Implement callbacks to Akka's `FileIO` whenever writing successfully adds a chunk to a file

[mdedetrich]

What is currently missing?

At some point we would like to add file based backups to Guardian.

How could this be improved?

Currently this is not possible because Akka's FileIO doesn't allow you to add a "callback" whenever a chunk of bytes have been successfully written to disk. The callback is required to commit the current Kafka cursor and hence the set of changes required are similar in concept to what was done with Alpakka's S3 client (see akka/alpakka#2770). Here is the location where the current FileIO actually writes the data to disk https://github.com/akka/akka/blob/7abc41cf4e7e8827393b181cd06c5f8ea684e696/akka-stream/src/main/scala/akka/stream/impl/io/FileOutputStage.scala#L63

Is this a feature you would work on yourself?

• I plan to open a pull request for this feature