Implement a PerThreadCache

This commit implements a per-request cache as a per-thread cache.

PerThreadCache is intended to cache objects that have a high
instantiation cost, are specific to the current request and
potentially need to be instantiated multiple times while serving a
request.

The implementation uses a ThreadLocal which limits the cache to
the serving thread. Given that fan-outs (like ChangeJson's executor)
run with high parallelization, it seems acceptable to re-do some
instantiation there in lieu of this cache.

This commit uses a the new cache in DefaultPermissionBackend to cache
a ProjectControl instance.

Change-Id: Id5de21ed8941c7e08eb37197b16ce2cadece7aea

java/com/google/gerrit/httpd/restapi/RestApiServlet.java

@@ -102,6 +102,7 @@ import com.google.gerrit.server.OptionUtil;
 import com.google.gerrit.server.OutputFormat;
 import com.google.gerrit.server.audit.AuditService;
 import com.google.gerrit.server.audit.ExtendedHttpAuditEvent;
+import com.google.gerrit.server.cache.PerThreadCache;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.git.LockFailureException;
 import com.google.gerrit.server.permissions.GlobalPermission;
@@ -278,7 +279,7 @@ public class RestApiServlet extends HttpServlet {
     RestResource rsrc = TopLevelResource.INSTANCE;
     ViewData viewData = null;
 
-    try {
+    try (PerThreadCache ignored = PerThreadCache.create()) {
       if (isCorsPreflight(req)) {
         doCorsPreflight(req, res);
         return;

java/com/google/gerrit/server/cache/PerThreadCache.java

@@ -0,0 +1,128 @@
+// Copyright (C) 2018 The Android Open Source Project
+//
+// 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.gerrit.server.cache;
+
+import static com.google.common.base.Preconditions.checkState;
+
+import com.google.common.base.Objects;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Maps;
+import com.google.gerrit.common.Nullable;
+import java.util.Map;
+import java.util.function.Supplier;
+
+/**
+ * Caches object instances for a request as {@link ThreadLocal} in the serving thread.
+ *
+ * <p>This class is intended to cache objects that have a high instantiation cost, are specific to
+ * the current request and potentially need to be instantiated multiple times while serving a
+ * request.
+ *
+ * <p>This is different from the key-value storage in {@code CurrentUser}: {@code CurrentUser}
+ * offers a key-value storage by providing thread-safe {@code get} and {@code put} methods. Once the
+ * value is retrieved through {@code get} there is not thread-safety anymore - apart from the the
+ * retrieved object guarantees. Depending on the implementation of {@code CurrentUser}, it might be
+ * shared between the request serving thread as well as sub- or background treads.
+ *
+ * <p>In comparison to that, this class guarantees thread safety even on non-thread-safe objects as
+ * it's cache is tied to the serving thread only. While allowing to cache non-thread-safe objects,
+ * it has the downside of not sharing any objects with background threads or executors.
+ *
+ * <p>Lastly, this class offers a cache, that requires callers to also provide a {@code Supplier} in
+ * case the object is not present in the cache, while {@code CurrentUser} provides a storage where
+ * just retrieving stored values is a valid operation.
+ */
+public class PerThreadCache implements AutoCloseable {
+  private static final ThreadLocal<PerThreadCache> CACHE = new ThreadLocal<>();
+
+  /**
+   * Unique key for key-value mappings stored in PerThreadCache. The key is based on the value's
+   * class and a list of identifiers that in combination uniquely set the object apart form others
+   * of the same class.
+   */
+  public static final class Key<T> {
+    private final Class<T> clazz;
+    private final ImmutableList<Object> identifiers;
+
+    /**
+     * Returns a key based on the value's class and an identifier that uniquely identify the value.
+     * The identifier needs to implement {@code equals()} and {@hashCode()}.
+     */
+    public static <T> Key<T> create(Class<T> clazz, Object identifier) {
+      return new Key<>(clazz, ImmutableList.of(identifier));
+    }
+
+    /**
+     * Returns a key based on the value's class and a set of identifiers that uniquely identify the
+     * value. Identifiers need to implement {@code equals()} and {@hashCode()}.
+     */
+    public static <T> Key<T> create(Class<T> clazz, Object... identifiers) {
+      return new Key<>(clazz, ImmutableList.copyOf(identifiers));
+    }
+
+    public Key(Class<T> clazz, ImmutableList<Object> identifiers) {
+      this.clazz = clazz;
+      this.identifiers = identifiers;
+    }
+
+    @Override
+    public int hashCode() {
+      return Objects.hashCode(clazz, identifiers);
+    }
+
+    @Override
+    public boolean equals(Object o) {
+      if (!(o instanceof Key)) {
+        return false;
+      }
+      Key other = (Key) o;
+      return this.clazz == other.clazz && this.identifiers.equals(other.identifiers);
+    }
+  }
+
+  public static PerThreadCache create() {
+    checkState(CACHE.get() == null, "called create() twice on the same request");
+    PerThreadCache cache = new PerThreadCache();
+    CACHE.set(cache);
+    return cache;
+  }
+
+  @Nullable
+  public static PerThreadCache get() {
+    return CACHE.get();
+  }
+
+  private final Map<Key, Object> cache = Maps.newHashMapWithExpectedSize(10);
+
+  private PerThreadCache() {}
+
+  /**
+   * Returns an instance of {@code T} that was either loaded from the cache or obtained from the
+   * provided {@link Supplier}.
+   */
+  public <T> T get(Key<T> key, Supplier<T> loader) {
+    T value = (T) cache.get(key);
+    if (value == null) {
+      value = loader.get();
+      cache.put(key, value);
+    }
+    return value;
+  }
+
+  @Override
+  public void close() {
+    CACHE.remove();
+  }
+}

