PowerShell : Use Case for MutEx

This post is to give you context on a practical use case of using MutEx in PowerShell.

From the MSDN documentation for the MutEx class , MutEx is :


"A synchronization primitive that can also be used for interprocess synchronization."

Mut - Mutually 
Ex - Exclusive


Recently while deploying AzureStack, I saw multiple failed deployments, partly because of me not paying attention. 


But since it failed, I had to go and look at the code in an effort to see what went wrong.

AzureStack runs all the deployment tasks for creating a POC by using scheduled tasks (runs in System context) heavily.

Also the status of the deployment  is tracked by using XML files (these are placed under C:\ProgramData\Microsoft\AzureStack\), so they have to avoid conflicts in reading and writing of these XML files from these tasks which are separate PowerShell processes.

Now this is a simple but very important part of the whole process. So while going through the code being used,I saw this little neat function in the AzureStakDeploymentStatus.psm1 file :

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## Lock the status reads\writes with a mutex to avoid conlicts function Invoke-ActionWithMutex {     param(         [ScriptBlock]$Action     )     $mutex = $null     try     {         $createdNew = $fasle # Typo here but this evaluates to False         $mutex = New-Object System.Threading.Mutex($true, "Global\AzureStackDeploymentStatus", [ref]$createdNew)         if (-not $createdNew)         {             try             {                $mutex.WaitOne() | Out-Null             }             catch [System.Threading.AbandonedMutexException]             {                 #AbandonedMutexException means another thread exit without releasing the mutex, and this thread has acquired the mutext, therefore, it can be ignored             }         }         Invoke-Command -ScriptBlock $Action -NoNewScope     }     finally     {         if ($mutex -ne $null)         {             $mutex.ReleaseMutex()             $mutex.Dispose()         }     } }

This function piqued my interest on the subject of MutEx. In the same .psm1 file this function is being used within the functions Get-AzureStackDeploymentStatus and Update-AzureStackDeploymentStatus.

Below is the Get-AzureStackDeploymentStatus defintion :

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function Get-AzureStackDeploymentStatus {     [CmdletBinding()]     param()     if (-not (Test-Path $statusFilePath)) {         Update-AzureStackDeploymentStatus $StatusTemplate | Out-Null     }     Invoke-ActionWithMutex -Action {         [xml](Get-Content $statusFilePath)     } }

and the Update-AzureStackDeploymentStatus definition is below:

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function Update-AzureStackDeploymentStatus {     [CmdletBinding()]     param(         [Xml]$Status     )     Invoke-ActionWithMutex -Action {         if (-not (Test-Path $statusFileFolder))         {             New-Item -Path $statusFileFolder -ItemType Directory | Out-Null         }         $Status.Save($statusFilePath)     } }

Now this is powerful stuff and since my first exposure to it, I have been reading up a lot on this topic and how to use this in my Scripts.

This post will soon follow a series of post on this particular topic.