[release/9.0] ProcessThreadTests.TestStartTimeProperty flaky failure - fix from #105526 not backported

Inspect src/libraries/System.Diagnostics.Process/tests/ProcessThreadTests.cs, especially TestStartTimeProperty, and compare release/9.0 with commit 4e5411701cf6c75ae461f67934493b3a27121047 from fix #125811. Backport that fix to release/9.0 and run the targeted Linux Mono Interpreter test; done means the reported failure is resolved.

Build Information

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1598063
Build error leg or test failing: build_linux_x64_Release_Mono_Interpreter_LibrariesTests (Ubuntu.2204.Amd64.Open) / System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty
Pull request: dotnet/runtime#133988 (release/9.0-staging -> release/9.0 merge)

System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty [FAIL]
  Assert.Single() Failure: The collection did not contain any matching items
  Expected:   (predicate expression)
  Collection: [System.Diagnostics.ProcessThread, System.Diagnostics.ProcessThread, System.Diagnostics.ProcessThread, System.Diagnostics.ProcessThread, System.Diagnostics.ProcessThread, ...]
  Stack Trace:
    /_/src/libraries/System.Diagnostics.Process/tests/ProcessThreadTests.cs(156,0): at System.Diagnostics.Tests.ProcessThreadTests.<>c__DisplayClass4_0.<TestStartTimeProperty>b__0()
    /_/src/libraries/System.Diagnostics.Process/tests/ProcessThreadTests.cs(149,0): at System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty()

Console log: https://helixr1107v0xd1eu3ibi6ka.blob.core.windows.net/dotnet-runtime-refs-heads-release-90-staging-55513970eee84bc486/System.Diagnostics.Process.Tests/1/console.61907546.log?helixlogtype=result

First reported here: https://github.com/dotnet/runtime/issues/105526
Fixed on main by #125811 ("Fix ProcessThreadTests.TestStartTimeProperty flakiness due to /proc visibility race", merged 2026-03-20, commit 4e5411701cf6c75ae461f67934493b3a27121047), but that fix was never backported to release/9.0 — confirmed neither release/9.0 nor release/9.0-staging contains that commit. Requesting the fix be backported to release/9.0.

Error Message

{
  "ErrorMessage": ["System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty [FAIL]", "The collection did not contain any matching items"],
  "ErrorPattern": "",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Known issue validation

Build: 🔎
Result validation: ⚠️ Validation could not be done without an Azure DevOps build URL on the issue. Please add it to the "Build: 🔎" line.
Validation performed at: 9/15/2026 11:21:38 PM UTC

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1598063
Error message validated: [System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty [FAIL] The collection did not contain any matching items]
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 9/15/2026 11:27:02 PM UTC

Report

Summary