Fixes for runBackground mutex and log management

[lihaoyi]

Fixes #3955.

• We make runBackground forward the stdout/stderr to $serverDir/{stdout,stderr} instead of os.Inherit. This is necessary because since we started using com-lihaoyi/os-lib@59b5fd9, os.Inherit is automatically pumped to the enclosing task logger, which for runBackground ends up being closed immediately so the logs are lost.

  • Now, the logs are instead picked up asynchronously by the FileToStreamTailer infrastructure, which picks them up and forwards them to any connected client regardless of who started the runBackground process

• Moved usage of FileToStreamTailer from the mill client to the server.

  • This allows better integration with the Mill logging infrastructure, e.g. ensuring tailed logs properly interact with the multi-line prompt by clearing the prompt before being printed and re-printing the prompt after.

• Simplified BackgroundWrapper

  • Renamed it MillBackgroundWrapper so it's more clear what it is when seen in jps
  • Use a file-lock for mutex, rather than polling on the process uuid/tombstone files
  • We still need to add a Thread.sleep after we take the lock, because the prior process seems to still hold on to sockets for some period of time. This defaults to 500ms (what is necessary experimentally) but is configurable by the new runBackgroundRestartDelayMillis: T[Int] task
  • Generally unified the creation/shutdown logic within MillBackgroundWrapper, rather than having it split between BackgroundWrapper and def backgroundSetup in the Mill server process

Tested manually by running rm -rf out && /Users/lihaoyi/Github/mill/target/mill-release -w runBackground inside example/javalib/web/1-hello-jetty. Forced updates via Enter in the terminal or via editing server source files. Verified that the runBackground server logs appear in the console and that they do not conflict with the multi-line status prompt