Editor - Prompt warning when overwriting a file that is modified on disk (#2783)

* added overwrite prevention to saving

* rearranging order of require variables and edit to rename

* added documentation, and fixed reload button

* followed suggestion by tom, started tests

* Revert "followed suggestion by tom, started tests"

This reverts commit 4d45ec7c1b.

* added back in reverted changes to editor.js

* Fix broken reference to 'this'

notebook/static/edit/js/editor.js

@@ -4,6 +4,8 @@
 define([
     'jquery',
     'base/js/utils',
+    'base/js/i18n',
+    'base/js/dialog',
     'codemirror/lib/codemirror',
     'codemirror/mode/meta',
     'codemirror/addon/comment/comment',
@@ -19,6 +21,8 @@ define([
 function(
     $,
     utils,
+    i18n,
+    dialog,
     CodeMirror
 ) {
     "use strict";
@@ -33,6 +37,8 @@ function(
         this.file_path = options.file_path;
         this.config = options.config;
         this.file_extension_modes = options.file_extension_modes || {};
+        this.last_modified = null;
+        this._changed_on_disk_dialog = null;
 
         this.codemirror = new CodeMirror($(this.selector)[0]);
         this.codemirror.on('changes', function(cm, changes){
@@ -106,6 +112,7 @@ function(
                 that.generation = cm.changeGeneration();
                 that.events.trigger("file_loaded.Editor", model);
                 that._clean_state();
+                that.last_modified = new Date(model.last_modified);
             }).catch(
             function(error) {
                 that.events.trigger("file_load_failed.Editor", error);
@@ -197,6 +204,11 @@ function(
         }
     };
 
+    /**
+     * Rename the file.
+     * @param  {string} new_name
+     * @return {Promise} promise that resolves when the file is renamed.
+     */
     Editor.prototype.rename = function (new_name) {
         /** rename the file */
         var that = this;
@@ -206,18 +218,32 @@ function(
             function (model) {
                 that.file_path = model.path;
                 that.events.trigger('file_renamed.Editor', model);
+                that.last_modified = new Date(model.last_modified);
                 that._set_mode_for_model(model);
                 that._clean_state();
             }
         );
     };
     
-    Editor.prototype.save = function () {
+
+    /**
+     * Save this file on the server.
+     *
+     * @param {boolean} check_last_modified - checks if file has been modified on disk
+     * @return {Promise} - promise that resolves when the notebook is saved.
+     */
+    Editor.prototype.save = function (check_last_modified) {
         /** save the file */
         if (!this.save_enabled) {
             console.log("Not saving, save disabled");
             return;
         }
+
+        // used to check for last modified saves
+        if (check_last_modified === undefined) {
+            check_last_modified = true;
+        }
+
         var model = {
             path: this.file_path,
             type: 'file',
@@ -225,13 +251,78 @@ function(
             content: this.codemirror.getValue(),
         };
         var that = this;
-        // record change generation for isClean
-        this.generation = this.codemirror.changeGeneration();
-        that.events.trigger("file_saving.Editor");
-        return this.contents.save(this.file_path, model).then(function(data) {
-            that.events.trigger("file_saved.Editor", data);
-            that._clean_state();
-        });
+
+        var _save = function () {
+            that.events.trigger("file_saving.Editor");
+            return that.contents.save(that.file_path, model).then(function(data) {
+                // record change generation for isClean
+                that.generation = that.codemirror.changeGeneration();
+                that.events.trigger("file_saved.Editor", data);
+                that.last_modified = new Date(data.last_modified);
+                that._clean_state();
+            });
+        };
+
+        /* 
+         * Gets the current working file, and checks if the file has been modified on disk. If so, it
+         * creates & opens a modal that issues the user a warning and prompts them to overwrite the file.
+         * 
+         * If it can't get the working file, it builds a new file and saves.
+         */ 
+        if (check_last_modified) {
+            return this.contents.get(that.file_path, {content: false}).then(
+                function check_if_modified(data) {
+                    var last_modified = new Date(data.last_modified);
+                    // We want to check last_modified (disk) > that.last_modified (our last save)
+                    // In some cases the filesystem reports an inconsistent time,
+                    // so we allow 0.5 seconds difference before complaining.
+                    if ((last_modified.getTime() - that.last_modified.getTime()) > 500) {  // 500 ms
+                        console.warn("Last saving was done on `"+that.last_modified+"`("+that._last_modified+"), "+
+                                     "while the current file seem to have been saved on `"+data.last_modified+"`");
+                        if (that._changed_on_disk_dialog !== null) {
+                            // since the modal's event bindings are removed when destroyed, we reinstate
+                            // save & reload callbacks on the confirmation & reload buttons
+                            that._changed_on_disk_dialog.find('.save-confirm-btn').click(_save);
+                            that._changed_on_disk_dialog.find('.btn-warning').click(function () {window.location.reload()});
+                            
+                            // redisplay existing dialog
+                            that._changed_on_disk_dialog.modal('show');
+                        } else {
+                            // create new dialog
+                            that._changed_on_disk_dialog = dialog.modal({
+                                keyboard_manager: that.keyboard_manager,
+                                title: i18n.msg._("File changed"),
+                                body: i18n.msg._("The file has changed on disk since the last time we opened or saved it. "
+                                        + "Do you want to overwrite the file on disk with the version open here, or load "
+                                        + "the version on disk (reload the page)?"),
+                                buttons: {
+                                    Reload: {
+                                        class: 'btn-warning',
+                                        click: function () {
+                                            window.location.reload();
+                                        }
+                                    },
+                                    Cancel: {},
+                                    Overwrite: {
+                                        class: 'btn-danger save-confirm-btn',
+                                        click: function () {
+                                            _save();
+                                        }
+                                    },
+                                }
+                            });
+                        }
+                    } else {
+                        return _save();
+                    }
+            }, function (error) {
+                console.log(error);
+                // maybe it has been deleted or renamed? Go ahead and save.
+                return _save();
+            })
+        } else {
+            return _save();
+        }
     };
 
     Editor.prototype._clean_state = function(){