Launched process from Scala terminates but Scala waits forever for its termination

Question

I launch a process from an Ammonite script in this way:

scala.sys.process.Process(Seq("executable", "arg1", "arg2")).run().exitValue()

The launched process terminates ok, but it doesn't know about. Normally it works ok and process termination is detected ok.

With jstack, I've taken the call stack:

"Thread-2" #15 prio=5 os_prio=0 tid=0x00007fb888023000 nid=0xc0f runnable [0x00007fb8ca696000]
   java.lang.Thread.State: RUNNABLE
        at java.io.FileInputStream.readBytes(Native Method)
        at java.io.FileInputStream.read(FileInputStream.java:255)
        at java.io.BufferedInputStream.read1(BufferedInputStream.java:284)
        at java.io.BufferedInputStream.read(BufferedInputStream.java:345)
        - locked <0x00000000ea926290> (a java.lang.UNIXProcess$ProcessPipeInputStream)
        at java.io.FilterInputStream.read(FilterInputStream.java:107)
        at scala.sys.process.BasicIO$.loop$1(BasicIO.scala:234)
        at scala.sys.process.BasicIO$.transferFullyImpl(BasicIO.scala:242)
        at scala.sys.process.BasicIO$.transferFully(BasicIO.scala:223)
        at scala.sys.process.BasicIO$.$anonfun$toStdErr$1(BasicIO.scala:212)
        at scala.sys.process.BasicIO$.$anonfun$toStdErr$1$adapted(BasicIO.scala:212)
        at scala.sys.process.BasicIO$$$Lambda$335/590671716.apply(Unknown Source)
        at scala.sys.process.ProcessBuilderImpl$Simple.$anonfun$run$4(ProcessBuilderImpl.scala:79)
        at scala.sys.process.ProcessBuilderImpl$Simple$$Lambda$345/1512415799.apply$mcV$sp(Unknown Source)
        at scala.sys.process.ProcessImpl$Spawn$$anon$1.run(ProcessImpl.scala:23)

"Thread-1" #14 prio=5 os_prio=0 tid=0x00007fb888021000 nid=0xc0e runnable [0x00007fb8ca797000]
   java.lang.Thread.State: RUNNABLE
        at java.io.FileInputStream.readBytes(Native Method)
        at java.io.FileInputStream.read(FileInputStream.java:255)
        at java.io.BufferedInputStream.read1(BufferedInputStream.java:284)
        at java.io.BufferedInputStream.read(BufferedInputStream.java:345)
        - locked <0x00000000ea9241d0> (a java.lang.UNIXProcess$ProcessPipeInputStream)
        at java.io.FilterInputStream.read(FilterInputStream.java:107)
        at scala.sys.process.BasicIO$.loop$1(BasicIO.scala:234)
        at scala.sys.process.BasicIO$.transferFullyImpl(BasicIO.scala:242)
        at scala.sys.process.BasicIO$.transferFully(BasicIO.scala:223)
        at scala.sys.process.BasicIO$.$anonfun$toStdOut$1(BasicIO.scala:217)
        at scala.sys.process.BasicIO$.$anonfun$toStdOut$1$adapted(BasicIO.scala:217)
        at scala.sys.process.BasicIO$$$Lambda$334/1481715712.apply(Unknown Source)
        at scala.sys.process.ProcessBuilderImpl$Simple.$anonfun$run$3(ProcessBuilderImpl.scala:76)
        at scala.sys.process.ProcessBuilderImpl$Simple$$Lambda$344/1948323324.apply$mcV$sp(Unknown Source)
        at scala.sys.process.ProcessImpl$Spawn$$anon$1.run(ProcessImpl.scala:23)

"process reaper" #12 daemon prio=10 os_prio=0 tid=0x00007fb88801c800 nid=0xc0b waiting for monitor entry [0x00007fb8ca8d1000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at java.lang.UNIXProcess$ProcessPipeInputStream.processExited(UNIXProcess.java:527)
        - waiting to lock <0x00000000ea9241d0> (a java.lang.UNIXProcess$ProcessPipeInputStream)
        at java.lang.UNIXProcess.lambda$initStreams$3(UNIXProcess.java:298)
        at java.lang.UNIXProcess$$Lambda$342/78086683.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at java.lang.Thread.run(Thread.java:748)

"scala-execution-context-global-10" #10 daemon prio=5 os_prio=0 tid=0x00007fb91a3eb000 nid=0xc08 in Object.wait() [0x00007fb8caad2000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x00000000ea938828> (a scala.sys.process.ProcessImpl$Spawn$$anon$1)
        at java.lang.Thread.join(Thread.java:1252)
        - locked <0x00000000ea938828> (a scala.sys.process.ProcessImpl$Spawn$$anon$1)
        at java.lang.Thread.join(Thread.java:1326)
        at scala.sys.process.ProcessImpl$SimpleProcess.$anonfun$exitValue$2(ProcessImpl.scala:241)
        at scala.sys.process.ProcessImpl$SimpleProcess.$anonfun$exitValue$2$adapted(ProcessImpl.scala:241)
        at scala.sys.process.ProcessImpl$SimpleProcess.exitValue(ProcessImpl.scala:241)

It is locked here:

override def exitValue() = {
  try p.waitFor()                   // wait for the process to terminate
  finally inputThread.interrupt()   // we interrupt the input thread to notify it that it can terminate
  outputThreads foreach (_.join())  // <=== locked here

  p.exitValue()
}

More details:

• Java 8
• Scala 2.12
• CentOS 7

How can I make such a simple and basic task more reliable? Is this a bug from scala.sys.Process?

Answer 1

Workaround is to use the Java API:

val p = Process(Seq(....)).run()
val cmp = p.getClass.getDeclaredField("p")
cmp setAccessible true
val proc = cmp.get(p).asInstanceOf[java.lang.Process]
proc.waitFor()
proc.exitValue()

In this way it doesn't lock anymore.