Fix scala-js#6: Throw a structured exception when a command fails to start

Author: gzm0

js-envs/src/main/scala/org/scalajs/jsenv/ExternalJSRun.scala

@@ -140,9 +140,18 @@ object ExternalJSRun {
 
     config.logger.debug("Starting process: " + command.mkString(" "))
 
-    builder.start()
+    try {
+      builder.start()
+    } catch {
+      case t: Throwable =>
+        throw new FailedToStartException(command, t)
+    }
   }
 
+  final case class FailedToStartException(
+      command: List[String], cause: Throwable)
+      extends Exception(s"failed to start command $command", cause)
+
   final case class NonZeroExitException(retVal: Int)
       extends Exception(s"exited with code $retVal")
 

js-envs/src/test/scala/org/scalajs/jsenv/ExternalJSRunTest.scala

@@ -0,0 +1,73 @@
+/*
+ * Scala.js JS Envs (https://github.com/scala-js/scala-js-js-envs)
+ *
+ * Copyright EPFL.
+ *
+ * Licensed under Apache License 2.0
+ * (https://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package org.scalajs.jsenv
+
+import org.junit.Test
+import org.junit.Assert._
+
+import scala.concurrent.{Await, Future}
+import scala.concurrent.duration._
+
+import scala.util.Failure
+
+/** Tests of failure behavior for ExternalJSRun.
+ *
+ *  Success behavior is tested via NodeJSSuite.
+ */
+class ExternalJSRunFailureTest {
+
+  def assertFails(future: Future[Unit])(
+      pf: PartialFunction[Throwable, Unit]) = {
+    Await.ready(future, 1.seconds).value.get match {
+      case Failure(t) if pf.isDefinedAt(t) => // OK
+
+      case result =>
+        result.get
+        fail("run succeeded unexpectedly")
+    }
+  }
+
+  @Test
+  def nonExistentCommand: Unit = {
+    val cmd = List("nonexistent-cmd")
+    val run = ExternalJSRun.start(cmd, ExternalJSRun.Config()) { _ =>
+      fail("unexpected call to input")
+    }
+
+    assertFails(run.future) {
+      case ExternalJSRun.FailedToStartException(`cmd`, _) => // OK
+    }
+  }
+
+  @Test
+  def failingCommand: Unit = {
+    val run = ExternalJSRun.start(List("false"), ExternalJSRun.Config())(_.close())
+
+    assertFails(run.future) {
+      case ExternalJSRun.NonZeroExitException(_) => // OK
+    }
+  }
+
+  @Test
+  def abortedCommand: Unit = {
+    val run = ExternalJSRun.start(List("cat"), ExternalJSRun.Config()) { _ =>
+      // Do not close stdin so cat keeps running
+    }
+
+    run.close()
+
+    assertFails(run.future) {
+      case ExternalJSRun.ClosedException() => // OK
+    }
+  }
+}