Introduce MultisetSemaphore: A concurrency primitive for managing access to at most K unique things at once.

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+// Copyright 2016 The Bazel Authors. All rights reserved.
+//
+// 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 com.google.devtools.build.lib.concurrent;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.collect.Collections2;
+import com.google.common.collect.ImmutableSet;
+import com.google.devtools.build.lib.util.Preconditions;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.atomic.AtomicInteger;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Tests for {@link MultisetSemaphore}. */
+@RunWith(JUnit4.class)
+public class MultisetSemaphoreTest {
+
+  @Test
+  public void testSimple_Serial() throws Exception {
+    // When we have a MultisetSemaphore
+    MultisetSemaphore<String> multisetSemaphore = MultisetSemaphore.newBuilder()
+        .concurrencyLevel(1)
+        // with 3 max num unique values,
+        .maxNumUniqueValues(3)
+        .build();
+
+    // And we serially acquire permits for 3 unique values
+    multisetSemaphore.acquireAll(ImmutableSet.of("a", "b", "c"));
+    // And then attempt to acquire permits for 2 of those same unique values,
+    // Then we don't deadlock.
+    multisetSemaphore.acquireAll(ImmutableSet.of("b", "c"));
+    // And then we release one of the permit for one of those unique values,
+    multisetSemaphore.releaseAll(ImmutableSet.of("c"));
+    // And then we release the other permit,
+    multisetSemaphore.releaseAll(ImmutableSet.of("c"));
+    // We are able to acquire a permit for a 4th unique value.
+    multisetSemaphore.acquireAll(ImmutableSet.of("d"));
+  }
+
+  @Test
+  public void testSimple_Concurrent() throws Exception {
+    // When we have N and M, with M > N and M|N.
+    final int n = 10;
+    int m = n * 2;
+    Preconditions.checkState(m > n && m % n == 0, "M=%d N=%d", m, n);
+    // When we have a MultisetSemaphore
+    final MultisetSemaphore<String> multisetSemaphore = MultisetSemaphore.newBuilder()
+        // With a concurrency level of M
+        .concurrencyLevel(m)
+        // And N max num unique values,
+        .maxNumUniqueValues(n)
+        .build();
+
+    // And a ExecutorService with M threads,
+    ExecutorService executorService = Executors.newFixedThreadPool(m);
+    // And a recorder for thrown exceptions,
+    ThrowableRecordingRunnableWrapper wrapper =
+        new ThrowableRecordingRunnableWrapper("testSimple_Concurrent");
+    final AtomicInteger numThreadsJustAfterAcquireInFirstRound = new AtomicInteger(0);
+    final AtomicInteger numThreadsJustAfterAcquireInSecondRound = new AtomicInteger(0);
+    final AtomicInteger secondRoundCompleted = new AtomicInteger(0);
+    final int napTimeMs = 42;
+    for (int i = 0; i < m; i++) {
+      final String val = "val" + i;
+      // And we submit M Runnables, each of which
+      executorService.submit(wrapper.wrap(new Runnable() {
+        @Override
+        public void run() {
+          try {
+            // Has two rounds
+
+            // Wherein the first round
+            //   The Runnable acquire a permit for a unique value (among M values),
+            ImmutableSet<String> valSet = ImmutableSet.of(val);
+            multisetSemaphore.acquireAll(valSet);
+            assertThat(numThreadsJustAfterAcquireInFirstRound.getAndIncrement()).isLessThan(n);
+            //   And then sleeps,
+            Thread.sleep(napTimeMs);
+            numThreadsJustAfterAcquireInFirstRound.decrementAndGet();
+            multisetSemaphore.releaseAll(valSet);
+
+            // And wherein the second round
+            //   The Runnable again acquires a permit for its unique value,
+            multisetSemaphore.acquireAll(valSet);
+            assertThat(numThreadsJustAfterAcquireInSecondRound.getAndIncrement()).isLessThan(n);
+            //   And then sleeps,
+            Thread.sleep(napTimeMs);
+            numThreadsJustAfterAcquireInSecondRound.decrementAndGet();
+            //   And notes that it has completed the second round,
+            secondRoundCompleted.incrementAndGet();
+            multisetSemaphore.releaseAll(valSet);
+          } catch (InterruptedException e) {
+            throw new IllegalStateException(e);
+          }
+        }
+      }));
+    }
+    // And we wait for all M Runnables to complete (that is, none of them were deadlocked),
+    boolean interrupted = ExecutorUtil.interruptibleShutdown(executorService);
+    // Then none of our Runnables threw any Exceptions.
+    assertThat(wrapper.getFirstThrownError()).isNull();
+    if (interrupted) {
+      Thread.currentThread().interrupt();
+      throw new InterruptedException();
+    }
+    // And the counters we used for sanity checks were correctly reset to 0.
+    assertThat(numThreadsJustAfterAcquireInFirstRound.get()).isEqualTo(0);
+    assertThat(numThreadsJustAfterAcquireInSecondRound.get()).isEqualTo(0);
+    // And all M Runnables completed the second round.
