strv: add helper to extend strv from both sides

Also, use the more correct type of 'const char* const*' for the input strv.
This requires adding the cast in a few places, but also allows to remove some
casts in others.

src/basic/conf-files.c

@@ -369,7 +369,7 @@ int conf_files_list_dropins(
         assert(dirs);
 
         suffix = strjoina("/", dropin_dirname);
-        r = strv_extend_strv_concat(&dropin_dirs, (char**) dirs, suffix);
+        r = strv_extend_strv_concat(&dropin_dirs, dirs, suffix);
         if (r < 0)
                 return r;
 

src/basic/path-lookup.c

@@ -214,7 +214,7 @@ static char** user_dirs(
                             persistent_config) < 0)
                 return NULL;
 
-        if (strv_extend_strv_concat(&res, config_dirs, "/systemd/user") < 0)
+        if (strv_extend_strv_concat(&res, (const char* const*) config_dirs, "/systemd/user") < 0)
                 return NULL;
 
         /* global config has lower priority than the user config of the same type */
@@ -232,7 +232,7 @@ static char** user_dirs(
                             data_home) < 0)
                 return NULL;
 
-        if (strv_extend_strv_concat(&res, data_dirs, "/systemd/user") < 0)
+        if (strv_extend_strv_concat(&res, (const char* const*) data_dirs, "/systemd/user") < 0)
                 return NULL;
 
         if (strv_extend_strv(&res, (char**) user_data_unit_paths, false) < 0)

src/basic/strv.c

@@ -242,13 +242,13 @@ rollback:
         return -ENOMEM;
 }
 
-int strv_extend_strv_concat(char ***a, char * const *b, const char *suffix) {
+int strv_extend_strv_biconcat(char ***a, const char *prefix, const char* const *b, const char *suffix) {
         int r;
 
         STRV_FOREACH(s, b) {
                 char *v;
 
-                v = strjoin(*s, suffix);
+                v = strjoin(strempty(prefix), *s, suffix);
                 if (!v)
                         return -ENOMEM;
 

src/basic/strv.h

@@ -43,7 +43,10 @@ int strv_copy_unless_empty(char * const *l, char ***ret);
 size_t strv_length(char * const *l) _pure_;
 
 int strv_extend_strv(char ***a, char * const *b, bool filter_duplicates);
-int strv_extend_strv_concat(char ***a, char * const *b, const char *suffix);
+int strv_extend_strv_biconcat(char ***a, const char *prefix, const char* const *b, const char *suffix);
+static inline int strv_extend_strv_concat(char ***a, const char* const *b, const char *suffix) {
+        return strv_extend_strv_biconcat(a, NULL, b, suffix);
+}
 int strv_prepend(char ***l, const char *value);
 
 /* _with_size() are lower-level functions where the size can be provided externally,