libzutil: add zfs_{base,dir}name()

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12105

include/libzutil.h

@@ -160,6 +160,9 @@ _LIBZUTIL_H void color_start(char *color);
 _LIBZUTIL_H void color_end(void);
 _LIBZUTIL_H int printf_color(char *color, char *format, ...);
 
+_LIBZUTIL_H const char *zfs_basename(const char *path);
+_LIBZUTIL_H ssize_t zfs_dirnamelen(const char *path);
+
 #ifdef	__cplusplus
 }
 #endif

lib/libzutil/zutil_device_path.c

@@ -31,6 +31,22 @@
 
 #include <libzutil.h>
 
+/* Substring from after the last slash, or the string itself if none */
+const char *
+zfs_basename(const char *path)
+{
+	const char *bn = strrchr(path, '/');
+	return (bn ? bn + 1 : path);
+}
+
+/* Return index of last slash or -1 if none */
+ssize_t
+zfs_dirnamelen(const char *path)
+{
+	const char *end = strrchr(path, '/');
+	return (end ? end - path : -1);
+}
+
 /*
  * Given a shorthand device name check if a file by that name exists in any
  * of the 'zpool_default_import_path' or ZPOOL_IMPORT_PATH directories.  If