Call `DownloadHandler::CanDownload` when downloading a PDF file from the PDF viewer

[lucamuscat]

Describe the bug
Typically, DownloadHandler::CanDownload is called before any download, giving the developer the opportunity to decide whether a file should be downloaded. DownloadHandler::CanDownload is not executed when a PDF file is downloaded through the PDF viewer plugin, circumventing the logic found in DownloadHandler::CanDownload.

Although one may argue that the PDF is technically already downloaded, thereby making this method call redundant, it subverts the exceptation that DownloadHandler::CanDownload method is called on all downloads.

My current workaround is to move my DownloadHandler::CanDownload logic into DownloadHandler::OnBeforeDownload, and returning false in DownloadHandler::OnBeforeDownload, allowing the alloy runtime to cancel the download by default. This is a hacky/dirty workaround which I wish to avoid.

To Reproduce
Steps to reproduce the behavior:

1. Clone my fork of cef-project containing the MVP of the bug
2. Build the project, and run the minimal executable. DownloadHandler::CanDownload will log a message starting with CanDownload when triggered, allowing you to grep for the log.
3. Download a normal file as a sanity check, and observe the CanDownload log
4. Open a PDF file, and press the download button, observe that the log is not emitted.

Expected behavior
DownloadHandler::CanDownload is called when downloading a PDF file through the PDF viewer.

Versions (please complete the following information):

• OS: Ubuntu 20.04.1
• CEF Version: 127.3.4+ge9e2e14+chromium-127.0.6533.100_linux64

Additional context
So far, I have only tried reproducing the bug in the minimal project in cef-project, but I also observe this behaviour in another project.

Let me know if I can provide more information. If you know where I can look to start fixing this bug, kindly let me know.

Regards,
Luca

[magreenblatt]

As background, the "Download" button in the PDF viewer should trigger a call to PdfViewWebPlugin::HandleSaveMessage (via onDownloadClick_ in the JS code). There is a different code path for original vs edited PDF files (SaveToFile vs SaveToBuffer). SaveToFile calls PDFDocumentHelper::SaveUrlAs and SaveToBuffer calls saveData_ (JS code). The SaveToFile case should call DownloadManagerImpl::DownloadUrl. However, the SaveToBuffer case would not trigger a download because it operates using local (in memory) PDF modifications.