fix issue with file put resume/append

Fixes issue with append uploads. Was opening the file with O_APPEND, but
it uses WriteAt() to write the data which doesn't work with a file
opened in append mode. Removing the append flag fixes the issue as the
client is sending the offsets anyways.

Also added a note to the Request server's FileWriter interface on
handling append flags.

request-interfaces.go

@@ -25,6 +25,8 @@ type FileReader interface {
 // The request server code will call Close() on the returned io.WriterAt
 // ojbect if an io.Closer type assertion succeeds.
 // Note in cases of an error, the error text will be sent to the client.
+// Note when receiving an Append flag it is important to not open files using
+// O_APPEND if you plan to use WriteAt, as they conflict.
 // Called for Methods: Put, Open
 type FileWriter interface {
 	Filewrite(*Request) (io.WriterAt, error)

server.go

@@ -398,9 +398,9 @@ func (p sshFxpOpenPacket) respond(svr *Server) responsePacket {
 		return statusFromError(p, syscall.EINVAL)
 	}
 
-	if p.hasPflags(sshFxfAppend) {
-		osFlags |= os.O_APPEND
-	}
+	// Don't use O_APPEND flag as it conflicts with WriteAt.
+	// The sshFxfAppend flag is a no-op here as the client sends the offsets.
+
 	if p.hasPflags(sshFxfCreat) {
 		osFlags |= os.O_CREATE
 	}