René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"ForEach-Object -Parallel situationally drops pipeline input","articleBody":"Note: I can only reproduce this _reliably_ on macOS 10.15.4.\r\nI see it _occasionally_ on Ubuntu 18.04, and can never reproduce it on Windows.\r\n\r\nIt may be connected to using `Start-Sleep` inside the script block; not sure if the fact that a _custom class_ is used across thread boundaries is relevant [_update_: [it is](https://github.com/PowerShell/PowerShell/issues/12801#issuecomment-634344380)]\r\n\r\n# Steps to reproduce\r\n\r\n```powershell\r\nclass Foo { static [object] Echo($val) { return $val } }\r\n$cls = [Foo]\r\nwhile ($true) { \r\n  $res = 1..10 | ForEach-Object -Parallel {\r\n    Start-Sleep -ms 100; ($using:cls)::echo($_) \r\n  }  |  Sort-Object\r\n  Compare-Object $res (1..10) | Should -BeNullOrEmpty\r\n  Write-Host -NoNewline . \r\n}\r\n```\r\n\r\n# Expected behavior\r\n\r\nThe loop should run indefinitely, because the test should alway succeed.\r\n\r\n# Actual behavior\r\n\r\nThe loop eventually - after a varying number of iterations - breaks due to intermittent test failures, stemming from a missing output; e.g.:\r\n\r\n```none\r\nExpected $null or empty, but got @(@{InputObject=5; SideIndicator==\u003e})\r\n```\r\n\r\nThat is, not all input objects were echoed.\r\n\r\n# Environment data\r\n\r\n```none\r\nPowerShell Core 7.1.0-preview.3  on macOS 10.15.4\r\n```\r\n","author":{"url":"https://github.com/mklement0","@type":"Person","name":"mklement0"},"datePublished":"2020-05-26T22:14:44.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":17},"url":"https://github.com/12801/PowerShell/issues/12801"}