Add method on Index to look up a single document by key

Now that we have Java 8, it is easy to provide a default
implementation using a default method. The only thing subinterfaces
need to provide is a way to construct a predicate matching exactly
one document by key. Index backend authors may also choose to provide
a more efficient implementation if necessary.

Change-Id: I7d9de2f9c947ced9411b69ed39606dc4ba0b599a

gerrit-server/src/main/java/com/google/gerrit/server/index/Index.java

@@ -17,8 +17,11 @@ package com.google.gerrit.server.index;
 import com.google.gerrit.server.query.DataSource;
 import com.google.gerrit.server.query.Predicate;
 import com.google.gerrit.server.query.QueryParseException;
+import com.google.gwtorm.server.OrmException;
 
 import java.io.IOException;
+import java.util.List;
+import java.util.Optional;
 
 /**
  * Secondary index implementation for arbitrary documents.
@@ -90,6 +93,44 @@ public interface Index<K, V> {
   DataSource<V> getSource(Predicate<V> p, QueryOptions opts)
       throws QueryParseException;
 
+  /**
+   * Get a single document from the index.
+   *
+   * @param key document key.
+   * @param opts query options. Options that do not make sense in the context of
+   *     a single document, such as start, will be ignored.
+   * @return a single document if present.
+   * @throws IOException
+   */
+  default Optional<V> get(K key, QueryOptions opts) throws IOException {
+    opts = opts.withStart(0).withLimit(2);
+    List<V> results;
+    try {
+      results = getSource(keyPredicate(key), opts).read().toList();
+    } catch (QueryParseException e) {
+      throw new IOException("Unexpected QueryParseException during get()", e);
+    } catch (OrmException e) {
+      throw new IOException(e);
+    }
+    switch (results.size()) {
+      case 0:
+        return Optional.empty();
+      case 1:
+        return Optional.of(results.get(0));
+      default:
+        throw new IOException("Multiple results found in index for key "
+            + key + ": " + results);
+    }
+  }
+
+  /**
+   * Get a predicate that looks up a single document by key.
+   *
+   * @param key document key.
+   * @return a single predicate.
+   */
+  Predicate<V> keyPredicate(K key);
+
   /**
    * Mark whether this index is up-to-date and ready to serve reads.
    *

gerrit-server/src/main/java/com/google/gerrit/server/index/account/AccountIndex.java

@@ -18,9 +18,16 @@ import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.server.account.AccountState;
 import com.google.gerrit.server.index.Index;
 import com.google.gerrit.server.index.IndexDefinition;
+import com.google.gerrit.server.query.Predicate;
+import com.google.gerrit.server.query.account.AccountPredicates;
 
 public interface AccountIndex extends Index<Account.Id, AccountState> {
   public interface Factory extends
       IndexDefinition.IndexFactory<Account.Id, AccountState, AccountIndex> {
   }
+
+  @Override
+  default Predicate<AccountState> keyPredicate(Account.Id id) {
+    return AccountPredicates.id(id);
+  }
 }

gerrit-server/src/main/java/com/google/gerrit/server/index/change/ChangeIndex.java

@@ -17,10 +17,17 @@ package com.google.gerrit.server.index.change;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.server.index.Index;
 import com.google.gerrit.server.index.IndexDefinition;
+import com.google.gerrit.server.query.Predicate;
 import com.google.gerrit.server.query.change.ChangeData;
+import com.google.gerrit.server.query.change.LegacyChangeIdPredicate;
 
 public interface ChangeIndex extends Index<Change.Id, ChangeData> {
   public interface Factory extends
       IndexDefinition.IndexFactory<Change.Id, ChangeData, ChangeIndex> {
   }
+
+  @Override
+  default Predicate<ChangeData> keyPredicate(Change.Id id) {
+    return new LegacyChangeIdPredicate(id);
+  }
 }

gerrit-server/src/main/java/com/google/gerrit/server/query/account/AccountPredicates.java

@@ -47,7 +47,7 @@ public class AccountPredicates {
     return Predicate.or(preds);
   }
 
-  static Predicate<AccountState> id(Account.Id accountId) {
+  public static Predicate<AccountState> id(Account.Id accountId) {
     return new AccountPredicate(AccountField.ID,
         AccountQueryBuilder.FIELD_ACCOUNT, accountId.toString());
   }

gerrit-server/src/main/java/com/google/gerrit/server/query/change/LegacyChangeIdPredicate.java

@@ -22,7 +22,7 @@ import com.google.gerrit.reviewdb.client.Change;
 public class LegacyChangeIdPredicate extends ChangeIndexPredicate {
   private final Change.Id id;
 
-  LegacyChangeIdPredicate(Change.Id id) {
+  public LegacyChangeIdPredicate(Change.Id id) {
     super(LEGACY_ID, ChangeQueryBuilder.FIELD_CHANGE, id.toString());
     this.id = id;
   }