Mailbox upload worker

The upload worker implementation is shared between the client for our own mailbox (#2290 (closed)) and the client for a contact's mailbox (#2289 (closed)). The worker's lifecycle is managed by the mailbox client, which in turn is managed by the mailbox client manager (#2228 (closed)).

Each upload worker is associated with a contact that has been assigned to a mailbox for upload. When a contact is reassigned to a different mailbox for upload, the old worker is destroyed and a new worker is created.

The worker can be in the following states:

• Checking for data to send
• Waiting for data to send
• Waiting for connectivity check
• Writing and uploading
• Destroyed

When the worker is created:

• Set the current state to "checking for data to send"
• Check for data to send

When the worker is destroyed:

• Set the current state to "destroyed"
• Cancel the current upload task, if any

To check for data to send:

• Load the time when data will next be ready to send to the contact
• If data is ready to send now:
  • Set the current state to "waiting for connectivity check"
  • Call the client's connectivity check method
• Else:
  • Set the current state to "waiting for data to send"
  • Schedule a wakeup task for the time when data will next be ready to send

When an event indicates that new data may be ready to send:

• If the current state is "waiting for data to send":
  • Cancel the wakeup task
  • Set the current state to "checking for data to send"
  • Check for data to upload

When a wakeup task runs:

• If the current state is "waiting for data to send":
  • Set the current state to "checking for data to send"
  • Check for data to upload

When a connectivity check succeeds:

• If the current state is "waiting for connectivity check":
  • Set the current state to "writing and uploading"
  • Write and upload a file

To write and upload a file:

• Create a temporary file in the upload directory
• Pass the file's path to the mailbox plugin to get a transport connection writer
• Decorate the transport connection writer to handle disposal
• Create a deferred send handler
• Pass the decorated transport connection writer and the deferred send handler to the connection manager
• Data will be written to the file asynchronously
• The IDs of any sent/acked messages will be recorded by the deferred send handler

When the transport connection writer is disposed of:

• If the write succeeded:
  • Start an upload task to upload the file to the mailbox (#2231 (closed))
• Else:
  • Delete the file
  • Set the current state to "checking for data to send"
  • Check for data to send

When an upload task succeeds:

• Retrieve the sent/acked message IDs from the deferred send handler
• Mark the messages as sent/acked
• Delete the file
• Set the current state to "checking for data to send"
• Check for data to send

When an upload task is cancelled:

• Delete the file