Eagerly add kernel metadata before the kernel is initialised

src/index.ts

@@ -2,7 +2,12 @@ import { ILabShell, JupyterFrontEnd, JupyterFrontEndPlugin } from '@jupyterlab/a
 import { INotebookTracker, NotebookPanel } from '@jupyterlab/notebook';
 import { Dialog, showDialog, ReactWidget, Notification } from '@jupyterlab/apputils';
 import { PageConfig } from '@jupyterlab/coreutils';
-import { INotebookContent } from '@jupyterlab/nbformat';
+import type {
+  INotebookContent,
+  INotebookMetadata,
+  IKernelspecMetadata,
+  ILanguageInfoMetadata
+} from '@jupyterlab/nbformat';
 
 import { customSidebar } from './sidebar';
 import { SharingService } from './sharing-service';
@@ -26,7 +31,12 @@ import {
   ViewOnlyNotebookPanel
 } from './view-only';
 
-import { KERNEL_DISPLAY_NAMES, switchKernel } from './kernels';
+import {
+  KERNEL_DISPLAY_NAMES,
+  KERNEL_URL_TO_NAME,
+  KERNEL_NAME_TO_URL,
+  switchKernel
+} from './kernels';
 
 /**
  * Generate a shareable URL for the currently active notebook.
@@ -75,6 +85,78 @@ async function showShareDialog(sharingService: SharingService, notebookContent:
   }
 }
 
+/**
+ * Ensure that the notebook metadata contains the necessary language and kernelspec information.
+ * This function will patch the metadata if it is missing or incomplete. This happens only for
+ * the initial load of the notebook, so that the metadata is always present even if the kernel
+ * has not started yet. Subsequent changes to the notebook will change the metadata we set here.
+ * @param panel - The notebook panel that we ensure the metadata for.
+ * @param content - The notebook content to patch, if necessary.
+ * @returns - a boolean indicating whether the metadata was patched.
+ */
+function ensureLanguageMetadata(
+  panel: NotebookPanel | ViewOnlyNotebookPanel,
+  content: INotebookContent
+): boolean {
+  const meta = (content.metadata ?? {}) as INotebookMetadata;
+  const before = JSON.stringify({ kernelspec: meta.kernelspec, languageInfo: meta.language_info });
+
+  // We are trying to extract the kernel name from the URL, session, or metadata; because it is
+  // not guaranteed that the URL param might not be sanitised by the time we access it.
+  const urlKernelParam = new URL(window.location.href).searchParams.get('kernel') ?? undefined;
+  const kernelFromUrl = urlKernelParam ? KERNEL_URL_TO_NAME[urlKernelParam] : undefined;
+  const kernelFromSession =
+    panel instanceof NotebookPanel ? panel.sessionContext.session?.kernel?.name : undefined;
+
+  const kernelName: 'xpython' | 'xr' =
+    (meta.kernelspec?.name as 'xpython' | 'xr' | undefined) ??
+    (kernelFromSession as 'xpython' | 'xr' | undefined) ??
+    (kernelFromUrl as 'xpython' | 'xr' | undefined) ??
+    'xpython';
+
+  const language: 'python' | 'r' = KERNEL_NAME_TO_URL[kernelName] as 'python' | 'r';
+  const display: string = KERNEL_DISPLAY_NAMES[kernelName] ?? (language === 'r' ? 'R' : 'Python');
+
+  const file_extension: string =
+    meta.language_info?.file_extension && meta.language_info.file_extension.trim() !== ''
+      ? meta.language_info.file_extension
+      : language === 'r'
+        ? '.r'
+        : '.py';
+
+  // We use a placeholder until kernel.info arrives and we override it later
+  const version: string =
+    typeof meta.language_info?.version === 'string' && meta.language_info.version.trim() !== ''
+      ? meta.language_info.version
+      : '0';
+
+  // Once grabbed, we patch the metadata with the kernelspec and language_info
+  // If the metadata already exists, we merge it with the existing metadata.
+  const kernelspec: IKernelspecMetadata = {
+    ...(meta.kernelspec ?? {}),
+    name: kernelName,
+    display_name: display,
+    language
+  };
+  const language_info: ILanguageInfoMetadata = {
+    ...(meta.language_info ?? {}),
+    name: language,
+    file_extension,
+    version
+  };
+
+  const patched: INotebookMetadata = {
+    ...(content.metadata ?? {}),
+    kernelspec,
+    language_info
+  };
+
+  content.metadata = patched;
+
+  const after = JSON.stringify({ ks: patched.kernelspec, li: patched.language_info });
+  return before !== after;
+}
+
 /**
  * Notebook share/save handler. This function handles both sharing a new notebook and
  * updating an existing shared notebook.
@@ -103,6 +185,12 @@ async function handleNotebookSharing(
       }
       return;
     }
+    // Ensure that the language metadata exists, even if the kernel has not started yet.
+    const patched = ensureLanguageMetadata(notebookPanel, notebookContent);
+    if (patched) {
+      notebookPanel.context.model.fromJSON(notebookContent);
+    }
+
     if (sharedId) {
       console.log('Updating notebook:', sharedId);
       await sharingService.update(sharedId, notebookContent);