capacity-limiter-api: add GradientCapacityLimiterTest (#3032)

Motivation:

We have have any specific tests for the gradient capacity limiter.

Modifications:

Add a test class with a few starter tests. We can add more tests
from there.

servicetalk-capacity-limiter-api/src/test/java/io/servicetalk/capacity/limiter/api/GradientCapacityLimiterTest.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright © 2024 Apple Inc. and the ServiceTalk project authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.servicetalk.capacity.limiter.api;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.time.Duration;
+import java.util.function.LongSupplier;
+import javax.annotation.Nullable;
+
+import static io.servicetalk.capacity.limiter.api.GradientCapacityLimiterProfiles.DEFAULT_INITIAL_LIMIT;
+import static io.servicetalk.capacity.limiter.api.GradientCapacityLimiterProfiles.DEFAULT_MIN_LIMIT;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.notNullValue;
+import static org.hamcrest.Matchers.nullValue;
+
+class GradientCapacityLimiterTest {
+
+    private static final Exception SAD_EXCEPTION = new Exception("sad");
+
+    private static final Classification DEFAULT = () -> 0;
+
+    @Nullable
+    private CapacityLimiter capacityLimiter;
+    @Nullable
+    private LongSupplier timeSource;
+    private volatile long currentTime;
+
+    @BeforeEach
+    void setup() {
+        if (timeSource == null) {
+            timeSource = () -> currentTime;
+        }
+        capacityLimiter = new GradientCapacityLimiterBuilder()
+                .timeSource(timeSource)
+                .build();
+    }
+
+    @Test
+    void canAcquireTicket() {
+        CapacityLimiter.Ticket ticket = capacityLimiter.tryAcquire(DEFAULT, null);
+        assertThat(ticket, notNullValue());
+    }
+
+    @Test
+    void capacityCanDepleteToTheMinLimit() {
+        for (;;) {
+            CapacityLimiter.Ticket ticket = capacityLimiter.tryAcquire(DEFAULT, null);
+            currentTime += Duration.ofMillis(10).toNanos();
+            int capacity = ticket.failed(SAD_EXCEPTION);
+            if (capacity == DEFAULT_MIN_LIMIT) {
+                break;
+            }
+        }
+    }
+
+    @Test
+    void canRejectTicketAcquisitions() {
+        CapacityLimiter.Ticket lastTicket = null;
+        for (int i = 0; i < DEFAULT_INITIAL_LIMIT; i++) {
+            // abandon all the tickets up to the limit
+            CapacityLimiter.Ticket ticket = capacityLimiter.tryAcquire(DEFAULT, null);
+            assertThat(ticket, notNullValue());
+            lastTicket = ticket;
+        }
+        CapacityLimiter.Ticket lastGoodTicket = lastTicket;
+        lastTicket = capacityLimiter.tryAcquire(DEFAULT, null);
+        assertThat(lastTicket, nullValue());
+
+        // complete all the tickets.
+        currentTime += Duration.ofMillis(10).toNanos();
+        lastGoodTicket.completed();
+
+        // We should be able to acquire a ticket again.
+        lastTicket = capacityLimiter.tryAcquire(DEFAULT, null);
+        assertThat(lastTicket, notNullValue());
+    }
+}