Test-TaskHost.ps1
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#Requires -Version 7 #Requires -Module TaskHost <# .SYNOPSIS Demonstrates usage of Subatomix.PowerShell.TaskHost. .DESCRIPTION The Test-TaskHost script demonstrates the usage of the Subatomix.PowerShell.TaskHost package. The script uses 'ForEach-Object -Parallel' to illustrate workarounds specific to that command. .NOTES Copyright 2023 Subatomix Research Inc. SPDX-License-Identifier: ISC #> [CmdletBinding()] param ( # Script block to execute in each parallel task. [Parameter(Position = 0)] [scriptblock] $ScriptBlock = { Write-Host "This is example output" } , # Count of parallel tasks to run. [Parameter()] [int] $Count = 4 , # Maximum number of tasks that [Parameter()] [int] $ThrottleLimit = [Environment]::ProcessorCount ) process { # Difficulties with the `ForEach-Object -Parallel { ... }` command: # # - The `$using:` prefix is required to reference variables defined outside # the -Parallel script block. # # - The `$using:` prefix does not support variables that themselves contain # script blocks. Convert such variables to strings before invoking # ForEach-Object, then convert back to script blocks once inside the # -Parallel script block. # # - Inside the -Parallel script block, any custom host active in the outer # scope is no longer active, having been supplanted by ForEach-Object's # own host wrapper. Get the custom host before invoking ForEach-Object, # then use Invoke-Task -OverrideHost to restore the custom host once # inside the -Parallel script block. # # - ForEach-Object -Parallel flows its output streams via a cross-thread # producer-consumer queue that does not preserve execution context and # thus forgets the current task. Use-TaskHost and Invoke-Task internally # implement a workaround for this. Invoke-Task injects the current task # id into the output before it is enqueued. Use-TaskHost extracts that # id and restores the current task. # Set up the host Use-TaskHost -WithElapsed { # TRAP: ForEach-Object -Parallel does not support $using:ScriptBlock. # Must stringify the script block here, then reconstitute it in each task. $ScriptText = $ScriptBlock.ToString() # TRAP: ForEach-Object -Parallel uses a new runspace with no loaded modules # Must reimport desired modules in each task. $ModulePath = Get-Module TaskHost | ForEach-Object Path | Split-Path | Join-Path -ChildPath TaskHost.psd1 # TRAP: ForEach-Object -Parallel overrides the host with its own. # Must get the host here, then restore it in each task. $MyHost = $Host # Do some parallel tasks 1..$Count | ForEach-Object -ThrottleLimit $ThrottleLimit -Parallel { # Reconstitute the -ScriptBlock parameter value from string. $ScriptBlock = [ScriptBlock]::Create($using:ScriptText) # Reimport desired modules Import-Module $using:ModulePath # Run the task, restoring the desired host Invoke-Task -Name "Task $_" -OverrideHost $using:MyHost $ScriptBlock } } } |