Comment 0 for bug 1902109

Rsync in Ubuntu 20.10 fails when /proc isn't mounted, while it worked before.
This happens because AC_CHECK_FUNC(lchmod) returns "yes" in 20.10, while it returned "no" before.

Steps to reproduce:

# Emulate /proc not being mounted
$ mount --bind / /mnt
$ chroot /mnt rsync -a /bin/ls .
rsync: [receiver] failed to set permissions on "/.ls.CDExhu": Operation not supported (95)
rsync error: some files/attrs were not transferred (see previous errors) (code 3) at main.c(1330) [sender=3.2.3]

I reported this issue upstream in https://github.com/WayneD/rsync/issues/109 but the rsync developer says it's a problem in libc, and it might well be.

Simple C code to reproduce the problem without rsync:

printf("lchmod returned: %d\n", lchmod("/tmp/ls", 0755));

If /tmp/ls is e.g. mode=0123, and needs to be changed, lchmod fails when /proc isn't mounted, yet it succeeds if it is mounted.

Python had a similar issue, and they ended up avoiding AC_CHECK_FUNC(lchmod) under Linux:

https://bugs.python.org/issue34652
https://github.com/python/cpython/commit/69e96910153219b0b15a18323b917bd74336d229#diff-49473dca262eeab3b4a43002adb08b4db31020d190caaad1594b47f1d5daa810R3140

```c
if test "$MACHDEP" != linux; then
  AC_CHECK_FUNC(lchmod)
fi
```

So I'm not sure which package is causing the bug here. Should autoconf return false? Should libc implement lchown without the bug? Or should rsync skip lchmod under Linux, like python did?