commit 0dd1daae2dffb1ebdc88e751766effb29c0c2288
author Christopher Collins <ccollins@apache.org> Tue Jul 23 17:32:36 2019 -0700
committer ccollins476ad <ccollins476ad@gmail.com> Mon Jul 29 09:51:49 2019 -0700
tree 64ea22f9eed57e6cfaeafbf0200e3a7f44822877
parent 0dcb0de1c9776010fbf56d598232b8fb88892082

nmxact: API change for CoAP operations

This commit changes the CoAP functions in the `sesn` interface.  In
particular, it replaces the old API with three low-level functions:

    // Creates a listener for incoming CoAP messages matching the
    // specified criteria.
    ListenCoap(mc nmcoap.MsgCriteria) (*nmcoap.Listener, error)

    // Cancels the CoAP listener with the specified criteria.
    StopListenCoap(mc nmcoap.MsgCriteria)

    // Transmits a CoAP message.
    TxCoap(m coap.Message) error

Helper functions that combine these operations in useful ways are
implemented in `sesn/sesn_util.go`.

Prior to this change, `sesn` contained only two CoAP functions:

    TxCoapOnce(m coap.Message,
        opt TxOptions) (coap.COAPCode, []byte, error)

    TxCoapObserve(m coap.Message, opt TxOptions, NotifCb GetNotifyCb,
        stopsignal chan int) (coap.COAPCode, []byte, []byte, error)

Both of these functions performed transmit followed by a blocking
receive.  There were two issues with these functions:

1. Lots of duplicated logic in each `sesn` implementation.
2. Some forms of application layer security are not possible.

Re: 2- for example, say secure notifications are implemented as follows:

    Notification is encrypted and wrapped in a second, unencrypted, CoAP
    notification.  The outer message always specifies the same resource
    and has an indeterminate token.

To receive these notifications, the application needs to set up a
permanent listener for the secure resource.  This listener needs to be
created independently; it should not be paired with a transmit
operation.  To make this possible, the "listen" operation needs to be a
standalone function.

More generally, an interface with low level functions leads to less code
duplication and a better design.