Make SkyKey cached hash code transient

SkyKey argument hashcodes are not stable, because they're composed
of values whose hashcodes may not be stable, such as Java enums.
Therefore a SkyKey's own hashcode isn't stable. And this is fine,
but if we try to serialize and then deserialize the SkyKey with its
cached hashcode, the deserialized SkyKey's cached hashcode won't
match a normally constructed SkyKey, despite them being equal.

Because a SkyKey will deserialize with a default value of 0 for its
cached hashcode, this change also introduces a transient boolean
guard to note whether the correct hashcode has been calculated and
cached.

--
MOS_MIGRATED_REVID=91985674

1 files changed, 88 insertions, 0 deletions

diff --git a/src/test/java/com/google/devtools/build/skyframe/SkyKeyTest.java b/src/test/java/com/google/devtools/build/skyframe/SkyKeyTest.java
new file mode 100644
index 0000000000..0eeed132c2
--- /dev/null
+++ b/src/test/java/com/google/devtools/build/skyframe/SkyKeyTest.java
@@ -0,0 +1,88 @@
+// Copyright 2015 Google Inc. 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.skyframe;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.devtools.build.lib.testutil.TestUtils;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+import java.io.Serializable;
+
+/**
+ * Unit test for the SkyKey class, checking hash code transience logic.
+ */
+@RunWith(JUnit4.class)
+public class SkyKeyTest {
+
+  @Test
+  public void testHashCodeTransience() throws Exception {
+    // Given a freshly constructed HashCodeSpy object,
+    HashCodeSpy hashCodeSpy = new HashCodeSpy();
+    assertThat(hashCodeSpy.getNumberOfTimesHashCodeCalled()).isEqualTo(0);
+
+    // When a SkyKey is constructed with that HashCodeSpy as its argument,
+    SkyKey originalKey = new SkyKey(new SkyFunctionName("TEMP", true), hashCodeSpy);
+
+    // Then the HashCodeSpy reports that its hashcode method was called once.
+    assertThat(hashCodeSpy.getNumberOfTimesHashCodeCalled()).isEqualTo(1);
+
+
+    // When the SkyKey's hashCode method is called,
+    originalKey.hashCode();
+
+    // Then the spy's hashCode method isn't called, because the SkyKey's hashCode was cached.
+    assertThat(hashCodeSpy.getNumberOfTimesHashCodeCalled()).isEqualTo(1);
+
+
+    // When that SkyKey is serialized and then deserialized,
+    SkyKey newKey = (SkyKey) TestUtils.deserializeObject(TestUtils.serializeObject(originalKey));
+
+    // Then the new SkyKey's HashCodeSpy has not had its hashCode method called.
+    HashCodeSpy spyInNewKey = (HashCodeSpy) newKey.argument();
+    assertThat(spyInNewKey.getNumberOfTimesHashCodeCalled()).isEqualTo(0);
+
+
+    // When the new SkyKey's hashCode method is called once,
+    newKey.hashCode();
+
+    // Then the new SkyKey's spy's hashCode method gets called.
+    assertThat(spyInNewKey.getNumberOfTimesHashCodeCalled()).isEqualTo(1);
+
+
+    // When the new SkyKey's hashCode method is called a second time,
+    newKey.hashCode();
+
+    // Then the new SkyKey's spy's hashCOde isn't called a second time, because the SkyKey's
+    // hashCode was cached.
+    assertThat(spyInNewKey.getNumberOfTimesHashCodeCalled()).isEqualTo(1);
+  }
+
+  private static class HashCodeSpy implements Serializable {
+    private transient int numberOfTimesHashCodeCalled;
+
+    public int getNumberOfTimesHashCodeCalled() {
+      return numberOfTimesHashCodeCalled;
+    }
+
+    @Override
+    public int hashCode() {
+      numberOfTimesHashCodeCalled++;
+      return 42;
+    }
+  }
+}