string-util: introduce STRING_FILENAME_PART flag for string_is_safe()

Whenever we are validating a string that shall appear in a filename
eventually we want to use filename_part_is_valid() rather than file
filename_is_valid(). Let's add explicit support for that to
string_is_safe(), since it's actually a really common case.

src/basic/string-util.c

@@ -1139,6 +1139,9 @@ bool string_is_safe(const char *p, StringSafeFlags flags) {
         if (FLAGS_SET(flags, STRING_FILENAME) && !filename_is_valid(p))
                 return false;
 
+        if (FLAGS_SET(flags, STRING_FILENAME_PART) && !filename_part_is_valid(p))
+                return false;
+
         return true;
 }
 

src/basic/string-util.h

@@ -229,7 +229,8 @@ typedef enum StringSafeFlags {
         STRING_ALLOW_QUOTES        = 1 << 4, /* Allow quotes (" or ') */
         STRING_ALLOW_GLOBS         = 1 << 5, /* Allow globs (?, * or [) */
         STRING_FILENAME            = 1 << 6, /* Verify the string is valid as regular filename */
-        STRING_DISALLOW_WHITESPACE = 1 << 7, /* Refuse whitespace (space, tab, newline, …) */
+        STRING_FILENAME_PART       = 1 << 7, /* Verify the string is valid as part of a regular filename */
+        STRING_DISALLOW_WHITESPACE = 1 << 8, /* Refuse whitespace (space, tab, newline, …) */
 } StringSafeFlags;
 
 bool string_is_safe(const char *p, StringSafeFlags flags) _pure_;