Make our Rename migration more lenient about having already been performed.

src/java/com/samskivert/jdbc/depot/EntityMigration.java

@@ -25,6 +25,8 @@ import java.sql.SQLException;
 import java.sql.Statement;
 import java.util.HashMap;
 
+import com.samskivert.jdbc.JDBCUtil;
+
 import static com.samskivert.jdbc.depot.Log.log;
 
 /**
@@ -75,9 +77,27 @@ public abstract class EntityMigration extends Modifier
             try {
                 log.info("Changing '" + _oldColumnName + "' to '" + _newColumnDef + "' in " +
                          _tableName);
-                return stmt.executeUpdate(
+
-                    "alter table " + _tableName + " change column " + _oldColumnName + " " +
+                if (!JDBCUtil.tableContainsColumn(conn, _tableName, _oldColumnName)) {
-                    _newColumnDef);
+                    if (JDBCUtil.tableContainsColumn(conn, _tableName, _newColumnName)) {
+                        // we'll accept this inconsistency
+                        log.warning("Column rename appears already performed.");
+                        return 0;
+                    }
+                    // but this is not OK
+                    throw new IllegalArgumentException(
+                        _tableName + " does not contain '" + _oldColumnName + "'");
+                }
+
+                // nor is this
+                if (JDBCUtil.tableContainsColumn(conn, _tableName, _newColumnName)) {
+                    throw new IllegalArgumentException(
+                        _tableName + " already contains '" + _newColumnName + "'");
+                }
+
+                return stmt.executeUpdate("alter table " + _tableName + " change column " +
+                                          _oldColumnName + " " + _newColumnDef);
+
             } finally {
                 stmt.close();
             }