A handy Powershell function to lock / unlock using .NET, to prevent concurrent read/writes to files or anything else you like.

function handleThreadLock{
    Param(
        [switch]$setLock,
        [switch]$releaseLock,
        [string]$lockName="defaultLockName",
        [int]$timeOut=600
    )
    if($setLock){
        #register a thread lock
        $script:threadLock = New-Object System.Threading.Mutex($false, $lockName)
        $waited = 0
        while($true){
            try{$lockState = $script:threadLock.WaitOne(1000)}catch{$lockState=$False}
            if($lockState){
                break
            }else{
                $waited+=1
                if($waited -gt $timeOut){
                    Throw "failed to get a thread within $timeOut seconds!"
                }
            }
        }
    }  
    if($releaseLock){
        #release a thread lock
        [void]$script:threadLock.ReleaseMutex()
    }  
}

In your script, call it like this:

try{
    handleThreadLock -setLock
}catch{Throw "Failed to set lock!"}

try{
    add-content -Path "c:\yourfile.txt" -Value "log entry" -ErrorAction Stop
}finally{
    handleThreadLock -releaseLock
}

While building some multithreading Azure Runbooks that log into multiple subscriptions simultaneously, I noticed that these multiple concurrent runs often end up on the same Azure Automation Host.

Apparently, these runbooks then don’t run in full isolation, and the following error may occur:

The running command stopped because the preference variable “ErrorActionPreference” or common parameter is set to Stop: The process cannot access the file ‘C:\Windows\system32\config\systemprofile\AppData\Roaming\Windows Azure Powershell\TokenCache.dat’ because it is being used by another process.

AzureProfile.json may also get locked. I resolved this by doing a retry on the Save-AzureRMContext, and using a randomized file name for the azure json profile:

$randomProfileName = [System.IO.Path]::GetRandomFileName()

Save-AzureRmContext -Path .\$randomProfileName -Force -Confirm:$False

And for my full Azure Login code snippet:

$randomProfileName = [System.IO.Path]::GetRandomFileName()
 $tries=0
    while($true){
        $tries++
        try{
            Write-Output "Logging in to Azure $azureSubscription"
            $res = Login-AzureRmAccount -Credential $azureCreds -SubscriptionId $azureSubscription -TenantId $tenantId -ErrorAction Stop
            Select-AzureRmSubscription -SubscriptionId $azureSubscription -ErrorAction Stop -TenantId $tenantId
            if($res.Context.Subscription.Id -eq $azureSubscription){
                Write-Output "Logged in to Azure subscription $($res.Context.Subscription.Id)"
            }else{
                Throw "Failed, we were logged in to $($res.Context.Subscription.Id) while trying to log in to $azureSubscription"
            }
            Save-AzureRmContext -Path .\$randomProfileName -Force -Confirm:$False
            break
        }catch{
            if($tries -ge 30){
                Throw
                Exit
            }
            #sleep on failed attempts, as the azure token cache gets locked by concurrent jobs
            sleep -s (Get-Random -minimum 1 -maximum 6)
        }
    }