GH-397: Fix exponential backoff for initial supplier

Fixes #397

`Retryable` with exponential backoff not working with `delayExpression`

The `ExponentialBackOffContext` when we are calling `getSleepAndIncrement` `sleep` value resolved from `intervalSupplier` if it is not null. 
But further when we resolving `getNextInterval`, we assigning it to the `interval` instead of `intervalSupplier` and `interval` keeps it initial value for the next iteration.

* Assign `intervalSupplier` result into an `interval` on a first `getInterval()` call

Author: R-Zer0

src/main/java/org/springframework/retry/backoff/ExponentialBackOffPolicy.java

@@ -42,6 +42,7 @@
  * @author Gary Russell
  * @author Artem Bilan
  * @author Marius Lichtblau
+ * @author Anton Aharkau
  */
 @SuppressWarnings("serial")
 public class ExponentialBackOffPolicy implements SleepingBackOffPolicy<ExponentialBackOffPolicy> {
@@ -259,7 +260,7 @@ static class ExponentialBackOffContext implements BackOffContext {
 
 		private final long maxInterval;
 
-		private Supplier<Long> intervalSupplier;
+		private Supplier<Long> initialIntervalSupplier;
 
 		private Supplier<Double> multiplierSupplier;
 
@@ -271,7 +272,7 @@ public ExponentialBackOffContext(long interval, double multiplier, long maxInter
 			this.interval = interval;
 			this.multiplier = multiplier;
 			this.maxInterval = maxInterval;
-			this.intervalSupplier = intervalSupplier;
+			this.initialIntervalSupplier = intervalSupplier;
 			this.multiplierSupplier = multiplierSupplier;
 			this.maxIntervalSupplier = maxIntervalSupplier;
 		}
@@ -297,7 +298,11 @@ public double getMultiplier() {
 		}
 
 		public long getInterval() {
-			return this.intervalSupplier != null ? this.intervalSupplier.get() : this.interval;
+			if (this.initialIntervalSupplier != null) {
+				this.interval = this.initialIntervalSupplier.get();
+				this.initialIntervalSupplier = null;
+			}
+			return this.interval;
 		}
 
 		public long getMaxInterval() {

src/test/java/org/springframework/retry/annotation/EnableRetryTests.java

@@ -59,6 +59,7 @@
  * @author Aldo Sinanaj
  * @author Henning Pöttker
  * @author Yanming Zhou
+ * @author Anton Aharkau
  * @since 1.1
  */
 public class EnableRetryTests {
@@ -295,7 +296,7 @@ void runtimeExpressions() throws Exception {
 		service.service6();
 		RuntimeConfigs runtime = context.getBean(RuntimeConfigs.class);
 		verify(runtime, times(5)).getMaxAttempts();
-		verify(runtime, times(2)).getInitial();
+		verify(runtime, times(1)).getInitial();
 		verify(runtime, times(2)).getMax();
 		verify(runtime, times(2)).getMult();
 

src/test/java/org/springframework/retry/backoff/ExponentialBackOffPolicyTests.java

@@ -26,6 +26,7 @@
  * @author Dave Syer
  * @author Gary Russell
  * @author Marius Lichtblau
+ * @author Anton Aharkau
  */
 public class ExponentialBackOffPolicyTests {
 
@@ -94,6 +95,22 @@ public void testMultiBackOff() {
 		}
 	}
 
+	@Test
+	public void testMultiBackOffWithInitialDelaySupplier() {
+		ExponentialBackOffPolicy strategy = new ExponentialBackOffPolicy();
+		long seed = 40;
+		double multiplier = 1.2;
+		strategy.initialIntervalSupplier(() -> 40L);
+		strategy.setMultiplier(multiplier);
+		strategy.setSleeper(sleeper);
+		BackOffContext context = strategy.start(null);
+		for (int x = 0; x < 5; x++) {
+			strategy.backOff(context);
+			assertThat(sleeper.getLastBackOff()).isEqualTo(seed);
+			seed *= multiplier;
+		}
+	}
+
 	@Test
 	public void testInterruptedStatusIsRestored() {
 		ExponentialBackOffPolicy strategy = new ExponentialBackOffPolicy();