Intermittent exceptions during concurrent custom-rule execution #2222

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Steps to reproduce

...\PSScriptAnalyzer-Run.ps1" -Path C:\_work\1\s -Recurse -Settings C:\_work\1\.gdn\rs\psscriptanalyzer\63df5559-3449-4d0b-a8b9-8c97aa98fe3b.ruleset -IgnorePattern .gdn -IncludeDefaultRules -RecurseCustomRulePath -BuildInCustomRules SDLRules -OutputFilePath C:\_work\1\.gdn\.r\psscriptanalyzer\001\psscriptanalyzer.sarif

Expected behavior

Executing PSScriptAnalyzer without any errors

Actual behavior

A customer is intermittently seeing errors from "Invoke-ScriptAnalyzer" while PSScriptAnalyzer runs external SDL rules:

Exception calling "Visit" with "1" argument(s):
"Object reference not set to an instance of an object."

They have also seen:

Exception calling "Visit" with "1" argument(s):
"Collection was modified; enumeration operation may not execute."

The affected rules use a PowerShell class derived from "AstVisitor2" and call "$ScriptBlockAst.Visit($customVisitor)". The problem is intermittent: the same source can fail in one run and pass when rerun. PSScriptAnalyzer reports the exception as a non-terminating error and the process still exits with code 0.

Environment data

Environment: Windows PowerShell 5.1 and PSScriptAnalyzer 1.19.1.

Is this a known concurrency issue in PSScriptAnalyzer’s external-rule execution path or in "Ast.Visit()" under Windows PowerShell 5.1?