How to convert infinity while loop to a pipeline statement - powershell

Question

I'm trying to use PowerShell pipeline for some recurring tasks and checks, like
perform certain checks X times or skip forward after the response in pipeline will have different state.
The simplest script I can write to do such checks is something like this:
do {
$result=Update-ACMEIdentifier dns1 -ChallengeType dns-01
if($result.Status -ne 'pending')
{
"Hurray"
break
}
"Still pending"
Start-Sleep -s 3
} while ($true)
The question is - how can I write this script as a single pipeline.
It looks like the only thing I need is infinity pipeline to start with:
1..Infinity |
%{ Start-Sleep -Seconds 1 | Out-Null; $_ } |
%{Update-ACMEIdentifier dns1 -ChallengeType dns-01 } |
Select -ExpandProperty Status | ?{$_ -eq 'pending'} |
#some code here to stop infinity producer or stop the pipeline
So is there any simple one-liner, which allows me to put infinity object producer on one side of the pipeline?
Good example of such object may be a tick generator that generates current timestamp into pipeline every 13 seconds

Answer 1

#PetSerAl gave the crucial pointer in a comment on the question: A script block containing an infinite loop, invoked with the call operator (&), creates an infinite source of objects that can be sent through a pipeline:
& { while ($true) { ... } }
A later pipeline segment can then stop the pipeline on demand.
Note:
As of PS v5, only Select-Object is capable of directly stopping a pipeline.
An imperfect generic pipeline-stopping function can be found in this answer of mine.
Using break to stop the pipeline is tricky, because it doesn't just stop the pipeline, but breaks out of any enclosing loop - safe use requires wrapping the pipeline in a dummy loop.
Alternatively, a Boolean variable can be used to terminate the infinite producer.
Here are examples demonstrating each approach:
A working example with Select-Object -First:
& { while ($true) { Get-Date; Start-Sleep 1 } } | Select-Object -First 5
This executes Get-Date every second indefinitely, but is stopped by Select-Object after 5 iterations.
An equivalent example with break and a dummy loop:
do {
& { while ($true) { Get-Date; Start-Sleep 1 } } |
% { $i = 0 } { $_; if (++$i -eq 5) { break } } # `break` stops the pipeline and
# breaks out of the dummy loop
} while ($false)
An equivalent example with a Boolean variable that terminates the infinite producer:
& { while (-not $done) { Get-Date; Start-Sleep 1 } } |
% { $done = $false; $i = 0 } { $_; if (++$i -eq 5) { $done = $true } }
Note how even though $done is only initialized in the 2nd pipeline segment - namely in the ForEach-Object (%) cmdlet's (implicit) -Begin block - that initialization still happens before the 1st pipeline segment - the infinite producer - starts executing.Thanks again, #PetSerAl.

Answer 2

Not sure why you'd want to use a pipeline over a loop in this scenario, but it is possible by using a bit of C#; e.g.
$Source = #"
using System.Collections.Generic;
public static class Counter
{
public static bool Running = false;
public static IEnumerable<long> Run()
{
Running = true;
while(Running)
{
for (long l = 0; l <= long.MaxValue; l++)
{
yield return l;
if (!Running) {
break;
}
}
}
}
}
"#
Add-Type -TypeDefinition $Source -Language CSharp
[Counter]::Run() | %{
start-sleep -seconds 1
$_
} | %{
"Hello $_"
if ($_ -eq 12) {
[Counter]::Running = $false;
}
}
NB: Because the numbers are generated in parallel with the pipeline execution it's possible that the generator may create a backlog of numbers before it's stopped. In my testing that didn't happen; but I believe that scenario is possible.
You'll also notice that I've stuck a for loop inside the while loop; that's to ensure that the values produced are valid; i.e. so I don't overrun the max value for the data type.
Update
Per #PetSerAl's comment above, here's an adapted version in pure PowerShell:
$run=$true; &{for($i=0;$run;$i++){$i}} | %{ #infinite loop outputting to pipeline demo
"hello $_";
if($_ -eq 10){"stop";$run=$false <# stopping condition demo #>}
}