More "semantic-versioning" compatible check for back-end schema

Summary

This MR refactors the checks for the mysql schema version.

Test Environment

Just put different values for MYSQL_SCHEMA_VERSION.

Check List for the Author

• All automated tests pass
• Reference related Issues
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.

src/main/java/org/caosdb/server/database/backend/implementation/MySQL/DatabaseConnectionPool.java

@@ -175,25 +175,69 @@ class DatabaseConnectionPool {
     logger.debug("####################");
   }
 
+  /**
+   * Check the version of the SQL server's database.
+   *
+   * <p>Current behaviour: Major versions must match and the database's minor version must be at
+   * least the expected minor version as defined in {@link
+   * ServerProperties.KEY_MYSQL_SCHEMA_VERSION}.
+   *
+   * <p>@todo Patch versions are not handled yet.
+   */
   private static void checkVersion(final Connection con)
       throws SQLException, ConnectionException, CaosDBException {
     try {
+      final String[] expected_version =
+          CaosDBServer.getServerProperty(ServerProperties.KEY_MYSQL_SCHEMA_VERSION)
+              .toLowerCase()
+              .split("\\.");
+      final String expected_major = expected_version[0];
+      final String expected_minor = expected_version[1];
+
       con.setReadOnly(false);
-      final PreparedStatement prepareStatement = con.prepareStatement("SELECT CaosDBVersion()");
+      final PreparedStatement prepared = con.prepareStatement("SELECT CaosDBVersion()");
       try {
-        final ResultSet executeQuery = prepareStatement.executeQuery();
+        final ResultSet executeQuery = prepared.executeQuery();
         if (executeQuery.next()) {
-          final String v_e =
-              CaosDBServer.getServerProperty(ServerProperties.KEY_MYSQL_SCHEMA_VERSION)
-                  .toLowerCase();
-          final String v_a = executeQuery.getString(1).toLowerCase();
-          if (!Objects.equal(v_a, v_e)) {
-            logger.error(
-                "Version of the MySQL schema is wrong.\n\tExpected: {}\n\tActual: {}n\nPlease upgrade the mysql backend.\n\n",
-                v_e,
-                v_a);
+
+          final String actual_version = executeQuery.getString(1).toLowerCase();
+          final String[] actual_version_split = actual_version.split("\\.");
+          final String actual_major = actual_version_split[0];
+          final String actual_minor = actual_version_split[1];
+
+          if (!Objects.equal(actual_major, expected_major)) {
+            if (Integer.parseInt(actual_major.replace("v", ""))
+                < Integer.parseInt(expected_major.replace("v", ""))) {
+              logger.error(
+                  "Version of the MySQL/MariaDB schema is incompatible.\n\tExpected: {}.{}\n\tActual: {}\nPlease upgrade the MySQL backend.\n\n",
+                  expected_major,
+                  expected_minor,
+                  actual_version);
+            } else {
+              logger.error(
+                  "Version of the MySQL/MariaDB schema is incompatible.\n\tExpected: {}.{}\n\tActual: {}\nPlease upgrade the CaosDB server.\n\n",
+                  expected_major,
+                  expected_minor,
+                  actual_version);
+            }
             System.exit(1);
           }
+          if (!Objects.equal(actual_minor, expected_minor)) {
+            if (Integer.parseInt(actual_minor) < Integer.parseInt(expected_minor)) {
+              logger.error(
+                  "Version of the MySQL/MariaDB schema is incompatible.\n\tExpected: {}.{}\n\tActual: {}\nPlease upgrade the MySQL backend.\n\n",
+                  expected_major,
+                  expected_minor,
+                  actual_version);
+              System.exit(1);
+            }
+          }
+        } else {
+          logger.error(
+              "Version of the MySQL schema could not be determined. This probably means that the version is too old.\n\tExpected: {}.{}\n\tActual: unknown\nPlease upgrade the MySQL backend.\n\n",
+              expected_major,
+              expected_minor);
+          System.exit(1);
         }
       } catch (final SQLException e) {
         logger.error("Could not check the version of the MySQL schema.", e);