Notebook cell execution buffering.

This buffering happens when the kernel is not set in the notebook/cell.

Author: jasongrout

notebook/static/notebook/js/codecell.js

@@ -299,41 +299,50 @@ define([
     /**
      * Execute current code cell to the kernel
      * @method execute
+     *
+     * If the cell cannot execute (e.g., the kernel does not exist),
+     * we return `false` and the cell will be left in a pending execution state.
+     * At some point, the cell should be executed again until execution succeeds.
+     *
+     * @returns true if execution was requested, and false if the cell cannot request execution.
      */
     CodeCell.prototype.execute = function (stop_on_error) {
-        if (!this.kernel || !this.kernel.is_connected()) {
-            console.log("Can't execute, kernel is not connected.");
-            return;
-        }
-
-        this.output_area.clear_output(false, true);
 
         if (stop_on_error === undefined) {
             stop_on_error = true;
         }
 
+        // Prepare for execution
+        this.output_area.clear_output(false, true);
         var old_msg_id = this.last_msg_id;
-
         if (old_msg_id) {
             this.kernel.clear_callbacks_for_msg(old_msg_id);
-            if (old_msg_id) {
-                delete CodeCell.msg_cells[old_msg_id];
-            }
+            delete CodeCell.msg_cells[old_msg_id];
+            this.last_msg_id = null;
         }
+
         if (this.get_text().trim().length === 0) {
             // nothing to do
             this.set_input_prompt(null);
-            return;
+            return true;
         }
-        this.set_input_prompt('*');
-        this.element.addClass("running");
-        var callbacks = this.get_callbacks();
 
-        this.last_msg_id = this.kernel.execute(this.get_text(), callbacks, {silent: false, store_history: true,
-            stop_on_error : stop_on_error});
-        CodeCell.msg_cells[this.last_msg_id] = this;
-        this.render();
-        this.events.trigger('execute.CodeCell', {cell: this});
+        this.set_input_prompt('*');
+        this.element.addClass("running");        
+
+        if (this.kernel) {
+            var callbacks = this.get_callbacks();
+            this.last_msg_id = this.kernel.execute(this.get_text(), callbacks, 
+                {silent: false,
+                 store_history: true,
+                 stop_on_error: stop_on_error});
+            CodeCell.msg_cells[this.last_msg_id] = this;
+            this.render();
+            this.events.trigger('execute.CodeCell', {cell: this});
+            return true;
+        } else {
+            return false;
+        }
     };
 
     /**

notebook/static/notebook/js/notebook.js

@@ -153,6 +153,8 @@ define(function (require) {
         this.dirty = null;
         this.trusted = null;
         this._fully_loaded = false;
+        
+        this.pending_execution = [];
 
         // Trigger cell toolbar registration.
         default_celltoolbar.register(this);
@@ -2090,6 +2092,7 @@ define(function (require) {
                 cell.set_kernel(this.session.kernel);
             }
         }
+        this.execute_pending_cells();
     };
 
     /**
@@ -2220,6 +2223,36 @@ define(function (require) {
         return promise;
     };
 
+    /**
+     * Execute a single cell
+     * 
+     * @param cell - the cell to execute
+     * 
+     * If the cell can not be executed, it is added to a list of 
+     * cells pending execution.
+     */
+    Notebook.prototype.execute = function (cell) {
+        var executed = cell.execute();
+        if (!executed) {
+            this.pending_execution.push(cell)
+        }
+    }
+    
+    /**
+     * Execute pending cells
+     * 
+     */
+    Notebook.prototype.execute_pending_cells = function () {
+        var cell;
+        while (cell = this.pending_execution.shift()) {
+            var executed = cell.execute();
+            if (!executed) {
+                console.error("Error trying to execute pending cells");
+                break;
+            }
+        }        
+    }
+
     /**
      * Execute cells corresponding to the given indices.
      *
@@ -2233,7 +2266,7 @@ define(function (require) {
         var cell;
         for (var i = 0; i < indices.length; i++) {
             cell = this.get_cell(indices[i]);
-            cell.execute();
+            this.execute(cell);
         }
 
         this.select(indices[indices.length - 1]);
@@ -2272,7 +2305,7 @@ define(function (require) {
         } else {
             var cell = this.get_selected_cell();
             cell_index = this.find_cell_index(cell);
-            cell.execute();
+            this.execute(cell);
         }
 
         // If we are at the end always insert a new cell and return
@@ -2305,7 +2338,7 @@ define(function (require) {
         } else {
             var cell = this.get_selected_cell();
             cell_index = this.find_cell_index(cell);
-            cell.execute();
+            this.execute(cell);
         }
 
         // If we are at the end always insert a new cell and return