core: do not create symlink to private directory if parent already exists

The very basic functinality of StateDirectory= or friends is creating
specified directories. That should work if one entry is a subdirectory
of another. However, it does not when combined with DynamicUser=yes.

To support such case, this adds ExecDirectoryItem.only_create flag, and
if it is set PID1 only create private directory, and not create the symlink
to the private directory.

Fixes #24783.

src/core/execute.h

@@ -135,6 +135,7 @@ typedef enum ExecDirectoryType {
 typedef struct ExecDirectoryItem {
         char *path;
         char **symlinks;
+        bool only_create;
 } ExecDirectoryItem;
 
 typedef struct ExecDirectory {
@@ -493,6 +494,7 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(ExecLoadCredential*, exec_load_credential_free);
 
 void exec_directory_done(ExecDirectory *d);
 int exec_directory_add(ExecDirectory *d, const char *path, const char *symlink);
+void exec_directory_sort(ExecDirectory *d);
 
 extern const struct hash_ops exec_set_credential_hash_ops;
 extern const struct hash_ops exec_load_credential_hash_ops;