", c1.LocalAddr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c2.Close()
+
+	b1 := []byte("READ SIZE ERROR TEST")
+	for _, genericRead := range []bool{false, true} {
+		n, err := c2.Write(b1)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if n != len(b1) {
+			t.Errorf("got %d; want %d", n, len(b1))
+		}
+		c1.SetReadDeadline(time.Now().Add(100 * time.Millisecond))
+		b2 := make([]byte, len(b1)-1)
+		if genericRead {
+			n, err = c1.(Conn).Read(b2)
+		} else {
+			n, _, err = c1.ReadFrom(b2)
+		}
+		switch err {
+		case nil: // ReadFrom succeeds
+		default: // Read may timeout, it depends on the platform
+			if nerr, ok := err.(Error); (!ok || !nerr.Timeout()) && runtime.GOOS != "windows" { // Windows returns WSAEMSGSIZE
+				t.Fatal(err)
+			}
+		}
+		if n != len(b1)-1 {
+			t.Fatalf("got %d; want %d", n, len(b1)-1)
+		}
+	}
+}

src/syscall/types_windows.go

@@ -27,6 +27,7 @@ const (
 	ERROR_NOT_FOUND           Errno = 1168
 	ERROR_PRIVILEGE_NOT_HELD  Errno = 1314
 	WSAEACCES                 Errno = 10013
+	WSAEMSGSIZE               Errno = 10040
 	WSAECONNABORTED           Errno = 10053
 	WSAECONNRESET             Errno = 10054
 )