Adds code to delete the revision.

Signed-off-by: Abijeet <abijeetpatro@gmail.com>

app/Exceptions/BadRequestException.php

@@ -0,0 +1,14 @@
+<?php namespace BookStack\Exceptions;
+
+class BadRequestException extends PrettyException
+{
+
+    /**
+     * BadRequestException constructor.
+     * @param string $message
+     */
+    public function __construct($message = 'Bad request')
+    {
+        parent::__construct($message, 400);
+    }
+}

app/Http/Controllers/PageController.php

@@ -2,6 +2,7 @@
 
 use Activity;
 use BookStack\Exceptions\NotFoundException;
+use BookStack\Exceptions\BadRequestException;
 use BookStack\Repos\EntityRepo;
 use BookStack\Repos\UserRepo;
 use BookStack\Services\ExportService;
@@ -454,6 +455,38 @@ class PageController extends Controller
         return redirect($page->getUrl());
     }
 
+
+    /**
+     * Deletes a revision using the id of the specified revision.
+     * @param string $bookSlug
+     * @param string $pageSlug
+     * @param int $revisionId
+     * @throws NotFoundException
+     * @throws BadRequestException
+     * @return \Illuminate\Http\RedirectResponse|\Illuminate\Routing\Redirector
+     */
+    public function destroyRevision($bookSlug, $pageSlug, $revId)
+    {
+        $page = $this->entityRepo->getBySlug('page', $pageSlug, $bookSlug);
+        $this->checkOwnablePermission('page-update', $page);
+
+        $revision = $page->revisions()->where('id', '=', $revId)->first();
+        if ($revision === null) {
+            throw new NotFoundException("Revision #{$revId} not found");
+        }
+
+        // Get the current revision for the page
+        $current = $revision->getCurrent();
+
+        // Check if its the latest revision, cannot delete latest revision.
+        if (intval($current->id) === intval($revId)) {
+            throw new BadRequestException("Cannot delete the current revision #{$revId}");
+        }
+
+        $revision->delete();
+        return view('pages/revisions', ['page' => $page, 'book' => $page->book, 'current' => $page]);
+    }
+
     /**
      * Exports a page to a PDF.
      * https://github.com/barryvdh/laravel-dompdf

app/PageRevision.php

@@ -48,6 +48,18 @@ class PageRevision extends Model
         return null;
     }
 
+    /**
+     * Get the current revision for the same page if existing
+     * @return \BookStack\PageRevision|null
+     */
+    public function getCurrent()
+    {
+        if ($id = static::where('page_id', '=', $this->page_id)->max('id')) {
+            return static::find($id);
+        }
+        return null;
+    }
+
     /**
      * Allows checking of the exact class, Used to check entity type.
      * Included here to align with entities in similar use cases.