Powershell Mutex not working as expected

PowerShell Help
1

Hello all,

I am attempting to work with mutexes in Powershell for inter-process synchronization.

Say I open one PS ISE and perform the below:

$mutex = New-Object -TypeName system.threading.mutex($true, "TestMutex")

Then if I open a new ISE tab or a new console session and issue the below:

$mutex = New-Object -TypeName system.threading.mutex($true, "TestMutex")

No error is thrown. I would expect an error since a mutex by this name should already exist. Furthermore, the below does not return the mutex like I would expect (when run on the new console):

[System.Threading.Mutex]::OpenExisting("TestMutex")

And finally, if I performed the below, I would expect a return code of “False” as the mutex should be busy. But I always get “True” (again, on the new console session):

$mutex = New-Object -TypeName system.threading.mutex($false, "TestMutex")
if ($mutex.WaitOne(5000)) { write-output "free" }
else {write-output "mutex busy"}

Any help would be appreciated.

2

We’ll see if anyone pops in with an answer, but you’re a bit outside the PowerShell-verse, here. StackOverflow might be a better spot to ask. Unfortunately, I know practically nothing about .NET’s implementation of mutexes to tell you if this is a problem related to PowerShell being essentially single-threaded, with its scope, the fact that PowerShell runs in the DLR, or something else entirely.

3

Use this to determine if you get mutex:

$CreatedMutex = ""
$Mutex = New-Object -TypeName system.threading.mutex($true, "TestMutex", [ref] $CreatedMutex)
if (!$CreatedMutex) {
    $Mutex.WaitOne()
}
# Do Stuff
$Mutex.ReleaseMutex()
$Mutex.Close()

You also might try “Global\TestMutex” as the name to make sure it is a global mutex. You can read more about it at https://msdn.microsoft.com/en-us/library/bwe34f1k.aspx

Timed out example:

$CreatedMutex = ""
$Mutex = New-Object -TypeName system.threading.mutex($true, "Global\TestMutex", [ref] $CreatedMutex)
if (!$CreatedMutex) {
    if (!$Mutex.WaitOne(5000)) {
        throw "Timed out waiting for TestMutex"
    }
}
# Do Stuff
$Mutex.ReleaseMutex()
$Mutex.Close()

Hope this helps…
SLDR
(Stephen L. De Rudder)

Note: It does look like that mutexes are always created as session level mutexes even when Global\TestMutex is used. So powershells that are on different sessions will not work. I don’t know if PowerShell run as admin will allow truly global mutexes to be created.