Throw exceptions when non Datasource classes are used as Datasources.

Using models as datasources can cause segmentation faults. Guard against
that by checking types and raising exceptions early.

Fixes #3694

lib/Cake/Error/exceptions.php

@@ -441,7 +441,7 @@ class MissingDatasourceConfigException extends CakeException {
  */
 class MissingDatasourceException extends CakeException {
 
-	protected $_messageTemplate = 'Datasource class %s could not be found.';
+	protected $_messageTemplate = 'Datasource class %s could not be found. %s';
 
 }
 

lib/Cake/Model/ConnectionManager.php

@@ -99,9 +99,17 @@ class ConnectionManager {
 		$conn = self::$_connectionsEnum[$name];
 		$class = $conn['classname'];
 
-		self::$_dataSources[$name] = new $class(self::$config->{$name});
-		self::$_dataSources[$name]->configKeyName = $name;
+		$instance = new $class(self::$config->{$name});
+		$instance->configKeyName = $name;
 
+		if (!$instance instanceof Datasource) {
+			throw new MissingDatasourceException(array(
+				'class' => $class,
+				'plugin' => null,
+				'message' => 'Only classes extending Datasource can be used as datasources.'
+			));
+		}
+		self::$_dataSources[$name] = $instance;
 		return self::$_dataSources[$name];
 	}
 

lib/Cake/View/Errors/missing_datasource.ctp

@@ -22,6 +22,9 @@ $pluginDot = empty($plugin) ? null : $plugin . '.';
 <p class="error">
 	<strong><?php echo __d('cake_dev', 'Error'); ?>: </strong>
 	<?php echo __d('cake_dev', 'Datasource class %s could not be found.', '<em>' . $pluginDot . $class . '</em>'); ?>
+	<?php if (isset($message)):  ?>
+		<?php echo h($message); ?>
+	<?php endif; ?>
 </p>
 <p class="notice">
 	<strong><?php echo __d('cake_dev', 'Notice'); ?>: </strong>