+    assertThat(secondRoundCompleted.get()).isEqualTo(m);
+    Set<String> newVals = new HashSet<>();
+    for (int i = 0; i < n; i++) {
+      newVals.add("newval" + i);
+    }
+    // And the main test thread is able to acquire permits for N new unique values (indirectly
+    // confirming that the MultisetSemaphore previously had no outstanding permits).
+    multisetSemaphore.acquireAll(newVals);
+  }
+
+  @Test
+  public void testConcurrentAtomicity() throws Exception {
+    int n = 100;
+    // When we have a MultisetSemaphore
+    final MultisetSemaphore<String> multisetSemaphore = MultisetSemaphore.newBuilder()
+        // With a concurrency level of N
+        .concurrencyLevel(n)
+        // And 2 max num unique values,
+        .maxNumUniqueValues(2)
+        .build();
+    // And a ExecutorService with N threads,
+    ExecutorService executorService = Executors.newFixedThreadPool(n);
+    // And a recorder for thrown exceptions,
+    ThrowableRecordingRunnableWrapper wrapper =
+        new ThrowableRecordingRunnableWrapper("testConcurrentAtomicity");
+    final int napTimeMs = 42;
+    // And a done latch with initial count N,
+    final CountDownLatch allDoneLatch = new CountDownLatch(n);
+    final String sameVal = "same-val";
+    for (int i = 0; i < n; i++) {
+      final String differentVal = "different-val" + i;
+      // And we submit N Runnables, each of which
+      executorService.submit(wrapper.wrap(new Runnable() {
+        @Override
+        public void run() {
+          try {
+            Set<String> vals = ImmutableSet.of(sameVal, differentVal);
+            // Tries to acquire a permit for a set of two values, one of which is the same for all
+            // the N Runnables and one of which is unique across all N Runnables.
+            multisetSemaphore.acquireAll(vals);
+            // And then sleeps
+            Thread.sleep(napTimeMs);
+            // And then releases its permits
+            multisetSemaphore.releaseAll(vals);
+            // And then counts down the done latch,
+            allDoneLatch.countDown();
+          } catch (InterruptedException e) {
+            throw new IllegalStateException(e);
+          }
+        }
+      }));
+    }
+    // Then all of our Runnables completed (without deadlock!), as expected,
+    boolean interrupted = ExecutorUtil.interruptibleShutdown(executorService);
+    // And thus were able to count down the done latch,
+    allDoneLatch.await();
+    // And also none of them threw any Exceptions.
+    assertThat(wrapper.getFirstThrownError()).isNull();
+    if (interrupted) {
+      Thread.currentThread().interrupt();
+      throw new InterruptedException();
+    }
+  }
+
+  @Test
+  public void testConcurrentRace() throws Exception {
+    // When we have N values
+    int n = 6;
+    ArrayList<String> vals = new ArrayList<>();
+    for (int i = 0; i < n; i++) {
+      vals.add("val-" + i);
+    }
+    // And we have all permutations of these N values
+    Collection<List<String>> permutations = Collections2.orderedPermutations(vals);
+    int numPermutations = permutations.size();
+    // And we have a MultisetSemaphore
+    final MultisetSemaphore<String> multisetSemaphore = MultisetSemaphore.newBuilder()
+        // With a concurrency level of N!
+        .concurrencyLevel(numPermutations)
+        // And with N max num unique values,
+        .maxNumUniqueValues(n)
+        .build();
+    // And a ExecutorService with N! threads,
+    ExecutorService executorService = Executors.newFixedThreadPool(numPermutations);
+    // And a recorder for thrown exceptions,
+    ThrowableRecordingRunnableWrapper wrapper =
+        new ThrowableRecordingRunnableWrapper("testConcurrentRace");
+    for (List<String> orderedVals : permutations) {
+      final Set<String> orderedSet = new LinkedHashSet<>(orderedVals);
+      // And we submit N! Runnables, each of which
+      executorService.submit(wrapper.wrap(new Runnable() {
+        @Override
+        public void run() {
+          try {
+            // Tries to acquire a permit for the set of N values, with a unique iteration order
+            // (across all the N! different permutations)
+            multisetSemaphore.acquireAll(orderedSet);
+            // And then immediately releases the permit.
+            multisetSemaphore.releaseAll(orderedSet);
+          } catch (InterruptedException e) {
+            throw new IllegalStateException(e);
+          }
+        }
+      }));
+    }
+    // Then all of our Runnables completed (without deadlock!), as expected,
+    boolean interrupted = ExecutorUtil.interruptibleShutdown(executorService);
+    // And also none of them threw any Exceptions.
+    assertThat(wrapper.getFirstThrownError()).isNull();
+    if (interrupted) {
+      Thread.currentThread().interrupt();
+      throw new InterruptedException();
+    }
+  }
+}
+