Introduce 'null object' timestamp and also parse it from JSON

In Gerrit, it is common practice to use empty Strings to unset a value
in contexts where using null is not possible. We need something similar
for timestamps as well.

Inside of the Gerrit backend, we introduce TimeUtil.never() to indicate
this value. For users of Gerrit's REST API, we introduce another mapping
for convenience: an empty timestamp string in JSON will be mapped to the
magic value. We deliberately don't add the reverse mapping as proper
implementations in Gerrit should never set this value internally and
hence it should never be returned to the user.

Change-Id: I533b793227de4315893ee99750b20a442ff3c081

java/com/google/gerrit/json/SqlTimestampDeserializer.java

@@ -44,7 +44,15 @@ class SqlTimestampDeserializer implements JsonDeserializer<Timestamp>, JsonSeria
       throw new JsonParseException("Expected string for timestamp type");
     }
 
-    return JavaSqlTimestampHelper.parseTimestamp(p.getAsString());
+    String input = p.getAsString();
+    if (input.trim().isEmpty()) {
+      // Magic timestamp to indicate no timestamp. (-> null object)
+      // Always create a new object as timestamps are mutable. Don't use TimeUtil.never() to not
+      // introduce an undesired dependency.
+      return new Timestamp(0);
+    }
+
+    return JavaSqlTimestampHelper.parseTimestamp(input);
   }
 
   @Override

java/com/google/gerrit/server/util/time/BUILD

@@ -3,6 +3,7 @@ java_library(
     srcs = glob(["**/*.java"]),
     visibility = ["//visibility:public"],
     deps = [
+        "//java/com/google/gerrit/common:annotations",
         "//lib:guava",
         "//lib/jgit/org.eclipse.jgit:jgit",
     ],

java/com/google/gerrit/server/util/time/TimeUtil.java

@@ -15,6 +15,8 @@
 package com.google.gerrit.server.util.time;
 
 import com.google.common.annotations.VisibleForTesting;
+import com.google.gerrit.common.UsedAt;
+import com.google.gerrit.common.UsedAt.Project;
 import java.sql.Timestamp;
 import java.time.Instant;
 import java.util.function.LongSupplier;
@@ -43,6 +45,17 @@ public class TimeUtil {
     return new Timestamp(nowMs());
   }
 
+  /**
+   * Returns the magic timestamp representing no specific time.
+   *
+   * <p>This "null object" is helpful in contexts where using {@code null} directly is not possible.
+   */
+  @UsedAt(Project.PLUGIN_CHECKS)
+  public static Timestamp never() {
+    // Always create a new object as timestamps are mutable.
+    return new Timestamp(0);
+  }
+
   public static Timestamp truncateToSecond(Timestamp t) {
     return new Timestamp((t.getTime() / 1000) * 1000);
   }