Update the method for determining kernel language (#354)

* Update the method for determining kernel language

The previous algorithm for `getNotebookType()` was based on getting the
kernel spec out of the current notebook. This is a problem when format
is triggered by, for example, `jupyterlab-autosave-on-focus-change`,
immediately after notebook creation. In that case, the language is
determined to be `null` and errors arise.

Now the `onSave()` handler waits for the `sessionContext` to be ready,
then `getNotebookType()` gets the same kernel spec info from the session
rather than the loaded notebook.

* Make metadata access safer at runtime

The `!` operator is a type assertion that the attribute is not null, but
does nothing at runtime to ensure this is the case. I haven't yet found
reproduction steps for this failure, but I encountered a case where
accessing `model!.sharedModel` threw an error. This commit adds optional
chaining operators, which do provide protection at runtime.

* Fall back to session check if metadata is missing

`getNotebookType()` has two ways to determine the language. If the
metadata check fails, it falls through to the current session check.
Previously, if metadata was `null`, the method would return early.

src/formatter.ts

@@ -92,34 +92,44 @@ export class JupyterlabNotebookCodeFormatter extends JupyterlabCodeFormatter {
     return codeCells;
   }
 
-  private getNotebookType() {
+  private getNotebookType(): string | null {
+    // If there is no current notebook, there is nothing to do
     if (!this.notebookTracker.currentWidget) {
       return null;
     }
 
+    // first, check the notebook's metadata for language info
     const metadata =
-      this.notebookTracker.currentWidget.content.model!.sharedModel.metadata;
-
-    if (!metadata) {
-      return null;
-    }
+      this.notebookTracker.currentWidget.content.model?.sharedModel?.metadata;
+
+    if (metadata) {
+      // prefer kernelspec language
+      if (
+        metadata.kernelspec &&
+        metadata.kernelspec.language &&
+        typeof metadata.kernelspec.language === 'string'
+      ) {
+        return metadata.kernelspec.language.toLowerCase();
+      }
 
-    // prefer kernelspec language
-    if (
-      metadata.kernelspec &&
-      metadata.kernelspec.language &&
-      typeof metadata.kernelspec.language === 'string'
-    ) {
-      return metadata.kernelspec.language.toLowerCase();
+      // otherwise, check language info code mirror mode
+      if (metadata.language_info && metadata.language_info.codemirror_mode) {
+        const mode = metadata.language_info.codemirror_mode;
+        if (typeof mode === 'string') {
+          return mode.toLowerCase();
+        } else if (typeof mode.name === 'string') {
+          return mode.name.toLowerCase();
+        }
+      }
     }
 
-    // otherwise, check language info code mirror mode
-    if (metadata.language_info && metadata.language_info.codemirror_mode) {
-      const mode = metadata.language_info.codemirror_mode;
-      if (typeof mode === 'string') {
-        return mode.toLowerCase();
-      } else if (typeof mode.name === 'string') {
-        return mode.name.toLowerCase();
+    // in the absence of metadata, look in the current session's kernel spec
+    const sessionContext = this.notebookTracker.currentWidget.sessionContext;
+    const kernelName = sessionContext?.session?.kernel?.name;
+    if (kernelName) {
+      const specs = sessionContext.specsManager.specs?.kernelspecs;
+      if (specs && kernelName in specs) {
+        return specs[kernelName]!.language;
       }
     }
 

src/index.ts

@@ -108,6 +108,7 @@ class JupyterLabCodeFormatter
     state: DocumentRegistry.SaveState
   ) {
     if (state === 'started' && this.config.formatOnSave) {
+      await context.sessionContext.ready;
       await this.notebookCodeFormatter.formatAllCodeCells(
         this.config,
         { saving: true },