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, 17 insertions, 3 deletions

diff --git a/src/main/java/com/google/devtools/build/skyframe/SkyKey.java b/src/main/java/com/google/devtools/build/skyframe/SkyKey.java
index cc1dd1fc88..1ac6d4457c 100644
--- a/src/main/java/com/google/devtools/build/skyframe/SkyKey.java
+++ b/src/main/java/com/google/devtools/build/skyframe/SkyKey.java
@@ -34,14 +34,20 @@ public final class SkyKey implements Serializable {
   private final Object argument;
 
   /**
-   * Cache the hash code for this object. It might be expensive to compute.
+   * Cache the hash code for this object. It might be expensive to compute. It is transient because
+   * argument's hash code might not be stable across JVM instances.
    */
-  private final int hashCode;
+  private transient int hashCode;
+
+  /**
+   * Whether the hash code is cached. Needed for {de,}serialization.
+   */
+  private transient boolean hashCodeCached;
 
   public SkyKey(SkyFunctionName functionName, Object valueName) {
     this.functionName = Preconditions.checkNotNull(functionName);
     this.argument = Preconditions.checkNotNull(valueName);
-    this.hashCode = 31 * functionName.hashCode() + argument.hashCode();
+    cacheHashCode();
   }
 
   public SkyFunctionName functionName() {
@@ -59,9 +65,17 @@ public final class SkyKey implements Serializable {
 
   @Override
   public int hashCode() {
+    if (!hashCodeCached) {
+      cacheHashCode();
+    }
     return hashCode;
   }
 
+  private void cacheHashCode() {
+    hashCode = 31 * functionName.hashCode() + argument.hashCode();
+    hashCodeCached = true;
+  }
+
   @Override
   public boolean equals(Object obj) {
     if (this == obj) {