[FEATURE] Let --download resume downloads rather than start over

[ZeLonewolf]

Is your feature request related to a problem? Please describe. When running planetiler with --download, if the run is stopped while a download is in progress, a partial file is left behind. It would save time if a partial file download could optionally be resumed, provided that the partially downloaded file is not too old (i.e. changed on the server).

Describe the solution you'd like A command-line argument, perhaps --resume-download, that instructs planetiler to resume a partial download.

[msbarry]

This should be doable, but definitely not trivial since the downloader could be downloading multiple segments of the file in parallel. The rough approach I'd take is:

• change the downloader to not delete the temp file we're downloading on exit
• also store a state file alongside the temp download file that contains some planetiler version number, remote file size (or etag?), the segment start/endpoints, and how many bytes we've downloaded for each segment
• as we're downloading, periodically flush the current state of the download segments to that state file

Then on resume check for a state file, if it exists and the planetiler version number and remote file size match then resume downloading each chunk where we left off.

if the run is stopped while a download is in progress, a partial file is left behind

Do you actually see the partial file after the JVM exits? It should be deleting that temp file on exit.

[ZeLonewolf]

Yes, I am seeing a partial file left behind on JVM exit:

This is when pressing the "stop" button in Eclipse, so it's possible that this is doing a non-graceful JVM shutdown.