Fix PDF export file path handling

[CNDY1390]

Fixes #16590

Fixes the long-standing bug where exporting a Jupyter notebook to PDF via the VS Code Jupyter extension fails, even though the PDF is correctly generated in the temp directory. The extension was looking for the wrong filename (missing or duplicating the .pdf extension), resulting in a file-not-found error and failed export.

Background and Motivation

• As detailed in issue #16590, exporting to PDF from the VS Code extension has been broken for years (at least since 2020).
• The bug is not present in the web interface or command-line tools (nbconvert), only in the VS Code extension.
• Many users believe Jupyter's export is convenient, but in practice, exporting to PDF from VS Code is unreliable and cumbersome. But if this commit is merged, exporting to PDF from VS Code would be much easier, especially for begginners.

Root Cause

• The extension generates a PDF file in the temp directory (e.g., tmp-xxxx.pdf), but then tries to access or copy a file with an incorrect name (e.g., tmp-xxxx.pdf.pdf or missing .pdf).
• This is due to incorrect filename/path handling in the export logic, specifically in src/notebooks/export/exportBase.node.ts:

  if ((await this.fs.stat(Uri.file(tempTarget.filePath))).size > 1) {
      await this.fs.copy(Uri.file(tempTarget.filePath), target);
  }

What has been changed

• The filename and path handling logic is corrected to ensure the extension always looks for the actual generated PDF file.

Additional context

• Thanks to the community for detailed analysis and temporary workarounds.
• My implementation might not be robust for non-Windows environment. Please follow the post if there is any problem. 👍

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Closes #16590

[amunger]

did you check why it creates a file that ends in .pdf.pdf?

[CNDY1390]

Thanks for the feedback. I’ve dug deeper into the issue and found that nbconvert automatically appends a .pdf suffix to the --output parameter, causing the filename error. Instead of modifying the judgment function, a better approach is to pass a filename without the suffix. I’ll close this PR and submit a new one with the improved fix.

[CNDY1390]

Hi there, I've opened a new PR with an improved fix for this issue. Please take a look at the updated solution here: #16651. Thank you!