Overview about message send process and delivery states

[HenrikJannsen]

When sending a direct message we show a message delivery state as icons and tooltips. In the process there are several steps where things can fail.
Most messages are supporting a ACK protocol, thus when the user receives an AckRequestingMessage message it has to send a AckMessage back. This gives the sender feedback about the successful delivery.
While sending we store the sent message locally, so that in case of a failure we can resend it.

Here an overview about the possible paths:

Both users online

1. Establish connection if not already established (CONNECTING)
2. Attempting to send message (SENT)
3. Send did not result in a failure. Waiting for ACK message
4. ACK message (ACK_RECEIVED)

Peer who was just online went offline

1. Use existing connection which has not been closed yet
2. Attempting to send message (SENT)
3. Send did result in a failure.
4. Try to send mailbox message (TRY_ADD_TO_MAILBOX)
5. Await until sufficient successful broadcast results have returned, then set state ADDED_TO_MAILBOX
6. Once peer goes online again, they will receive the mailbox message (could be also the case that peer was online but message sent failed, then peer gets the mailbox message via the broadcast and process it). When processing the mailbox message send the ACK message back
7. Receive ACK message of peer (MAILBOX_MSG_RECEIVED)

Offline peer

1. Trying to establish a connection, but it fails (this can take up to 2 minutes in the worst case, for optimisation see [1]) (CONNECTING)
2. Try to send mailbox message (TRY_ADD_TO_MAILBOX)
3. Await until sufficient successful broadcast results have returned, then set state ADDED_TO_MAILBOX
4. Once peer goes online again, they will receive the mailbox message (could be also the case that peer was online but message sent failed, then peer gets the mailbox message via the broadcast and process it). When processing the mailbox message send the ACK message back
5. Receive ACK message of peer (MAILBOX_MSG_RECEIVED)

Failed due too fast close while connecting

1. Trying to establish a connection (CONNECTING)
2. User close app. We have stored the message locally
3. When user starts up app again we lookup the local store for unsuccessfully send attempts. We see that state is CONNECTING and show resend button as well we try to resend after a delay (better connectivity)

Failed due too fast close while adding to mailbox

1. Trying to establish a connection, but it fails (this can take up to 2 minutes in the worst case, for optimisation see [1]) (CONNECTING)
2. Try to send mailbox message (TRY_ADD_TO_MAILBOX)
3. User close app. We have stored the message locally
4. When user starts up app again we lookup the local store for unsuccessfully send attempts. We see that state is TRY_ADD_TO_MAILBOX and show resend button as well we try to resend after a delay (better connectivity)

Other failure cases

If a failure happens at send we show the FAILED state. We show the resend button

If we have not received a ACK message after state SENT (expecting user is online), we also resend and show resend button at restart.

enum MessageDeliveryStatus
    CONNECTING,
    SENT,
    ACK_RECEIVED,
    TRY_ADD_TO_MAILBOX,
    ADDED_TO_MAILBOX,
    MAILBOX_MSG_RECEIVED,
    FAILED;

[1] #2536