commit 9191b1038a03e2ae8d8dff9c20ff35285924790c
Author: Patrick Hiesel <hiesel@google.com>
Date:   Fri Oct 16 09:05:46 2020 +0000

    Revert "Remove PerThreadCache"
    
    Revert "Adjust to changes in Gerrit core"
    
    Revert submission 283559-currentuser-remove-cache-key
    
    Reason for revert: Causes a latency regression for some hosts
    Reverted Changes:
    I76bfd3ebc:Adjust to changes in Gerrit core
    If7ccfd9a4:Remove unused CurrentUser#cacheKey method
    I1378ad083:Remove PerThreadCache
    
    Change-Id: I2bfe41d8acc4b325bb59b3ae099661300d84454e

diff --git a/java/com/google/gerrit/httpd/restapi/RestApiServlet.java b/java/com/google/gerrit/httpd/restapi/RestApiServlet.java
index a1197ef..404f5e4 100644
--- a/java/com/google/gerrit/httpd/restapi/RestApiServlet.java
+++ b/java/com/google/gerrit/httpd/restapi/RestApiServlet.java
@@ -108,6 +108,7 @@ import com.google.gerrit.server.OptionUtil;
 import com.google.gerrit.server.RequestInfo;
 import com.google.gerrit.server.RequestListener;
 import com.google.gerrit.server.audit.ExtendedHttpAuditEvent;
+import com.google.gerrit.server.cache.PerThreadCache;
 import com.google.gerrit.server.change.ChangeFinder;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.group.GroupAuditService;
@@ -327,7 +328,7 @@ public class RestApiServlet extends HttpServlet {
     try (TraceContext traceContext = enableTracing(req, res)) {
       List<IdString> path = splitPath(req);
 
-      try {
+      try (PerThreadCache ignored = PerThreadCache.create()) {
         RequestInfo requestInfo = createRequestInfo(traceContext, requestUri(req), path);
         globals.requestListeners.runEach(l -> l.onRequest(requestInfo));
 
diff --git a/java/com/google/gerrit/server/cache/PerThreadCache.java b/java/com/google/gerrit/server/cache/PerThreadCache.java
new file mode 100644
index 0000000..b4f79d1
--- /dev/null
+++ b/java/com/google/gerrit/server/cache/PerThreadCache.java
@@ -0,0 +1,146 @@
+// 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
+ * 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
+ * its 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.
+ *
+ * <p>To prevent OOM errors on requests that would cache a lot of objects, this class enforces an
+ * internal limit after which no new elements are cached. All {@code get} calls are served by
+ * invoking the {@code Supplier} after that.
+ */
+public class PerThreadCache implements AutoCloseable {
+  private static final ThreadLocal<PerThreadCache> CACHE = new ThreadLocal<>();
+  /**
+   * Cache at maximum 25 values per thread. This value was chosen arbitrarily. Some endpoints (like
+   * ListProjects) break the assumption that the data cached in a request is limited. To prevent
+   * this class from accumulating an unbound number of objects, we enforce this limit.
+   */
+  private static final int PER_THREAD_CACHE_SIZE = 25;
+
+  /**
+   * 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));
+    }
+
+    private 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();
+  }
+
+  public static <T> T getOrCompute(Key<T> key, Supplier<T> loader) {
+    PerThreadCache cache = get();
+    return cache != null ? cache.get(key, loader) : loader.get();
+  }
+
+  private final Map<Key<?>, Object> cache = Maps.newHashMapWithExpectedSize(PER_THREAD_CACHE_SIZE);
+
+  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) {
+    @SuppressWarnings("unchecked")
+    T value = (T) cache.get(key);
+    if (value == null) {
+      value = loader.get();
+      if (cache.size() < PER_THREAD_CACHE_SIZE) {
+        cache.put(key, value);
+      }
+    }
+    return value;
+  }
+
+  @Override
+  public void close() {
+    CACHE.remove();
+  }
+}
diff --git a/java/com/google/gerrit/server/permissions/DefaultPermissionBackend.java b/java/com/google/gerrit/server/permissions/DefaultPermissionBackend.java
index 7934d60..d10c139 100644
--- a/java/com/google/gerrit/server/permissions/DefaultPermissionBackend.java
+++ b/java/com/google/gerrit/server/permissions/DefaultPermissionBackend.java
@@ -34,6 +34,7 @@ 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.ProjectCache;
 import com.google.gerrit.server.project.ProjectState;
 import com.google.inject.Inject;
@@ -108,7 +109,11 @@ public class DefaultPermissionBackend extends PermissionBackend {
     public ForProject project(Project.NameKey project) {
       try {
         ProjectState state = projectCache.get(project).orElseThrow(illegalState(project));
-        return projectControlFactory.create(user, state).asForProject();
+        ProjectControl control =
+            PerThreadCache.getOrCompute(
+                PerThreadCache.Key.create(ProjectControl.class, project, user.getCacheKey()),
+                () -> projectControlFactory.create(user, state));
+        return control.asForProject();
       } catch (Exception e) {
         Throwable cause = e.getCause() != null ? e.getCause() : e;
         return FailedPermissionBackend.project(
diff --git a/javatests/com/google/gerrit/server/cache/PerThreadCacheTest.java b/javatests/com/google/gerrit/server/cache/PerThreadCacheTest.java
new file mode 100644
index 0000000..5d420d3
--- /dev/null
+++ b/javatests/com/google/gerrit/server/cache/PerThreadCacheTest.java
@@ -0,0 +1,103 @@
+// 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 static com.google.gerrit.testing.GerritJUnit.assertThrows;
+
+import java.util.function.Supplier;
+import org.junit.Test;
+
+public class PerThreadCacheTest {
+
+  @SuppressWarnings("TruthIncompatibleType")
+  @Test
+  public void key_respectsClass() {
+    assertThat(PerThreadCache.Key.create(String.class))
+        .isEqualTo(PerThreadCache.Key.create(String.class));
+    assertThat(PerThreadCache.Key.create(String.class))
+        .isNotEqualTo(
+            /* expected: Key<String>, actual: Key<Integer> */ 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()) {
+      IllegalStateException thrown =
+          assertThrows(IllegalStateException.class, () -> PerThreadCache.create());
+      assertThat(thrown).hasMessageThat().contains("called create() twice on the same request");
+    }
+  }
+
+  @Test
+  public void enforceMaxSize() {
+    try (PerThreadCache cache = PerThreadCache.create()) {
+      // Fill the cache
+      for (int i = 0; i < 50; i++) {
+        PerThreadCache.Key<String> key = PerThreadCache.Key.create(String.class, i);
+        cache.get(key, () -> "cached value");
+      }
+      // Assert that the value was not persisted
+      PerThreadCache.Key<String> key = PerThreadCache.Key.create(String.class, 1000);
+      cache.get(key, () -> "new value");
+      String value = cache.get(key, () -> "directly served");
+      assertThat(value).isEqualTo("directly served");
+    }
+  }
+}