Make Run.ParallelThrottleLimit = 1 run sequentially in the current session

[nohwnd]

Comes out of #2971, where @DarkLite1 described the pattern he uses in his own code: one function that runs a scriptblock over items, and when you set -ThrottleLimit 1 it drops to a plain foreach on the main thread so you can step through it. Same knob for concurrency and for debugging.

Run.ParallelThrottleLimit = 1 did not do that. It ran one file at a time, but still inside a worker runspace, so you got no speedup and no debugging: breakpoints are per runspace, a breakpoint set in the session that calls Invoke-Pester never hits inside a worker.

Now 1 falls back to the normal sequential path, next to the fallbacks we already have (Windows PowerShell 5.1, ScriptBlock containers, Run.SkipRemainingOnFailure = 'Run'). The banner also stops saying "in parallel", because the run is not parallel.

No new configuration option, and no new switch on Invoke-Pester. Turn the throttle you already have down to 1 and you can debug the file.

Before:

Invoke-Pester -Configuration @{ Run = @{ Parallel = $true; ParallelThrottleLimit = 1 } }
# file 1 ran in runspace 2, breakpoint hits: 0

After:

Invoke-Pester -Configuration @{ Run = @{ Parallel = $true; ParallelThrottleLimit = 1 } }
# file 1 ran in runspace 1, breakpoint hits: 1

Tests added in tst/Pester.RSpec.Parallel.ts.ps1: the breakpoint hits, the banner does not claim parallel, and the counts match a parallel run. test.ps1 -File tst/Pester.RSpec.Parallel.ts.ps1 passes 25/25, PesterConfiguration.Tests.ps1 and Help.Tests.ps1 pass, and Invoke-ScriptAnalyzer on src/Main.ps1 reports the same 9 pre-existing findings as main.

Open question: someone might want 1 file at a time but still isolated in a runspace, to tell an isolation problem apart from a concurrency problem. That is what this takes away. I think the debugging case is worth more, LMK if you disagree.

🤖

[nohwnd]

Not the direction we want to take. Enabling Run.Parallel means the files run in runspaces, throttling to 1 just means one runspace at a time. If you don't want parallel, don't enable it.

🤖