fs-util: unify code we use to check if dirent's d_name is "." or ".."

We use different idioms at different places. Let's replace this is the
one true new idiom, that is even a bit faster...

src/basic/cgroup-util.c

@@ -182,8 +182,7 @@ int cg_read_subgroup(DIR *d, char **fn) {
                 if (de->d_type != DT_DIR)
                         continue;
 
-                if (streq(de->d_name, ".") ||
-                    streq(de->d_name, ".."))
+                if (dot_or_dot_dot(de->d_name))
                         continue;
 
                 b = strdup(de->d_name);

src/basic/copy.c

@@ -331,7 +331,7 @@ static int fd_copy_directory(
                 struct stat buf;
                 int q;
 
-                if (STR_IN_SET(de->d_name, ".", ".."))
+                if (dot_or_dot_dot(de->d_name))
                         continue;
 
                 if (fstatat(dirfd(d), de->d_name, &buf, AT_SYMLINK_NOFOLLOW) < 0) {

src/basic/path-util.c

@@ -699,10 +699,7 @@ bool filename_is_valid(const char *p) {
         if (isempty(p))
                 return false;
 
-        if (streq(p, "."))
-                return false;
-
-        if (streq(p, ".."))
+        if (dot_or_dot_dot(p))
                 return false;
 
         e = strchrnul(p, '/');
@@ -720,14 +717,17 @@ bool path_is_safe(const char *p) {
         if (isempty(p))
                 return false;
 
-        if (streq(p, "..") || startswith(p, "../") || endswith(p, "/..") || strstr(p, "/../"))
+        if (dot_or_dot_dot(p))
+                return false;
+
+        if (startswith(p, "../") || endswith(p, "/..") || strstr(p, "/../"))
                 return false;
 
         if (strlen(p)+1 > PATH_MAX)
                 return false;
 
         /* The following two checks are not really dangerous, but hey, they still are confusing */
-        if (streq(p, ".") || startswith(p, "./") || endswith(p, "/.") || strstr(p, "/./"))
+        if (startswith(p, "./") || endswith(p, "/.") || strstr(p, "/./"))
                 return false;
 
         if (strstr(p, "//"))
@@ -892,3 +892,16 @@ int systemd_installation_has_version(const char *root, unsigned minimal_version)
 
         return false;
 }
+
+bool dot_or_dot_dot(const char *path) {
+        if (!path)
+                return false;
+        if (path[0] != '.')
+                return false;
+        if (path[1] == 0)
+                return true;
+        if (path[1] != '.')
+                return false;
+
+        return path[2] == 0;
+}

src/basic/path-util.h

@@ -141,3 +141,5 @@ bool is_device_path(const char *path);
 bool is_deviceallow_pattern(const char *path);
 
 int systemd_installation_has_version(const char *root, unsigned minimal_version);
+
+bool dot_or_dot_dot(const char *path);

src/basic/rm-rf.c

@@ -83,7 +83,7 @@ int rm_rf_children(int fd, RemoveFlags flags, struct stat *root_dev) {
                 bool is_dir;
                 struct stat st;
 
-                if (streq(de->d_name, ".") || streq(de->d_name, ".."))
+                if (dot_or_dot_dot(de->d_name))
                         continue;
 
                 if (de->d_type == DT_UNKNOWN ||

src/basic/socket-util.c

@@ -877,7 +877,7 @@ bool ifname_valid(const char *p) {
         if (strlen(p) >= IFNAMSIZ)
                 return false;
 
-        if (STR_IN_SET(p, ".", ".."))
+        if (dot_or_dot_dot(p))
                 return false;
 
         while (*p) {