Implement Progress Reporting

[rgaudin]

image-creator being a machine-tool to be used by other systems as well as it being task-oriented and sensible to exofailures (downloads), it should be able to report its status (where it is in the state machine), what it's doing and overall progress in a machine-readable format.

As a crucial SPOF, it should also allow post-mortem investigations.

Progress report and History, although regularly mixed for simplicity, respond to different needs and constraint:

• History is to be used by humans, so text format is preferred
• Progress Report is to be used by machines so JSON is preferred
• Textual Progress Report should also be possible

Options for it includes stdout/stderr output, output to file or socket, file or TCP socket to read or query status.

Let's keep in mind that it will mostly be used from within a container (I suppose).

At this moment, my proposal would be:

• Textual history + progress to stdout.
• JSON live progress to a specified file.

stdout is very easy to integrate and work-with. Kind of how it's always being used, so history can easily be read/fetched/archived.
Progress would be dynamic, using CR character so that it doesn't pollute history (not visible in docker logs).

Using a specified file for machine-readable progress has several advantages:

• easy to consume
• passive, synchronous. no need to query or wait for answer
• it's atomic: you just read the whole file instead of wondering where you picked the stream at or how much you should be reading from the socket.
• Previous point is super useful when parsing JSON
• can be set to a RAMfs (/dev/shm for instance) so you're not hitting IOs
• easy to bind and reuse when using containers

It's important that long-lasting tasks report both to the history and periodically on the progress to avoid the common ambiguity of the last running task: is it still running or is the process stalled?