More commentary and clarification

storage/mysql/admin_storage.go

@@ -51,8 +51,8 @@ const (
 			Description,
 			CreateTimeMillis,
 			UpdateTimeMillis,
-			PrivateKey,
-			PublicKey,
+			PrivateKey, -- Unused
+			PublicKey, -- Used to store StorageSettings
 			MaxRootDurationMillis,
 			Deleted,
 			DeleteTimeMillis
@@ -273,8 +273,8 @@ func (t *adminTX) CreateTree(ctx context.Context, tree *trillian.Tree) (*trillia
 			Description,
 			CreateTimeMillis,
 			UpdateTimeMillis,
-			PrivateKey,
-			PublicKey,
+			PrivateKey, -- Unused
+			PublicKey, -- Used to store StorageSettings
 			MaxRootDurationMillis)
 		VALUES(?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`)
 	if err != nil {

storage/mysql/schema/storage.sql

@@ -18,8 +18,8 @@ CREATE TABLE IF NOT EXISTS Trees(
   CreateTimeMillis      BIGINT NOT NULL,
   UpdateTimeMillis      BIGINT NOT NULL,
   MaxRootDurationMillis BIGINT NOT NULL,
-  PrivateKey            MEDIUMBLOB NOT NULL,
-  PublicKey             MEDIUMBLOB NOT NULL,
+  PrivateKey            MEDIUMBLOB NOT NULL, -- Unused.
+  PublicKey             MEDIUMBLOB NOT NULL, -- This is now used to store settings.
   Deleted               BOOLEAN,
   DeleteTimeMillis      BIGINT,
   PRIMARY KEY(TreeId)

storage/mysql/tree_storage.go

@@ -318,11 +318,12 @@ func (t *treeTX) storeSubtrees(ctx context.Context, subtrees []*storagepb.Subtre
 	// a really large number of subtrees to store.
 	args := make([]interface{}, 0, len(subtrees))
 
-	subtreeRev := t.writeRevision
-	if !t.subtreeRevs {
-		// If we aren't doing subtree revisions then always fill in the subtree
-		// revision with a constant, such as 0.
-		subtreeRev = 0
+	// If not using subtree revisions then default value of 0 is fine. There is no
+	// significance to this value, other than it cannot be NULL in the DB.
+	var subtreeRev int64
+	if t.subtreeRevs {
+		// We're using subtree revisions, so ensure we write at the correct revision
+		subtreeRev = t.writeRevision
 	}
 	for _, s := range subtrees {
 		s := s