This PR adds retries for transfers, along with the ability to resume partially downloaded files.

Here are the docs:

Retry Behavior

Failed downloads can be retried automatically up to the configured number of attempts. If an attempt fails initially, the application delays the second attempt by the configured delay, and an exponential backoff is used to compute the delay for all subsequent events, up to the configured maximum delay.

By default, partial downloads are resumed based on the size of the incomplete file. Users can choose to always overwrite files if they wish.

YAML

transfers:
  download:
    retry:
      incomplete: resume # 'overwrite' or 'resume'
      attempts: 3
      delay: 5000 # initial time between retries, in milliseconds
      max_delay: 60000 # maximum time between retries, in milliseconds

This PR also introduces a BatchId column for Transfers, but doesn't do anything with it yet.

Testing

I ran another copy of slskd with the following adjustment to the upload code:

governor: (tx, req, ct) =>
{
    if (Random.Shared.NextDouble() < 0.5)
    {
        Log.Error("Unlucky!!!");
        throw new Exception("unlucky");
    }
    return Governor.GetBytesAsync(tx.Username, req, ct);
},

This gives uploads a 50% chance of failing each time the governor is asked for a block of bytes, which is every iteration of the send loop. With this in place, I downloaded a directory:

Each file failed and was automatically retried, up to 96 times. I found and fixed a bug with progress reporting in Soulseek.NET here, and with that, everything seems to be working perfectly.

Source files:

Downloaded files: