Running external command inside akka actor. Catch exception - scala

Question

I don't know how to reproduce my problem in a simple way.
I have an actor that executes external command by 'sys.process' package.
object FileHelper {
def downloadFile(url: String, filename: String): Either[String, Unit] = {
println(s"MyThread: ${Thread.currentThread().getName}")
util.Try {
import scala.language.postfixOps
new URL(url) #> new File(filename) !
} match {
case util.Failure(err) => Left(s"Download error: $err")
case util.Success(code) => if (code != 0) Left("Can't download file") else Right({})
}
}
}
So when i call dowloadFile within actor Try statement doesn't work!
router MyThread: app-akka.actor.default-dispatcher-3
router[ERROR] Exception in thread "Thread-10" java.io.FileNotFoundException: /home/alex/dumpss/456.tar.bz2 (No such file or directory)
router[ERROR] at java.io.FileOutputStream.open0(Native Method)
router[ERROR] at java.io.FileOutputStream.open(FileOutputStream.java:270)
router[ERROR] at java.io.FileOutputStream.<init>(FileOutputStream.java:213)
router[ERROR] at scala.sys.process.ProcessBuilderImpl$FileOutput$$anonfun$$lessinit$greater$3.apply(ProcessBuilderImpl.scala:33)
router[ERROR] at scala.sys.process.ProcessBuilderImpl$FileOutput$$anonfun$$lessinit$greater$3.apply(ProcessBuilderImpl.scala:33)
router[ERROR] at scala.sys.process.ProcessBuilderImpl$OStreamBuilder$$anonfun$$lessinit$greater$4.apply(ProcessBuilderImpl.scala:38)
router[ERROR] at scala.sys.process.ProcessBuilderImpl$OStreamBuilder$$anonfun$$lessinit$greater$4.apply(ProcessBuilderImpl.scala:38)
router[ERROR] at scala.sys.process.ProcessBuilderImpl$ThreadBuilder$$anonfun$1.apply$mcV$sp(ProcessBuilderImpl.scala:58)
router[ERROR] at scala.sys.process.ProcessImpl$Spawn$$anon$1.run(ProcessImpl.scala:23)
As you see external command has been executed in thread 'Thread-10' but Try is catching exception in 'app-akka.actor.default-dispatcher-3'.

Answer 1

With scala process api, url downloading and file redirection are implemented by threads instead of real processes: https://github.com/scala/scala/blob/2.12.x/src/library/scala/sys/process/ProcessBuilderImpl.scala#L31-L64
So, when this line gets executed,
new URL(url) #> new File(filename) !
two more threads is spawned, one for downloading the url and writing the result to the pipe, the other reads from the pipe and writes whatever it reads to the file. And the parent thread (in which the actor is running) waits for their exit values, and returns either of them accordingly: https://github.com/scala/scala/blob/2.12.x/src/library/scala/sys/process/ProcessImpl.scala#L151
Unfortunately, exit value for file redirection is always ignored. So you cannot tell whether the operation succeeds or not by checking the return code of the pipe. https://github.com/scala/scala/blob/2.12.x/src/library/scala/sys/process/ProcessBuilderImpl.scala#L39
Instead of using scala process api, you can do the work with the help of commons-io library:
Try {
IOUtils.copy(url.openStream, new FileOutputStream(file))
} match {
case Success(_) => ...
case Failure(ex) => ...
}