Why directory in unmanagedClasspath is copied to system temporary directory?

[ngocdaothanh]

I'm using SBT 1.2.1 on Mac for a multimodule project.

My project directory looks like this:

project/
  module1/
    src/
  module2/
    src/
  app/
    config/
      xitrum.conf
    src/
  build.sbt

In my build.sbt, I'm adding app/config directory to classpath of module app like this:

unmanagedClasspath in Compile += baseDirectory.value / "config"
unmanagedClasspath in Runtime += baseDirectory.value / "config"

When I run the project with sbt run, I see that the config directory is copied to system temporary directory like this:

/var/folders/gc/14jrc47n1k17cz4zfx2f9t6h0000gp/T/sbt_d931459f/target/dbed45b7/config

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Question:
Why SBT 1.2.1 doesn't simply use the config file as is? Why does it need to copy (unlike SBT 0.13.x)?

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I'm asking because in the multimodule project, path to the directory containing the config directory is calculated like this:

val url = Thread.currentThread.getContextClassLoader.getResource("xitrum.conf")
val fileName = url.getFile
val rootDir = fileName.replace("/config/xitrum.conf", "")

The code works with SBT 0.13.x, but not with SBT 1.2.1.

Note: System.getProperty("user.dir") can be used, but it's not flexible for multimodule SBT project.

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Question: If modifying SBT to use the config directory as-is is not an option, in a multimodule SBT project, what should I do to get path to directory containing the config directory dynamically (that is, do not hardcode System.getProperty("user.dir") + "/app")?

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Also, it seems that SBT doesn't clean up the temporary directory it created when it exits (#502).

[eed3si9n]

sbt run now uses bgRun (background run). If you would like the same behavior as 0.13, please use fgRun.

[ngocdaothanh]

I searched about bgRun and found this: #2936

Is there more official doc about bgRun and fgRun?