Reject non-empty directories for new WSLC session storage

[beena352]

Summary of the Pull Request

Prevent users from accidentally reusing existing non-empty directories as WSLC session storage. For new sessions, validate that the storage directory is either empty or non-existent. Existing sessions identified by their storage.vhdx VHD bypass this check, so users can re-enter sessions freely even if additional files are present in the storage directory.

PR Checklist

• Closes: Link to issue #62259817
• 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

Validate session storage paths at initialization time:

• If creating a new session and the directory already exists and is non-empty, reject with E_INVALIDARG and a user-facing error message
• If the directory is empty or doesn't exist, create it and proceed
• If a session's VHD already exists in the directory, skip the emptiness check (re-entry allowed)
  This prevents data loss by ensuring new sessions don't accidentally overwrite unrelated user files placed in the storage directory. Once a session is created (VHD exists), the directory can contain other files without blocking re-entry.

Validation Steps Performed

[Copilot]

Pull request overview

Adds a wslcsession marker-file convention at the root of a WSLC session storage directory so the service can distinguish directories it owns from arbitrary user content, and rejects new session creation into a non-empty, unmarked directory. Also wires IErrorInfo/COMServiceExecutionContext through the per-user WSLCSessionFactory and WSLCSessionManager so the user-facing rejection message reaches the CLI/test caller.

Changes:

• Add EnsureSessionMarker helper in WSLCSession::ConfigureStorage (new sessions require empty dir + stamp marker; existing VHD paths get a legacy upgrade stamp).
• Add ISupportErrorInfo + COMServiceExecutionContext to WSLCSessionFactory and propagate IErrorInfo from factory to caller in WSLCSessionManagerImpl::CreateSession; add ISupportErrorInfo to WSLCSessionManager.
• Add API-level and e2e tests for marker rejection and legacy auto-upgrade; add localized MessageWslcSessionStorageMustBeEmpty.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/windows/wslcsession/WSLCSession.cpp	New EnsureSessionMarker helper and wiring into ConfigureStorage for new/existing storage paths.
src/windows/wslcsession/WSLCSessionFactory.cpp	Establish COMServiceExecutionContext around CreateSession and implement InterfaceSupportsErrorInfo.
src/windows/wslcsession/WSLCSessionFactory.h	Add ISupportErrorInfo base and method declaration.
src/windows/service/exe/WSLCSessionManager.cpp	Inspect IErrorInfo after factory failure and re-throw with user-facing message; implement InterfaceSupportsErrorInfo.
src/windows/service/exe/WSLCSessionManager.h	Add ISupportErrorInfo base and method declaration.
test/windows/WSLCTests.cpp	New API-level cases: non-empty dir rejection and wslcsession-as-directory rejection with COM error validation.
test/windows/wslc/e2e/WSLCE2ESessionEnterTests.cpp	E2E test verifying marker creation on default session and legacy auto-upgrade.
localization/strings/en-US/Resources.resw	New MessageWslcSessionStorageMustBeEmpty localized string.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated no new comments.

[OneBlue]

I think refusing to create a session storage if the directory is non-empty makes sense (although I worry a bit about this putting us in a corner, like if there's ever a thumdb.db, desktop.ini or similar file created, this could "lock a user out" of using that folder permanently)

I'm not sure about the marker file though. Whenever we create a storage folder, we always create a storage VHD, so could we use that directly instead ?

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.