bug#70586: cp walks dir differently than rm and is hitting "File name too long" where this could be avoided

Paul Eggert <[email protected]> writes:

>> is there a reason why savewd and
>> mkancesdirs don't use openat + mkdirat instead of using chdir in a child
>> process? Do these functions just predate the *at interfaces
>
> Yes, precisely. Some day it will be time to give up on those old
> platforms, I suppose. We haven't done that yet because there hasn't
> been motivation to get rid of them, or rewrite them.

I see. With this trivial test it seems that chdir is faster than open on
GNU/Linux:

    $ cat main.c 
    #define _GNU_SOURCE 1
    #include <fcntl.h>
    #include <unistd.h>
    #include <stdlib.h>
    int
    main (void)
    {
      for (int i = 0; i < 100000; ++i)
        {
    #ifdef DO_OPEN
          int fd = open ("/", O_DIRECTORY | O_PATH);
          if (fd < 0 || close (fd) < 0)
            abort ();
    #else
          if (chdir ("/") < 0)
            abort ();
    #endif
        }
      return 0;
    }
    $ gcc main.c
    $ perf stat --repeat 10 ./a.out 2>&1 | grep -F 'seconds time'
    $ perf stat --repeat 10 ./a.out 2>&1 | grep -F 'seconds time'
           0.08036 +- 0.00194 seconds time elapsed  ( +-  2.41% )
    $ gcc -DDO_OPEN main.c
    $ perf stat --repeat 10 ./a.out 2>&1 | grep -F 'seconds time'
           0.13929 +- 0.00102 seconds time elapsed  ( +-  0.73% )

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