rm: improve efficiency of rm -r (without -f) from O(N^2) to O(N)

where N is the depth of the deepest hierarchy rm is processing.
* src/remove.c (write_protected_non_symlink): Use faccessat to
avoid O(N)-per-entry cost of calling euidaccess.
* m4/jm-macros.m4 (coreutils_MACROS): Check for faccessat.
* NEWS (Improvements): Mention it.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/remove.c b/src/remove.c
index 32f67a181..2db385909 100644
--- a/src/remove.c
+++ b/src/remove.c
@@ -172,6 +172,13 @@ write_protected_non_symlink (int fd_cwd,
         mess up with long file names). */
 
   {
+    /* Use faccessat if possible, so as to avoid the expense
+       of processing an N-component name.  */
+#if HAVE_FACCESSAT && AT_EACCESS
+    if (faccessat (fd_cwd, file, W_OK, AT_EACCESS) == 0)
+      return 0;
+#endif
+
     /* This implements #5: */
     size_t file_name_len = strlen (full_name);