SBT hangs waiting for orphaned subprocess to complete - scala

Question

In my SBT (0.13.16) build, I have the following task:
startThing := {
var bin_path = s"${file(".").getAbsolutePath}/bin"
val result = s"$bin_path/start_thing".!
if (result != 0)
throw new RuntimeException("Could not start Thing..")
true
}
And start_thing contains:
(run_subprocess &)
and my build hangs.
I can see that start_thing exits (the process table does not have it as an entry) but adding some printlns to the task shows that it's stuck on val result = s"$bin_path/start_thing".!.
If I kill the run_subprocess process then SBT unblocks and runs normally.
In this particular case, run_subprocess has set up some Kubernetes port-forwarding that needs to be there in order for subsequent tests to work.

Answer 1

Try daemonising the background process like so
(run_subprocess >/dev/null 2>&1 &)
The issue could be output from run_subprocess still going to sbt parent as suggested here.
I was able to replicate the issue in both sbt 0.13.17 and 1.0.2. Daemonising worked in both.

Answer 2

Regardless of my comment, in my case the reason for the hanging was ACTUALLY the potential leaving of open STDOUT , STDERR handles in a daemon started by the script , that is OK:
/usr/local/bin/minio server "$minio_data_dir" > /dev/null 2>&1 & # and start the server
and NOT ok:
/usr/local/bin/minio server "$minio_data_dir" 2>&1 & # and start the server
So the hanging occurred randomly EVEN with the accepted answers start in the background ...
Thus this solution needed NOT, any wrapper bash scripts ... This is how the code looked in the build.sbt
lazy val startLocalS3 = inputKey[Unit]("localS3")
lazy val startLocalS3Task = TaskKey[Unit]("localS3", "create local s3")
lazy val core: Project = project
.in(file("."))
.settings(
name := "rfco-core",
startLocalS3Task := {
val cmd: Seq[String] = Seq("bash" , "-c" , "./CI/start-s3-svr.sh")
import sys.process._
cmd.mkString(" ").!!
},
fork in startLocalS3Task := true,
compile.in(Compile) := (compile in Compile).dependsOn((startLocalS3Task)).value
// you might want to use Test scope ^^ here
)