[release/2.7] Host plugin Plan9 shares as the session user - #41557

This backport changes how host plugin folder mounts are serviced so that host paths are accessed using the session user’s Windows permissions (instead of the service identity), by introducing a dedicated per-user Plan9 server on a separate port and updating mini_init mounting accordingly.

Changes:

• Create and reuse a per-session-user Plan9 server for plugin folder shares and mount them through a new dedicated Plan9 port.
• Extend mini_init’s Plan9 mounting helper to connect to a specified host port, and route plugin “mount folder” messages to the plugin Plan9 port.
• Add a regression test that validates allowed vs denied writes through a plugin-mounted folder.

Suppressed comments (1)

src/windows/service/exe/WslCoreVm.cpp:2075

• If m_pluginPlan9Server exists but IsRunning() is not S_OK, the code creates a new server and overwrites m_pluginPlan9Server without explicitly tearing down the old instance. If the old server still holds resources (e.g., bound port / shares), this can leak and/or make reinitialization flaky. Teardown/reset the existing server before creating the replacement.

        if (!m_pluginPlan9Server || m_pluginPlan9Server->IsRunning() != S_OK)
        {
            auto server =
                wsl::windows::common::wslutil::CreateComServerAsUser<p9fs::Plan9FileSystem, IPlan9FileSystem>(m_userToken.get());
            THROW_IF_FAILED(server->Init(&m_runtimeId, LX_INIT_UTILITY_VM_PLAN9_PLUGIN_PORT));
            THROW_IF_FAILED(server->Resume());
            m_pluginPlan9Server = std::move(server);
        }

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Lite

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

src/windows/service/exe/WslCoreVm.cpp:2084

• This path now propagates ERROR_ALREADY_EXISTS from AddSharePath, whereas the existing Plan9-share path treats that result as success (see WslCoreVm.cpp:960-966). A plugin that retries a mount after a partial mount, or mounts two folders with the same share name, will therefore fail before mini_init is asked to mount it; normalize this result to S_OK here as well.

• Files reviewed: 7/7 changed files
• Comments generated: 0 new
• Review effort level: Lite