more robust treatment of execution with interrupts;
tuned comments;

--- a/src/HOL/Tools/Nitpick/kodkod.scala	Mon Aug 24 22:30:34 2020 +0200
+++ b/src/HOL/Tools/Nitpick/kodkod.scala	Tue Aug 25 13:44:09 2020 +0200
@@ -16,6 +16,8 @@
 
 object Kodkod
 {
+  /** result **/
+
   sealed case class Result(rc: Int, out: String, err: String)
   {
     def ok: Boolean = rc == 0
@@ -29,6 +31,10 @@
     }
   }
 
+
+
+  /** execute **/
+
   def execute(source: String,
     solve_all: Boolean = false,
     prove: Boolean = false,
@@ -37,11 +43,18 @@
     timeout: Time = Time.zero,
     max_threads: Int = 0): Result =
   {
+    /* executor */
+
     val pool_size = if (max_threads == 0) Isabelle_Thread.max_threads() else max_threads
-    val executor = Executors.newFixedThreadPool(pool_size)
 
+    val executor = Executors.newFixedThreadPool(pool_size)
+    val executor_killed = Synchronized(false)
     def executor_kill(): Unit =
-      if (!executor.isShutdown) Isabelle_Thread.fork() { executor.shutdownNow() }
+      executor_killed.change(b =>
+        if (b) b else { Isabelle_Thread.fork() { executor.shutdownNow() }; true })
+
+
+    /* system context */
 
     class Exit extends Exception("EXIT")
 
@@ -52,8 +65,19 @@
         private val err = new StringBuilder
 
         def return_code(i: Int): Unit = synchronized { rc = rc max i}
-        override def output(s: String): Unit = synchronized { out ++= s; out += '\n' }
-        override def error(s: String): Unit = synchronized { err ++= s; err += '\n' }
+
+        override def output(s: String): Unit = synchronized {
+          Exn.Interrupt.expose()
+          out ++= s
+          out += '\n'
+        }
+
+        override def error(s: String): Unit = synchronized {
+          Exn.Interrupt.expose()
+          err ++= s
+          err += '\n'
+        }
+
         override def exit(i: Int): Unit =
           synchronized {
             return_code(i)
@@ -64,6 +88,9 @@
         def result(): Result = synchronized { Result(rc, out.toString, err.toString) }
       }
 
+
+    /* main */
+
     try {
       val lexer = new KodkodiLexer(new ANTLRInputStream(Bytes(source).stream))
       val parser =
@@ -101,6 +128,8 @@
         context.return_code(1)
     }
 
+    executor.shutdownNow()
+
     context.result()
   }
 
@@ -110,7 +139,8 @@
       File.read(Path.explode("$KODKODI/examples/weber3.kki"))).check
 
 
-  /* scala function */
+
+  /** scala function **/
 
   object Fun extends Scala.Fun("kodkod")
   {