tmpfiles: fix up and complain paths referring to /var/run (instead of /run) automatically

tmpfiles.d relies on merging lines for the same file paths, as well as
detecting conflicts between lines for the same file paths. /var/run is a
prominent case that is an alias for /run, any many tmpfiles snippets
refer to paths in /var/run rather than /run currently, which breaks the
conflict detection and merging.

We can't really fix this comprehensively, as doing so would require us
to resolve symlinks early on, but that's precisely not what we want to
do, as tmpfiles is usually run very early on where the paths might not
be fully available yet (in particular as we might likely create them
ourselves).

Hence, let's at least detect and fix this case for the most prominent
case of this ambiguity, and also log explicitly about this, asking users
to fix the snippets in question so that the merging and conflict
detection works properly again.

src/tmpfiles/tmpfiles.c

@@ -2115,6 +2115,43 @@ static int specifier_expansion_from_arg(Item *i) {
         return 0;
 }
 
+static int patch_var_run(const char *fname, unsigned line, char **path) {
+        const char *k;
+        char *n;
+
+        assert(path);
+        assert(*path);
+
+        /* Optionally rewrites lines referencing /var/run/, to use /run/ instead. Why bother? tmpfiles merges lines in
+         * some cases and detects conflicts in others. If files/directories are specified through two equivalent lines
+         * this is problematic as neither case will be detected. Ideally we'd detect these cases by resolving symlinks
+         * early, but that's precisely not what we can do here as this code very likely is running very early on, at a
+         * time where the paths in question are not available yet, or even more importantly, our own tmpfiles rules
+         * might create the paths that are intermediary to the listed paths. We can't really cover the generic case,
+         * but the least we can do is cover the specific case of /var/run vs. /run, as /var/run is a legacy name for
+         * /run only, and we explicitly document that and require that on systemd systems the former is a symlink to
+         * the latter. Moreover files below this path are by far the primary usecase for tmpfiles.d/. */
+
+        k = path_startswith(*path, "/var/run/");
+        if (isempty(k)) /* Don't complain about other paths than /var/run, and not about /var/run itself either. */
+                return 0;
+
+        n = strjoin("/run/", k);
+        if (!n)
+                return log_oom();
+
+        /* Also log about this briefly. We do so at LOG_NOTICE level, as we fixed up the situation automatically, hence
+         * there's no immediate need for action by the user. However, in the interest of making things less confusing
+         * to the user, let's still inform the user that these snippets should really be updated. */
+
+        log_notice("[%s:%u] Line references path below legacy directory /var/run/, updating %s → %s; please update the tmpfiles.d/ drop-in file accordingly.", fname, line, *path, n);
+
+        free(*path);
+        *path = n;
+
+        return 0;
+}
+
 static int parse_line(const char *fname, unsigned line, const char *buffer, bool *invalid_config) {
 
         _cleanup_free_ char *action = NULL, *mode = NULL, *user = NULL, *group = NULL, *age = NULL, *path = NULL;
@@ -2193,6 +2230,10 @@ static int parse_line(const char *fname, unsigned line, const char *buffer, bool
                 return log_error_errno(r, "[%s:%u] Failed to replace specifiers: %s", fname, line, path);
         }
 
+        r = patch_var_run(fname, line, &i.path);
+        if (r < 0)
+                return r;
+
         switch (i.type) {
 
         case CREATE_DIRECTORY: