NEW DatabaselessKernel to support operation without DB

This is required for GraphQL code generation in CI (without a working runtime database/webserver environment).
Context: silverstripe/silverstripe-graphql#388

composer.json

@@ -32,6 +32,7 @@
         "psr/container": "1.0.0",
         "silverstripe/config": "^1@dev",
         "silverstripe/assets": "^1@dev",
+        "silverstripe/event-dispatcher": "^0.1.0",
         "silverstripe/vendor-plugin": "^1.4",
         "sminnee/callbacklist": "^0.1",
         "swiftmailer/swiftmailer": "~5.4",

src/Core/DatabaselessKernel.php

@@ -0,0 +1,53 @@
+<?php
+
+namespace SilverStripe\Core;
+
+use SilverStripe\EventDispatcher\Dispatch\Dispatcher;
+use SilverStripe\EventDispatcher\Event\EventContextInterface;
+use SilverStripe\EventDispatcher\Event\EventHandlerInterface;
+use SilverStripe\ORM\Connect\NullDatabase;
+use SilverStripe\ORM\DB;
+
+/**
+ * Boot a kernel without requiring a database connection.
+ * This is a workaround for the lack of composition in the boot stages
+ * of CoreKernel, as well as for the framework's misguided assumptions
+ * around the availability of a database for every execution path.
+ *
+ * @internal
+ */
+class DatabaselessKernel extends CoreKernel
+{
+    protected $queryErrorMessage = 'Booted with DatabaseLessKernel, cannot execute query: %s';
+
+    /**
+     * Allows disabling of the configured error handling.
+     * This can be useful to ensure the execution context (e.g. composer)
+     * can consistently use its own error handling.
+     *
+     * @var boolean
+     */
+    protected $bootErrorHandling = true;
+
+    public function setBootErrorHandling(bool $bool)
+    {
+        $this->bootErrorHandling = $bool;
+        return $this;
+    }
+
+    public function boot($flush = false)
+    {
+        $this->flush = $flush;
+
+        $this->bootPHP();
+        $this->bootManifests($flush);
+
+        if ($this->bootErrorHandling) {
+            $this->bootErrorHandling();
+        }
+
+        $this->bootConfigs();
+
+        $this->booted = true;
+    }
+}

src/ORM/Connect/Database.php

@@ -9,6 +9,8 @@
 use SilverStripe\ORM\PaginatedList;
 use SilverStripe\ORM\Queries\SQLUpdate;
 use SilverStripe\ORM\Queries\SQLInsert;
+use SilverStripe\EventDispatcher\Dispatch\Dispatcher;
+use SilverStripe\EventDispatcher\Symfony\Event;
 use BadMethodCallException;
 use Exception;
 use SilverStripe\Dev\Backtrace;
@@ -151,6 +153,13 @@ public function query($sql, $errorLevel = E_USER_ERROR)
             return null;
         }
 
+        Dispatcher::singleton()->trigger('database.query', Event::create(
+            null,
+            [
+                'sql' => $sql
+            ]
+        ));
+
         // Benchmark query
         $connector = $this->connector;
         return $this->benchmarkQuery(
@@ -177,6 +186,14 @@ public function preparedQuery($sql, $parameters, $errorLevel = E_USER_ERROR)
             return null;
         }
 
+        Dispatcher::singleton()->trigger('database.preparedQuery', Event::create(
+            null,
+            [
+                'sql' => $sql,
+                'parameters' => $parameters
+            ]
+        ));
+
         // Benchmark query
         $connector = $this->connector;
         return $this->benchmarkQuery(