Wait for init to exit before trying to remount the distro VHD by OneBlue · Pull Request #41669 · microsoft/WSL

Blue (OneBlue) · 2026-09-21T23:59:59Z

Summary of the Pull Request

This change solves a race condition that has been observed in the CI. If a distribution is force-terminated, its LUN can be reused by a future instance creation while init is still in the process of unmounting its filesystem, which can fail the instance creation.

This change solves this by actually waiting for init to exit to consider that the distribution is stopped. The timeout reused the distribution start timeout for now (we can add a .wslconfig entry for it later if needed), and the timeout only logs a warning for now, so a single broken distro doesn't deadlock the entire user session

PR Checklist

Closes: Link to issue #xxx
Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
Tests: Added/updated if needed and all pass
Localization: All end user facing strings can be localized
Dev docs: Added/updated if needed
Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

Copilot

Copilot review overview

🟡 Changes recommended

Normal instance creation can still reuse the VHD/LUN while init is unmounting, so the reported race remains unresolved.

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Review effort: Lite
Findings: 2

Open (2)

CreateInstance skips waiting for pending termination · New
_CreateInstance does not wait before reusing the VHD · New

What changed in this PR

Adds termination tracking so distro operations can wait for WSL2 init to exit before reusing VHDs.

Changes:

Tracks pending init termination events.
Waits during selected conversion and VHD operations.
Logs timeout warnings without blocking the session.

File	Review
`src/windows/service/exe/LxssUserSession.h`	Adds termination tracking state and wait APIs. Critical: normal instance creation does not wait before reusing the distro VHD/LUN.
`src/windows/service/exe/LxssUserSession.cpp`	Implements termination tracking and waits for selected operations. Critical: the `CreateInstance` path still does not consume pending termination waits.

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Copilot

Copilot review overview

🟡 Changes recommended

PID reuse can associate exit notifications with the wrong distro, and unregister can still eject the VHD before init exits.

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Review effort: Lite
Findings: 1

Open (1)

PID reuse can associate exit notifications with the wrong instance · New

Resolved since last review (2)

_CreateInstance does not wait before reusing the VHD
CreateInstance skips waiting for pending termination

Copilot

Copilot review overview

🟡 Changes recommended

Unresolved termination ordering and default-distribution selection issues remain.

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Review effort: Lite
Findings: 1

Open (1)

Unregister deletes VHD before pending termination completes · New

Resolved since last review (1)

PID reuse can associate exit notifications with the wrong instance

Copilot

Copilot review overview

🔵 Needs a closer look

An unregister cleanup path can still delete the VHD before init has exited.

Review effort: Lite
Findings: None

Resolved since last review (1)

Unregister deletes VHD before pending termination completes

Copilot

Copilot review overview

🔵 Needs a closer look

The cross-process protocol and concurrent VM lifecycle changes warrant final human validation.

Review effort: Balanced
Findings: None

Copilot

Copilot review overview

🟡 Changes recommended

Timed-out termination records remain active and repeatedly delay subsequent operations.

Review effort: Balanced
Findings: 1

Open (1)

Timed-out termination record remains waitable and blocks later launches · New

Ben Hillis (benhillis)

A timed-out termination remains in m_pidTerminations, so each later launch or conversion waits the full timeout again. Please make a timed-out record non-waitable while retaining whatever identity is needed to correlate a late exit.

Copilot

Copilot review overview

🔵 Needs a closer look

The concurrency-sensitive VM lifecycle and host/guest protocol changes warrant final human review.

Review effort: Balanced
Findings: None

Resolved since last review (1)

Timed-out termination record remains waitable and blocks later launches

…ation-vhd-sync

Copilot

Copilot review overview

🟡 Changes recommended

Several changed C++ lines exceed the repository’s enforced 130-column limit.

Review effort: Balanced
Findings: 4

Open (4)

Declaration exceeds 130-column limit · New
Function signature exceeds 130-column limit · New
Declaration exceeds 130-column limit · New
Assertion exceeds 130-column limit · New

+int ProcessMessage(
+    wsl::shared::Transaction& Transaction, LX_MESSAGE_TYPE Type, gsl::span<gsl::byte> Buffer, VmConfiguration& Config, std::map<pid_t, GUID>& DistroInstances);



 _Requires_lock_held_(m_instanceLock)
-void LxssUserSessionImpl::_ConversionBegin(_In_ GUID DistroGuid, _In_ LxssDistributionState State)
+std::vector<LxssUserSessionImpl::PidTermination> LxssUserSessionImpl::_ConversionBegin(_In_ GUID DistroGuid, _In_ LxssDistributionState State)


+void WslCoreVm::RegisterCallbacks(
+    _In_ const std::function<void(const LX_MINI_INIT_CHILD_EXIT_MESSAGE&)>& DistroExitCallback, _In_ const std::function<void(GUID)>& TerminationCallback)


+            VERIFY_ARE_EQUAL(
+                LxsstuLaunchWsl(std::format(L"--install --from-file \"{}\" --no-launch --name {} --version 2", g_testDistroPath, distroName)), 0L);


Blue (OneBlue) added 3 commits September 21, 2026 14:34

Save state

cebe0be

Cover all code paths

1df7fe3

Cleanup diff

83e8fb7

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Comment thread src/windows/service/exe/LxssUserSession.cpp

Comment thread src/windows/service/exe/LxssUserSession.h

Feng Wang (chemwolf6922) mentioned this pull request Sep 22, 2026

Prevent potential invalid access for string array pretty print #41673

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Comment thread src/windows/service/exe/LxssUserSession.cpp

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Blue (OneBlue) marked this pull request as ready for review September 23, 2026 22:22

Blue (OneBlue) requested a review from a team as a code owner September 23, 2026 22:22

Ben Hillis (benhillis) requested a balanced review from Copilot September 29, 2026 21:12

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Comment thread src/windows/service/exe/LxssUserSession.cpp

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Ben Hillis (benhillis) previously approved these changes Oct 1, 2026

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Merge remote-tracking branch 'origin/master' into user/oneblue/termin…

578fee4

…ation-vhd-sync

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Wait for init to exit before trying to remount the distro VHD - #41669

Wait for init to exit before trying to remount the distro VHD#41669
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		int ProcessMessage(
		wsl::shared::Transaction& Transaction, LX_MESSAGE_TYPE Type, gsl::span<gsl::byte> Buffer, VmConfiguration& Config, std::map<pid_t, GUID>& DistroInstances);

		void WslCoreVm::RegisterCallbacks(
		_In_ const std::function<void(const LX_MINI_INIT_CHILD_EXIT_MESSAGE&)>& DistroExitCallback, _In_ const std::function<void(GUID)>& TerminationCallback)

		VERIFY_ARE_EQUAL(
		LxsstuLaunchWsl(std::format(L"--install --from-file \"{}\" --no-launch --name {} --version 2", g_testDistroPath, distroName)), 0L);

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Blue (OneBlue) commented Sep 21, 2026

Summary of the Pull Request

PR Checklist

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