install: allow removing symlinks even for units that are gone

If a symlink is leftover, still allow cleaning it up via 'disable'. This
happens when a unit is stopped and removed, but not disabled, and a reload
has already happened. At that point, cleaning up the old symlinks becomes
impossible through the APIs, and needs to be done manually. Always allow
cleaning up symlinks, if they exist, by only erroring out if there is an
OOM.

Follow-up for f31f10a

(cherry picked from commit 5163c9b)
(cherry picked from commit c26e56d)

Author: bluca

src/shared/install.c

@@ -2232,7 +2232,9 @@ static int install_context_mark_for_removal(
                         else {
                                 log_debug_errno(r, "Unit %s not found, removing name.", i->name);
                                 r = install_changes_add(changes, n_changes, r, i->path ?: i->name, NULL);
-                                if (r < 0)
+                                /* In case there's no unit, we still want to remove any leftover symlink, even if
+                                 * the unit might have been removed already, hence treating ENOENT as non-fatal. */
+                                if (r != -ENOENT)
                                         return r;
                         }
                 } else if (r < 0) {
@@ -2830,9 +2832,13 @@ static int do_unit_file_disable(
                 r = install_info_add(&ctx, *name, NULL, lp->root_dir, /* auxiliary= */ false, &info);
                 if (r >= 0)
                         r = install_info_traverse(&ctx, lp, info, SEARCH_LOAD|SEARCH_FOLLOW_CONFIG_SYMLINKS, NULL);
-
-                if (r < 0)
-                        return install_changes_add(changes, n_changes, r, *name, NULL);
+                if (r < 0) {
+                        r = install_changes_add(changes, n_changes, r, *name, NULL);
+                        /* In case there's no unit, we still want to remove any leftover symlink, even if
+                         * the unit might have been removed already, hence treating ENOENT as non-fatal. */
+                        if (r != -ENOENT)
+                                return r;
+                }
 
                 /* If we enable multiple units, some with install info and others without,
                  * the "empty [Install] section" warning is not shown. Let's make the behavior

test/units/testsuite-26.sh

@@ -311,6 +311,12 @@ systemctl cat "$UNIT_NAME"
 systemctl help "$UNIT_NAME"
 systemctl service-watchdogs
 systemctl service-watchdogs "$(systemctl service-watchdogs)"
+# Ensure that the enablement symlinks can still be removed after the user is gone, to avoid having leftovers
+systemctl enable "$UNIT_NAME"
+systemctl stop "$UNIT_NAME"
+rm -f "/usr/lib/systemd/system/$UNIT_NAME"
+systemctl daemon-reload
+systemctl disable "$UNIT_NAME"
 
 # show/set-environment
 # Make sure PATH is set