Fix: add systemd unit to make /mnt/wsl recursive slave before umount

src/linux/init/init.cpp

@@ -344,6 +344,11 @@ int GenerateSystemdUnits(int Argc, char** Argv)
             File.reset();
         }
 
+        if (automountRoot.empty() || automountRoot.back() != '/')
+        {
+            automountRoot += '/';
+        }
+
         // Mask systemd-networkd-wait-online.service since WSL always ensures that networking is configured during boot.
         // That unit can cause systemd boot timeouts since WSL's network interface is unmanaged by systemd.
         THROW_LAST_ERROR_IF(symlink("/dev/null", std::format("{}/systemd-networkd-wait-online.service", installPath).c_str()) < 0);
@@ -355,6 +360,24 @@ int GenerateSystemdUnits(int Argc, char** Argv)
         // When multiple distros are running, the second distro's getty fails because the tty is already held.
         THROW_LAST_ERROR_IF(symlink("/dev/null", std::format("{}/console-getty.service", installPath).c_str()) < 0);
 
+        const auto sharedMountPath = automountRoot + "wsl";
+        const auto mountGuardUnitContent = std::format(
+            R"(# Note: This file is generated by WSL to prevent shutdown unmounts from propagating to other distributions.
+
+[Unit]
+Description=WSL Cross-Distribution Mount Guard
+After=local-fs.target remote-fs.target
+ConditionPathIsMountPoint={}
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/bin/true
+ExecStop=-/bin/mount --make-rslave {})",
+            sharedMountPath,
+            sharedMountPath);
+        InstallSystemdUnit(installPath, "wsl-mnt-guard", mountGuardUnitContent.c_str());
+
         // Only create the wslg unit if both enabled in wsl.conf, and if the wslg folder actually exists.
         if (enableGuiApps && access("/mnt/wslg/runtime-dir", F_OK) == 0)
         {

test/windows/UnitTests.cpp

@@ -516,6 +516,49 @@ class UnitTests
         }
     }
 
+    WSL2_TEST_METHOD(SharedMountSurvivesDistroTermination)
+    {
+        constexpr auto peerDistroName = L"mount-guard-peer-test";
+
+        auto validate = [&](const std::string& automountRoot) {
+            const auto extraConfig = automountRoot.empty() ? "" : std::format("[automount]\nroot={}\n", automountRoot);
+            const auto effectiveAutomountRoot = automountRoot.empty() ? "/mnt" : automountRoot;
+            const auto mountPoint = std::format(L"{}/wsl/mount-guard-test", wsl::shared::string::MultiByteToWide(effectiveAutomountRoot));
+
+            auto cleanupVm = wil::scope_exit_log(WI_DIAGNOSTICS_INFO, []() { WslShutdown(); });
+            auto cleanupSystemd = EnableSystemd(extraConfig);
+
+            LxsstuLaunchWsl(std::format(L"--unregister {}", peerDistroName));
+            VERIFY_ARE_EQUAL(LxsstuLaunchWsl(std::format(L"--import {} . \"{}\" --version 2", peerDistroName, g_testDistroPath)), 0L);
+            auto cleanupPeer = wil::scope_exit_log(
+                WI_DIAGNOSTICS_INFO, [&]() { LxsstuLaunchWsl(std::format(L"--unregister {}", peerDistroName)); });
+
+            auto cleanupPeerSystemd = EnableSystemd(extraConfig, peerDistroName);
+
+            VERIFY_ARE_EQUAL(
+                LxsstuLaunchWsl(std::format(L"-d {} -- sh -c \"systemctl is-system-running | grep -Eq 'running|degraded'\"", peerDistroName)), 0L);
+
+            VERIFY_ARE_EQUAL(
+                LxsstuLaunchWsl(std::format(
+                    L"sh -c 'mkdir -p {0} && mount -t tmpfs -o size=4M mount-guard-test {0} && echo survived > {0}/marker'", mountPoint)),
+                0L);
+            auto cleanupMount = wil::scope_exit_log(WI_DIAGNOSTICS_INFO, [&]() {
+                LxsstuLaunchWsl(std::format(L"sh -c 'umount {0} 2>/dev/null || true; rmdir {0} 2>/dev/null || true'", mountPoint));
+            });
+
+            VERIFY_ARE_EQUAL(LxsstuLaunchWsl(std::format(L"-d {} -- findmnt -n {}", peerDistroName, mountPoint)), 0L);
+            VERIFY_ARE_EQUAL(LxsstuLaunchWsl(std::format(L"-d {} -- grep -qx survived {}/marker", peerDistroName, mountPoint)), 0L);
+
+            TerminateDistribution(peerDistroName);
+
+            VERIFY_ARE_EQUAL(LxsstuLaunchWsl(std::format(L"findmnt -n {}", mountPoint)), 0L);
+            VERIFY_ARE_EQUAL(LxsstuLaunchWsl(std::format(L"grep -qx survived {}/marker", mountPoint)), 0L);
+        };
+
+        validate("");
+        validate("/wsl-test-mount");
+    }
+
     WSL2_TEST_METHOD(ConfigUpdateLanguage)
     {
         // Validates that init populates $LANG from the distro locale configuration file.