chase: CHASE_MKDIR_0755 requires CHASE_NONEXISTENT and/or CHASE_PARENT

When CHASE_MKDIR_0755 is specified without CHASE_NONEXISTENT and
CHASE_PARENT, then chase() succeeds only when the file specified by
the path already exists, and in that case, chase() does not create
any parent directories, and CHASE_MKDIR_0755 is meaningless.

Let's mention that CHASE_MKDIR_0755 needs to be specified with
CHASE_NONEXISTENT or CHASE_PARENT, and adds a assertion about that.

src/basic/chase.c

@@ -85,6 +85,7 @@ int chaseat(int dir_fd, const char *path, ChaseFlags flags, char **ret_path, int
         assert(!FLAGS_SET(flags, CHASE_PREFIX_ROOT));
         assert(!FLAGS_SET(flags, CHASE_STEP|CHASE_EXTRACT_FILENAME));
         assert(!FLAGS_SET(flags, CHASE_TRAIL_SLASH|CHASE_EXTRACT_FILENAME));
+        assert(!FLAGS_SET(flags, CHASE_MKDIR_0755) || (flags & (CHASE_NONEXISTENT | CHASE_PARENT)) != 0);
         assert(dir_fd >= 0 || dir_fd == AT_FDCWD);
 
         /* Either the file may be missing, or we return an fd to the final object, but both make no sense */

src/basic/chase.h

@@ -24,8 +24,14 @@ typedef enum ChaseFlags {
                                              * full path is still stored in ret_path and only the returned
                                              * file descriptor will point to the parent directory. Note that
                                              * the result path is the root or '.', then the file descriptor
-                                             * also points to the result path even if this flag is set. */
-        CHASE_MKDIR_0755         = 1 << 11, /* Create any missing parent directories in the given path. */
+                                             * also points to the result path even if this flag is set.
+                                             * When this specified, chase() will succeed with 1 even if the
+                                             * file points to the last path component does not exist. */
+        CHASE_MKDIR_0755         = 1 << 11, /* Create any missing parent directories in the given path. This
+                                             * needs to be set with CHASE_NONEXISTENT and/or CHASE_PARENT.
+                                             * Note, chase_and_open() or friends always add CHASE_PARENT flag
+                                             * when internally call chase(), hence CHASE_MKDIR_0755 can be
+                                             * safely set without CHASE_NONEXISTENT and CHASE_PARENT. */
         CHASE_EXTRACT_FILENAME   = 1 << 12, /* Only return the last component of the resolved path */
 } ChaseFlags;