Allow users to download multiple VM images at the same time from the Hyper-V gallery

[bbonaby]

Summary of the pull request

There are two steps in creating a VM from the VM gallery that involves a file.

1. When downloading the archive file that contains the virtual disk file to the users temp folder.
2. When extracting the archive file to the users default virtual hard disk location.

Currently when users attempt to create a VM multiple times simultaneously, this error is thrown: "The process cannot access the file because it is being used by another process. The file is in use. Please close the file before continuing". This is thrown during the download portion. To prevent this from happening, we can reuse the download progress for subsequent download requests of the same file after one has been initiated.

Changes in the Hyper-V extension:

• Added a FileDownloadMonitor class that is used to monitor the progress of a download. Clients can subscribe to the monitor who will publish the progress of the download as it receives them to these subscribers.
• Updated the DownloaderService to use the FileDownloadMonitor to start the download and add clients to the monitor based on the absolute file path of the file that will be downloaded.
• Updated the DownloaderService class to allow for two possibilities. One where it handles the case where a new file is requested to be downloaded and two where it handles the case where the requested file is already being downloaded.
• Added a retry attempt if the first call of VMGalleryVMCreationOperation's StartAsync's method fails.

Here is a video of me attempting to create two of the same VMs BEFORE this change:

Video.showing.file.in.use.error.when.multiple.downloads.are.in.progress.mp4

Here is a video of me attempting to create two of the same VMs AFTER this change:
Timestamps:

• 0:21 first VM creation starts by downloading ubuntu image
• 0:37 second VM creation starts which subscribes to the same download
• 1:46 Download of ubuntu image completes and both VM creation objects continue separately checking the file hash and extracting the file
• 2:33 Both VM creation objects finish extracting archive file and move on to creating a new VM with the newly extracted virtual disk

Video.showing.me.being.able.to.create.two.Hyper-V.VMs.at.the.same.time.mp4

Added cancelation from the Dev Home side:

Video.of.me.showing.that.you.can.now.cancel.downloads.from.Dev.Home.for.Hyper-V.mp4

References and relevant issues

Note: I did think about using the AsyncManualResetEvent to allow threads waiting for the download to complete to wait asynchronously. See the nuget dependency here: https://www.nuget.org/packages/Microsoft.VisualStudio.Threading/. But adding this dependency causes a lot of async errors and warnings (via Microsoft.VisualStudio.Threading.Analyzers) with regards to Dev Home's usage of certain async patterns. So instead I used the WaitAsync method of the SemaphoreSlim class. In the future we should look into that library as it has some good async functionality.

Detailed description of the pull request / Additional comments

Validation steps performed

PR checklist

• Closes [Environments reliability] Hyper-V VM Creation Error: File in use by another process  #3657
• Tests added/passed
• Documentation updated

[sshilov7]

We should think about adding a way (and UX) to cancel this download. It can get stuck forever.

[bbonaby]

Yea I thought about adding a timeout, but I figure it depends on the users internet connection so I can't really say for sure how long it'll take. I'll make a GitHub issue for this to update the UI. We need to get the cancellation token from the IAsyncOperation<CreateComputeSystemResult> StartAsync() method above and use it here to cancel the operation.

[bbonaby]

Good news, looks like I actually originally do send a cancelation request from Dev Home to the extension here:

devhome/common/Environments/Models/CreateComputeSystemOperation.cs

Line 120 in 7b65abf

CreateComputeSystemResult = await _createComputeSystemOperation.StartAsync().AsTask(_cancellationTokenSource.Token);

So I have updated the code to allow for cancellation: See below video as we now only use async/awaits:

Video.of.me.showing.that.you.can.now.cancel.downloads.from.Dev.Home.for.Hyper-V.mp4

As you can see we can cancel them now. We'll just need to add a content dialog to ask the user if they're sure.

[sshilov7]

In StartDownloadAsync this lock is taken for the duration of the downloading process, so this method won't return until downloading is done.

[bbonaby]

From what I see I don't see how that would be the case. I don't lock the entire StartDownloadAsync method. From my testing, this doesn't get locked for the entirety of the StartDownloadAsync call. But please let me know if I'm missing something. Thanks.

Edit, While my first comment still stands as I'd appreciate some insights in case I missed something. I moved the first lock outside the try/catch in StartDownloadAsync, so the finally block doesn't inadvertently remove the monitor from the map after subsequent download requests for the same file.

Here is me creating 6 VMs using the same image consecutively with no errors. Note that all of them share the same download progress.

Video.of.me.creating.6.VMs.with.the.same.image.consecutively.mp4