homed: keep "pinning" fd open while home dir active

The pin fd keeps the mount busy, ensuring that unmount requests need to
go through us.

Note that this doesn't change too much IRL, since a logged in user
generally has processes keeping the home dir busy anyway. However, in
some corner cases it is safer to protect from accidental unmounts this
way. (e.g. if user manually called "homectl activate" first).

src/home/homed-home.c

@@ -130,6 +130,7 @@ int home_new(Manager *m, UserRecord *hr, const char *sysfs, Home **ret) {
                 .worker_stdout_fd = -1,
                 .sysfs = TAKE_PTR(ns),
                 .signed_locally = -1,
+                .pin_fd = -1,
         };
 
         r = hashmap_put(m->homes_by_name, home->user_name, home);
@@ -203,6 +204,8 @@ Home *home_free(Home *h) {
 
         h->current_operation = operation_unref(h->current_operation);
 
+        safe_close(h->pin_fd);
+
         return mfree(h);
 }
 
@@ -317,6 +320,48 @@ int home_unlink_record(Home *h) {
         return 0;
 }
 
+static void home_unpin(Home *h) {
+        assert(h);
+
+        if (h->pin_fd < 0)
+                return;
+
+        h->pin_fd = safe_close(h->pin_fd);
+        log_debug("Successfully closed pin fd on home for %s.", h->user_name);
+}
+
+static void home_pin(Home *h) {
+        const char *path;
+
+        assert(h);
+
+        if (h->pin_fd >= 0) /* Already pinned? */
+                return;
+
+        path = user_record_home_directory(h->record);
+        if (!path) {
+                log_warning("No home directory path to pin for %s, ignoring.", h->user_name);
+                return;
+        }
+
+        h->pin_fd = open(path, O_RDONLY|O_DIRECTORY|O_CLOEXEC);
+        if (h->pin_fd < 0) {
+                log_warning_errno(errno, "Couldn't open home directory '%s' for pinning, ignoring: %m", path);
+                return;
+        }
+
+        log_debug("Successfully pinned home directory '%s'.", path);
+}
+
+static void home_update_pin_fd(Home *h, HomeState state) {
+        assert(h);
+
+        if (state < 0)
+                state = home_get_state(h);
+
+        return HOME_STATE_IS_ACTIVE(state) ? home_pin(h) : home_unpin(h);
+}
+
 static void home_set_state(Home *h, HomeState state) {
         HomeState old_state, new_state;
 
@@ -331,6 +376,8 @@ static void home_set_state(Home *h, HomeState state) {
                  home_state_to_string(old_state),
                  home_state_to_string(new_state));
 
+        home_update_pin_fd(h, new_state);
+
         if (HOME_STATE_IS_EXECUTING_OPERATION(old_state) && !HOME_STATE_IS_EXECUTING_OPERATION(new_state)) {
                 /* If we just finished executing some operation, process the queue of pending operations. And
                  * enqueue it for GC too. */
@@ -1253,9 +1300,14 @@ static int home_deactivate_internal(Home *h, bool force, sd_bus_error *error) {
 
         assert(h);
 
+        home_unpin(h); /* unpin so that we can deactivate */
+
         r = home_start_work(h, force ? "deactivate-force" : "deactivate", h->record, NULL);
-        if (r < 0)
+        if (r < 0) {
+                /* Operation failed before it even started, reacquire pin fd, if state still dictates so */
+                home_update_pin_fd(h, _HOME_STATE_INVALID);
                 return r;
+        }
 
         home_set_state(h, HOME_DEACTIVATING);
         return 0;

src/home/homed-home.h

@@ -126,6 +126,9 @@ struct Home {
 
         /* Used to coalesce bus PropertiesChanged events */
         sd_event_source *deferred_change_event_source;
+
+        /* An fd to the top-level home directory we keep while logged in, to keep the dir busy */
+        int pin_fd;
 };
 
 int home_new(Manager *m, UserRecord *hr, const char *sysfs, Home **ret);