Testing of ForEach-Object -Parallel

[JanElholm]

Checklist

• Feature request has a meaningful title
• I have searched the existing issues. See all issues
• I have tested using the latest version of Pester. See Installation and update guide.

Summary of the feature request

$TimeToSleep = 5
1..10 | ForEach-Object -Parallel {
    $TimeToSleep = $using:TimeToSleep
    Start-Sleep $TimeToSleep
} -ThrottleLimit 5

I would like to be able to mock a function inside of a "ForEach-Object -Parallel". In the above rather contrieved example I would like to be able to mock the Start-Sleep function

Thx

How should it work?

No response

[fflaten]

Unfortunately that's not possible.

Using -Parallel runs the scriptblock-code in new PowerShell runspaces (threads). The new runspaces doesn't share any state like imported modules, variables etc. with your normal session. So the Pester-module and any knowledge of existing mocks are invisble inside.

[JanElholm]

Sorry to hear that it's not possible. Any recommendation or best practises on how to implement multithreading in powershell in order to make it testable with Pester?

[fflaten]

It's mostly mocking objects and commands that won't work.

You can test the results of any code using multithreading, so maybe focus on integration tests against test-data/environments?

[DarkLite1]

Running into this same limitation of Pester. Some easy repro code:

Describe 'it should be' {
    BeforeAll {
        $realCmdLet = @{
            InvokeCommand = Get-Command Invoke-Command
        }

        Mock Invoke-Command {
            & $realCmdLet.InvokeCommand -Scriptblock {
                1
            } -ComputerName $env:COMPUTERNAME
        }

        $testData = @(
            @{ComputerName = 'PC1' }
            @{ComputerName = 'PC2' }
        )
    }
    It 'green' {
        $testData | ForEach-Object {
            Invoke-Command -ComputerName $_.ComputerName -ScriptBlock { 1 }
        }

        Should -Invoke Invoke-Command -Times 1 -Exactly -ParameterFilter {
            $ComputerName -eq 'PC1'
        }

        Should -Invoke Invoke-Command -Times 1 -Exactly -ParameterFilter {
            $ComputerName -eq 'PC2'
        }
    }
    It 'green' {
        $testData | ForEach-Object -Parallel {
            Invoke-Command -ComputerName $_.ComputerName -ScriptBlock { 1 }
        }

        Should -Invoke Invoke-Command -Times 1 -Exactly -ParameterFilter {
            $ComputerName -eq 'PC1'
        }

        Should -Invoke Invoke-Command -Times 1 -Exactly -ParameterFilter {
            $ComputerName -eq 'PC2'
        }
    }
}

The last test fails because of Foreach-Object -Parallel creating its own runspaces in which Pester can't utilise mocks.

@nohwnd @fflaten is there really no way around this?

It would maybe be an idea if Pester could remove the -Parallel switch and run it in the same thread just for debugging purposes. But that would also entail replacing variables that start with $using:xxx.

Related StackOverflow question.