Add onTimeout support for polling conditions

spock-core/src/main/java/spock/util/concurrent/PollingConditions.java

@@ -53,6 +53,7 @@ public class PollingConditions {
   private double initialDelay = 0;
   private double delay = 0.1;
   private double factor = 1.0;
+  private Closure<String> timeoutMessage = null;
 
   /**
    * Returns the timeout (in seconds) until which the conditions have to be satisfied.
@@ -128,6 +129,21 @@ public void setFactor(double factor) {
     this.factor = factor;
   }
 
+  /**
+   * Sets the closure that is evaluated when a timeout is reached. 
+   * <p>
+   * The closure can use a {@link Throwable} as an input parameter,
+   * which is thrown by the test conditions when a timeout is reached. The result of this
+   * closure is added to the {@link SpockTimeoutError} message. Calling it with null resets the timeout message.
+   *
+   * @param timeoutMessage the closure that is evaluated when a timeout is reached
+   * @since 2.4
+   */
+   @Beta
+  public void onTimeout(Closure<String> timeoutMessage) {
+    this.timeoutMessage = timeoutMessage;
+  }
+
   /**
    * Repeatedly evaluates the specified conditions until they are satisfied or the timeout has elapsed.
    *
@@ -182,6 +198,9 @@ public void within(double seconds, Closure<?> conditions) throws InterruptedExce
     }
 
     String msg = String.format("Condition not satisfied after %1.2f seconds and %d attempts", elapsedTime / 1000d, attempts);
+    if (timeoutMessage != null) {
+      msg = String.format("%s: %s", msg, GroovyRuntimeUtil.invokeClosure(timeoutMessage, testException));
+    }
     throw new SpockTimeoutError(seconds, msg, testException);
   }
 

spock-specs/src/test/groovy/spock/util/concurrent/PollingConditionsSpec.groovy

@@ -28,6 +28,9 @@ class PollingConditionsSpec extends Specification {
   volatile int num = 0
   volatile String str = null
 
+  private static def noArgClosure = { "test" }
+  private static def throwableArgClosure = { Throwable err -> err.getClass().simpleName }
+
   def "defaults"() {
     expect:
     with(conditions) {
@@ -168,4 +171,47 @@ class PollingConditionsSpec extends Specification {
     then: "there will be no second one"
     thrown SpockTimeoutError
   }
+
+  def "correctly creates timeout error message"() {
+    given:
+    PollingConditions conditions = new PollingConditions()
+    conditions.onTimeout(onTimeoutClosure)
+
+    when:
+    conditions.eventually {
+      1 == 0
+    }
+
+    then:
+    def e = thrown(SpockTimeoutError)
+    e.message ==~ /Condition not satisfied after \d+(\.\d+)? seconds and \d+ attempts/ + expectedMessageSuffix
+
+    where:
+    onTimeoutClosure    || expectedMessageSuffix
+    null                || ""
+    noArgClosure        || ": test"
+    throwableArgClosure || ": ConditionNotSatisfiedError"
+  }
+
+  def "correctly creates timeout error message when onTimeout called multiple times"() {
+    given:
+    PollingConditions conditions = new PollingConditions()
+    conditions.onTimeout(onTimeoutClosure)
+    conditions.onTimeout(secondOnTimeoutClosure)
+
+    when:
+    conditions.eventually {
+      1 == 0
+    }
+
+    then:
+    def e = thrown(SpockTimeoutError)
+    e.message ==~ /Condition not satisfied after \d+(\.\d+)? seconds and \d+ attempts/ + expectedMessageSuffix
+
+    where:
+    onTimeoutClosure    | secondOnTimeoutClosure || expectedMessageSuffix
+    noArgClosure        | null                   || ""
+    null                | noArgClosure           || ": test"
+    throwableArgClosure | noArgClosure           || ": test"
+  }
 }