Shard refs/draft-comments/* by change instead of account ID

The motivation is the same as in Ib81d462: scanning the whole
refs/draft-comments/* space (with grows without bound) when reindexing
a change is potentially too slow.

Instead of refs/draft-comments/<sharded account>/<change>, use
refs/draft-comments/<sharded change>/<account>. This is mostly
straightforward, since there are relatively few places that scan
multiple refs; most callers just use the helper method in RefNames.

Change-Id: I063e2c496751f637d968c89c1501743074fb9025

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/client/Account.java

@@ -125,6 +125,21 @@ public final class Account {
       Integer id = RefNames.parseShardedRefPart(name);
       return id != null ? new Account.Id(id) : null;
     }
+
+    /**
+     * Parse an Account.Id out of the last part of a ref name.
+     * <p>
+     * The input is a ref name of the form {@code ".../1234"}, where the suffix
+     * is a non-sharded account ID. Ref names using a sharded ID should use
+     * {@link #fromRefPart(String)} instead for greater safety.
+     *
+     * @param name ref name
+     * @return account ID, or null if not numeric.
+     */
+    public static Id fromRefSuffix(String name) {
+      Integer id = RefNames.parseRefSuffix(name);
+      return id != null ? new Account.Id(id) : null;
+    }
   }
 
   @Column(id = 1)

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/client/Change.java

@@ -163,6 +163,11 @@ public final class Change {
       return new Change.Id(Integer.parseInt(id));
     }
 
+    public static Id fromRefPart(String ref) {
+      Integer id = RefNames.parseShardedRefPart(ref);
+      return id != null ? new Change.Id(id) : null;
+    }
+
     static int startIndex(String ref) {
       if (ref == null || !ref.startsWith(REFS_CHANGES)) {
         return -1;

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/client/RefNames.java

@@ -107,15 +107,15 @@ public class RefNames {
     return r.toString();
   }
 
-  public static String refsDraftComments(Account.Id accountId,
-      Change.Id changeId) {
-    StringBuilder r = buildRefsPrefix(REFS_DRAFT_COMMENTS, accountId.get());
-    r.append(changeId.get());
+  public static String refsDraftComments(Change.Id changeId,
+      Account.Id accountId) {
+    StringBuilder r = buildRefsPrefix(REFS_DRAFT_COMMENTS, changeId.get());
+    r.append(accountId.get());
     return r.toString();
   }
 
-  public static String refsDraftCommentsPrefix(Account.Id accountId) {
-    return buildRefsPrefix(REFS_DRAFT_COMMENTS, accountId.get()).toString();
+  public static String refsDraftCommentsPrefix(Change.Id changeId) {
+    return buildRefsPrefix(REFS_DRAFT_COMMENTS, changeId.get()).toString();
   }
 
   public static String refsStarredChanges(Change.Id changeId,
@@ -217,6 +217,26 @@ public class RefNames {
     return id;
   }
 
+  static Integer parseRefSuffix(String name) {
+    if (name == null) {
+      return null;
+    }
+    int i = name.length();
+    while (i > 0) {
+      char c = name.charAt(i - 1);
+      if (c == '/') {
+        break;
+      } else if (!Character.isDigit(c)) {
+        return null;
+      }
+      i--;
+    }
+    if (i == 0) {
+      return null;
+    }
+    return Integer.valueOf(name.substring(i, name.length()));
+  }
+
   private RefNames() {
   }
 }