#792 #814 Implement SQLite migration, allow last IP to be nullable in MySQL

- Old SQLite setups have the last IP column as NOT NULL but without a default value. With the new concept (where we don't set a last IP on player registration) it fails.
  - Create an /authme debug child that allows to migrate SQLite (tricky because SQLite does not support dropping or modifying columns)
  - Allow last IP column to be NOT NULL in MySQL as well (extend MySQL /authme debug child)
- Add TODO comments with follow-up issue to extend our commands with new registration IP field

src/main/java/fr/xephi/authme/datasource/MySQL.java

@@ -192,7 +192,7 @@ public class MySQL implements DataSource {
 
             if (isColumnMissing(md, col.LAST_IP)) {
                 st.executeUpdate("ALTER TABLE " + tableName
-                    + " ADD COLUMN " + col.LAST_IP + " VARCHAR(40) CHARACTER SET ascii COLLATE ascii_bin NOT NULL;");
+                    + " ADD COLUMN " + col.LAST_IP + " VARCHAR(40) CHARACTER SET ascii COLLATE ascii_bin;");
             }
 
             if (isColumnMissing(md, col.LAST_LOGIN)) {

src/main/java/fr/xephi/authme/datasource/SQLite.java

@@ -97,7 +97,7 @@ public class SQLite implements DataSource {
 
             if (isColumnMissing(md, col.LAST_IP)) {
                 st.executeUpdate("ALTER TABLE " + tableName
-                    + " ADD COLUMN " + col.LAST_IP + " VARCHAR(40) NOT NULL DEFAULT '';");
+                    + " ADD COLUMN " + col.LAST_IP + " VARCHAR(40);");
             }
 
             if (isColumnMissing(md, col.LAST_LOGIN)) {
@@ -152,10 +152,30 @@ public class SQLite implements DataSource {
                 st.executeUpdate("ALTER TABLE " + tableName
                     + " ADD COLUMN " + col.HAS_SESSION + " INT NOT NULL DEFAULT '0';");
             }
+
+            if (isMigrationRequired(md)) {
+                ConsoleLogger.warning("READ ME! Your SQLite database is outdated and cannot save new players.");
+                ConsoleLogger.warning("Run /authme debug migratesqlite after making a backup");
+            }
         }
         ConsoleLogger.info("SQLite Setup finished");
     }
 
+    /**
+     * Returns whether the database needs to be migrated.
+     * <p>
+     * Background: Before commit 22911a0 (July 2016), new SQLite databases initialized the last IP column to be NOT NULL
+     * without a default value. Allowing the last IP to be null (#792) is therefore not compatible.
+     *
+     * @param metaData the database meta data
+     * @return true if a migration is necessary, false otherwise
+     * @throws SQLException .
+     */
+    public boolean isMigrationRequired(DatabaseMetaData metaData) throws SQLException {
+        return SqlDataSourceUtils.isNotNullColumn(metaData, tableName, col.LAST_IP)
+            && SqlDataSourceUtils.getColumnDefaultValue(metaData, tableName, col.LAST_IP) == null;
+    }
+
     private boolean isColumnMissing(DatabaseMetaData metaData, String columnName) throws SQLException {
         try (ResultSet rs = metaData.getColumns(null, null, tableName, columnName)) {
             return !rs.next();

src/main/java/fr/xephi/authme/datasource/SqlDataSourceUtils.java

@@ -2,13 +2,14 @@ package fr.xephi.authme.datasource;
 
 import fr.xephi.authme.ConsoleLogger;
 
+import java.sql.DatabaseMetaData;
 import java.sql.ResultSet;
 import java.sql.SQLException;
 
 /**
  * Utilities for SQL data sources.
  */
-final class SqlDataSourceUtils {
+public final class SqlDataSourceUtils {
 
     private SqlDataSourceUtils() {
     }
@@ -18,7 +19,7 @@ final class SqlDataSourceUtils {
      *
      * @param e the exception to log
      */
-    static void logSqlException(SQLException e) {
+    public static void logSqlException(SQLException e) {
         ConsoleLogger.logException("Error during SQL operation:", e);
     }
 
@@ -31,8 +32,55 @@ final class SqlDataSourceUtils {
      * @return the value (which may be null)
      * @throws SQLException :)
      */
-    static Long getNullableLong(ResultSet rs, String columnName) throws SQLException {
+    public static Long getNullableLong(ResultSet rs, String columnName) throws SQLException {
         long longValue = rs.getLong(columnName);
         return rs.wasNull() ? null : longValue;
     }
+
+    /**
+     * Returns whether the given column has a NOT NULL constraint.
+     *
+     * @param metaData the database meta data
+     * @param tableName the name of the table in which the column is
+     * @param columnName the name of the column to check
+     * @return true if the column is NOT NULL, false otherwise
+     * @throws SQLException :)
+     */
+    public static boolean isNotNullColumn(DatabaseMetaData metaData, String tableName,
+                                          String columnName) throws SQLException {
+        try (ResultSet rs = metaData.getColumns(null, null, tableName, columnName)) {
+            if (!rs.next()) {
+                throw new IllegalStateException("Did not find meta data for column '"
+                    + columnName + "' while checking for not-null constraint");
+            }
+
+            int nullableCode = rs.getInt("NULLABLE");
+            if (nullableCode == DatabaseMetaData.columnNoNulls) {
+                return true;
+            } else if (nullableCode == DatabaseMetaData.columnNullableUnknown) {
+                ConsoleLogger.warning("Unknown nullable status for column '" + columnName + "'");
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Returns the default value of a column (as per its SQL definition).
+     *
+     * @param metaData the database meta data
+     * @param tableName the name of the table in which the column is
+     * @param columnName the name of the column to check
+     * @return the default value of the column (may be null)
+     * @throws SQLException :)
+     */
+    public static Object getColumnDefaultValue(DatabaseMetaData metaData, String tableName,
+                                               String columnName) throws SQLException {
+        try (ResultSet rs = metaData.getColumns(null, null, tableName, columnName)) {
+            if (!rs.next()) {
+                throw new IllegalStateException("Did not find meta data for column '"
+                    + columnName + "' while checking its default value");
+            }
+            return rs.getObject("COLUMN_DEF");
+        }
+    }
 }