Serialize CommandInfo lookups onto a single dedicated runspace

[Copilot]

Follow-up to PowerShell#2206 / issue PowerShell#2205. CommandInfoCache resolved commands through a RunspacePool(1, 10), allowing up to ten Get-Command invocations to run concurrently across separate runspaces. The PowerShell engine's command discovery state is not thread safe (PowerShell#4003), which surfaced as intermittent CommandNotFoundException for Get-Command itself and non-deterministic diagnostic counts.

A runspace pool doesn't help here: it already guarantees a given runspace is used by only one PowerShell instance at a time, and that per-runspace serialization is precisely the configuration that fails. The unsafe state is engine-global, so the gate has to be global too — at which point additional pooled runspaces are never concurrently active.

Changes

• Engine/CommandInfoCache.cs

  • Replaced the runspace pool with a single long-lived Runspace.
  • Added a re-entrant instance monitor around the engine invocation, including the retry loop. A monitor rather than a SemaphoreSlim so a future re-entrant lookup can't self-deadlock.
  • Cache hits remain lock-free — only misses reach the engine and serialize.
  • Dispose acquires the same lock; a lookup that acquires it after disposal returns null rather than touching a disposed runspace.
  • Retained the retry / CommandNotFoundException fallback from Fix intermittent "The term 'Get-Command' is not recognized" failures during recursive analysis PowerShell/PSScriptAnalyzer#2206 as a safety net for hosts that drive the engine from other threads.

• Tests/Engine/CommandInfoCacheConcurrency.tests.ps1 (new)

  • Resolves 40 commands from thread pool threads via a small C# driver and asserts each lookup resolves. The driver is C# because invoking a script block on a thread pool thread introduces runspace affinity problems of its own.
  • Runs the analyzer once first: touching Helper.Instance before the cmdlet does installs a helper with no command invocation context, which breaks every later analysis in the process. Pre-existing engine fragility, called out here so the test doesn't trip it.

// Only cache misses reach this; hits are served from the ConcurrentDictionary lock-free.
lock (_runspaceLock)
{
    if (disposed) { return null; }

    using (var ps = System.Management.Automation.PowerShell.Create())
    {
        ps.Runspace = _runspace;
        // ...
    }
}

Performance

Invoke-ScriptAnalyzer -Path Tests -Recurse, warm, alternating runs:

	before	after
steady state	15.0s / 14.5s	15.1 – 15.8s
outliers	81s, 106s, 116s	none
diagnostics	4187 – 4199	4199

Steady-state cost is within noise. The pre-change build also produced large outliers and varying diagnostic counts, so serializing appears to trade little or no throughput for determinism.