java/com/google/gerrit/server/permissions/DefaultPermissionBackend.java

@@ -31,6 +31,7 @@ import com.google.gerrit.server.CurrentUser;
 import com.google.gerrit.server.IdentifiedUser;
 import com.google.gerrit.server.PeerDaemonUser;
 import com.google.gerrit.server.account.CapabilityCollection;
+import com.google.gerrit.server.cache.PerThreadCache;
 import com.google.gerrit.server.project.NoSuchProjectException;
 import com.google.gerrit.server.project.ProjectCache;
 import com.google.gerrit.server.project.ProjectState;
@@ -108,7 +109,15 @@ public class DefaultPermissionBackend extends PermissionBackend {
       try {
         ProjectState state = projectCache.checkedGet(project);
         if (state != null) {
-          return projectControlFactory.create(user, state).asForProject().database(db);
+          PerThreadCache perThreadCache = PerThreadCache.get();
+          if (perThreadCache == null) {
+            return projectControlFactory.create(user, state).asForProject().database(db);
+          }
+          PerThreadCache.Key<ProjectControl> cacheKey =
+              PerThreadCache.Key.create(ProjectControl.class, project, user.getCacheKey());
+          ProjectControl control =
+              perThreadCache.get(cacheKey, () -> projectControlFactory.create(user, state));
+          return control.asForProject().database(db);
         }
         return FailedPermissionBackend.project("not found", new NoSuchProjectException(project));
       } catch (IOException e) {

javatests/com/google/gerrit/server/cache/BUILD

@@ -0,0 +1,12 @@
+load("//tools/bzl:junit.bzl", "junit_tests")
+
+junit_tests(
+    name = "tests",
+    srcs = ["PerThreadCacheTest.java"],
+    deps = [
+        "//java/com/google/gerrit/server",
+        "//lib:guava",
+        "//lib:junit",
+        "//lib:truth",
+    ],
+)

javatests/com/google/gerrit/server/cache/PerThreadCacheTest.java

@@ -0,0 +1,86 @@
+// Copyright (C) 2018 The Android Open Source Project
+//
+// 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.gerrit.server.cache;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import java.util.function.Supplier;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+
+public class PerThreadCacheTest {
+  @Rule public ExpectedException exception = ExpectedException.none();
+
+  @Test
+  public void key_respectsClass() {
+    assertThat(PerThreadCache.Key.create(String.class))
+        .isEqualTo(PerThreadCache.Key.create(String.class));
+    assertThat(PerThreadCache.Key.create(String.class))
+        .isNotEqualTo(PerThreadCache.Key.create(Integer.class));
+  }
+
+  @Test
+  public void key_respectsIdentifiers() {
+    assertThat(PerThreadCache.Key.create(String.class, "id1"))
+        .isEqualTo(PerThreadCache.Key.create(String.class, "id1"));
+    assertThat(PerThreadCache.Key.create(String.class, "id1"))
+        .isNotEqualTo(PerThreadCache.Key.create(String.class, "id2"));
+  }
+
+  @Test
+  public void endToEndCache() {
+    try (PerThreadCache ignored = PerThreadCache.create()) {
+      PerThreadCache cache = PerThreadCache.get();
+      PerThreadCache.Key<String> key1 = PerThreadCache.Key.create(String.class);
+
+      String value1 = cache.get(key1, () -> "value1");
+      assertThat(value1).isEqualTo("value1");
+
+      Supplier<String> neverCalled =
+          () -> {
+            throw new IllegalStateException("this method must not be called");
+          };
+      assertThat(cache.get(key1, neverCalled)).isEqualTo("value1");
+    }
+  }
+
+  @Test
+  public void cleanUp() {
+    PerThreadCache.Key<String> key = PerThreadCache.Key.create(String.class);
+    try (PerThreadCache ignored = PerThreadCache.create()) {
+      PerThreadCache cache = PerThreadCache.get();
+      String value1 = cache.get(key, () -> "value1");
+      assertThat(value1).isEqualTo("value1");
+    }
+
+    // Create a second cache and assert that it is not connected to the first one.
+    // This ensures that the cleanup is actually working.
+    try (PerThreadCache ignored = PerThreadCache.create()) {
+      PerThreadCache cache = PerThreadCache.get();
+      String value1 = cache.get(key, () -> "value2");
+      assertThat(value1).isEqualTo("value2");
+    }
+  }
+
+  @Test
+  public void doubleInstantiationFails() {
+    try (PerThreadCache ignored = PerThreadCache.create()) {
+      exception.expect(IllegalStateException.class);
+      exception.expectMessage("called create() twice on the same request");
+      PerThreadCache.create();
+    }
+  }
+}