Properly handle io.EOF error conditions when reading

Previously, the Server.Serve method would never return nil,
because the infinite for-loop handling request packets would
only break if reading a packet reported an error.
A common termination condition is when the underlying connection
is closed and recvPacket returns io.EOF.
In which case Serve should ignore io.EOF and
treat it as a normal shutdown.

However, this means that recvPacket must correctly handle io.EOF
such that it never reports io.EOF if a packet is partially read.
There are two calls to io.ReadFull in recvPacket.
The first call correctly forwards an io.EOF error
if no additional bytes of the next packet are read.
However, the second call incorrectly forwards io.EOF
when no bytes of the payload could be read.
This is incorrect since we already read the length and
should convert the io.EOF into an io.ErrUnexpectedEOF.

conn.go

@@ -18,7 +18,9 @@ type conn struct {
 }
 
 // the orderID is used in server mode if the allocator is enabled.
-// For the client mode just pass 0
+// For the client mode just pass 0.
+// It returns io.EOF if the connection is closed and
+// there are no more packets to read.
 func (c *conn) recvPacket(orderID uint32) (uint8, []byte, error) {
 	return recvPacket(c, c.alloc, orderID)
 }

packet.go

@@ -290,6 +290,9 @@ func recvPacket(r io.Reader, alloc *allocator, orderID uint32) (uint8, []byte, e
 		b = make([]byte, length)
 	}
 	if _, err := io.ReadFull(r, b[:length]); err != nil {
+		if err == io.EOF {
+			err = io.ErrUnexpectedEOF
+		}
 		debug("recv packet %d bytes: err %v", length, err)
 		return 0, nil, err
 	}

server.go

@@ -327,7 +327,7 @@ func handlePacket(s *Server, p orderedRequest) error {
 }
 
 // Serve serves SFTP connections until the streams stop or the SFTP subsystem
-// is stopped.
+// is stopped. It returns nil if the server exits cleanly.
 func (svr *Server) Serve() error {
 	defer func() {
 		if svr.pktMgr.alloc != nil {
@@ -353,6 +353,10 @@ func (svr *Server) Serve() error {
 	for {
 		pktType, pktBytes, err = svr.serverConn.recvPacket(svr.pktMgr.getNextOrderID())
 		if err != nil {
+			// Check whether the connection terminated cleanly in-between packets.
+			if err == io.EOF {
+				err = nil
+			}
 			// we don't care about releasing allocated pages here, the server will quit and the allocator freed
 			break
 		}