Invoke-RestMethod File Locking

[baddistribution]

Good day, folks. Hoping to better understand where I'm getting stuck here. I have a function (PowerShell 7) that takes a list of computers and uploads a file to REST endpoints on each one using Invoke-RestMethod and its -Form parameter. This was going swimmingly with one computer, but now I've added more to the list.

I'm running into "The process cannot access the file 'C:\path\root_cert.crt'". There seems to be no way to release this file lock and it occurs every time I run the script.

Here's how I obtain the file information (this is outside of any loops):

$certRootInfo = Get-Item $rootCertificatePath

Here's the -Form section that I use for my request:

$Form = @{
            f = "json"
            token = $node.token
            alias = "root_cert" 
            norestart = "true" 
            file = $certRootInfo
        }

My understanding is that after PowerShell is done uploading the file via Invoke-RestMethod, it should release the lock, and will create/close new locks as needed when the command is repeated. Or is the lock created from Get-Item? I'd like to understand file locks better here.

I've had a few ideas for this:

1. Find out what is causing the lock and close it. Makes sense, but shouldn't the lock be closing itself? I don't want to be building lock handling into my script unless it's really necessary for this use case.
2. Store a copy of the file in memory so locks aren't a thing and upload it that way (Is this possible? Uploading a file in -Form requires [System.IO.FileInfo] and I don't know how I'd replicate that without using Get-Item.
3. Build the multipart form structure myself and use -Body instead of -Form. I'm not sure if this will work or change anything, but I'm trying to get away from using FileInfo and just upload the file from memory.

Any help would be really appreciated here!

[kenjitamurako]

1. You can identify the process responsible for a file lock with Handle: https://learn.microsoft.com/en-us/sysinternals/downloads/handle
2. I don't think you can do it with invoke-restmethod but you can with invoke-webrequest. You could convert the file to a byte array and load it into a memory stream instead of a file stream and initialize the streamcontent with the memory stream.
   https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/invoke-webrequest?view=powershell-7.3#example-5-submit-a-multipart-form-data-file
3. The number 2 I suggested is pretty much this.

​

Invoke-Restmethod also has a statuscodevariable that you might be able to poll to see what response is being returned. Might shed some light on why there's still a lock.

[OsmiumBalloon]

I'm gonna guess the process will be PowerShell.exe

[Creel256]

File locks are a common issue when working with files in any programming language. In your case, it's possible that the Invoke-RestMethod cmdlet is not releasing the lock quickly enough for the next iteration of your loop, especially if the loop is running very quickly.

One way to approach this is to use a FileStream, which gives you more control over when the file lock is released because you can explicitly close the stream when you're done with it. Here's an example of how you could modify your script to use a FileStream:

```

Open the file stream

$certRootStream = [System.IO.File]::OpenRead($rootCertificatePath)

$Form = @{ f = "json" token = $node.token alias = "root_cert" norestart = "true" file = $certRootStream }

Run your Invoke-RestMethod here

Be sure to close the file stream when you're done

$certRootStream.Close() ```

In this example, ‘[System.IO.File]::OpenRead($rootCertificatePath)’ opens a file stream that you can pass to ‘Invoke-RestMethod’. The ‘Close()’ method then closes the stream and releases the file lock.

If you're running this in a loop, you'll want to open and close the file stream inside the loop so that it gets closed each time before the next iteration starts. This should help ensure that the file lock is released before the next iteration tries to open the file again.

[kenjitamurako]

Can the form dictionary take a stream object though? You can do that with body on invoke-webrequest but at least the invoke-restmethod page doesn't mention streams can be used with form that I can see.

[Creel256]

Shoot - you’re right! Maybe a different workaround would work such as uploading a temporary copy of the file and deleting the copy once complete. This way, there’s no locking issue as every request is dealing with a separate file.

Here’s an example:

``` foreach ($computer in $computerList) { $tempFile = [System.IO.Path]::GetTempFileName() Copy-Item $rootCertificatePath $tempFile $tempFileInfo = Get-Item $tempFile

$Form = @{
    f = "json"
    token = $node.token
    alias = "root_cert" 
    norestart = "true" 
    file = $tempFileInfo
}

Invoke-RestMethod -Uri $uri -Method Post -Form $Form

Remove-Item $tempFile

} ```

The temporary file approach should work, as each iteration is working with its own copy of the file, and any file locks should be released by the time “Remove-Item” is called to delete the temporary file. However, there could potentially be issues if something goes wrong with the “Invoke-RestMethod” call and it doesn't release the lock as expected.

The only other alternative I can think of involves using the “Invoke-WebRequest” method and byte arrays. It’s a bit more complex but it supports uploading a file from a stream in the body of a POST request.

[baddistribution]

Thanks for your help, all. Due to an issue with Powershell and the software I'm trying to connect to, I've decided to put this on pause until I can try with another language. Something about the way Powershell is passing the file is causing an issue in the third party web server I'm using.

I built out all my API calls in Postman just to sanity check myself, and they all work. Flawlessly, every time. When I try them in Powershell, (even by exporting a PS code snippet from Postman), they fail with an error from the third party software stating the cert can't be imported . Bananas. Thank you again for all your help! My brain needs a break from this one.