F_SETLK blocks on nfsv4

[Kenneth Johansson <ken@kenjo.org>]

I was trying to change over to using nfsv4 from nfsv3 and hit a problem 
with VLC. after some strace digging I came up with a test program that 
has the same effect and works on nfs3 but hangs on nfsv4.

Even more interesting when run on a nfsv4 mounted home directory it is 
no longer possible to open new connections in google-chrome so the error 
is visible not only for the test program but probably all program that 
does locking after this program is started on the same mount. or 
something I have not investigated what really happens.

anyway the program is below and it hangs on last the line "ret = 
"fcntl(fd, F_SETLK, &fl);" if run on a nfsv4 mount.

server; debain 9.5 with kernel version 4.9 or 4.17.0-0.bpo.1-amd64.

client: ubuntu 18.04. kernel 4.18.0-041800rc6-generic

--------------

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/file.h>
#include <string.h>

char *lfile="lock";

int main(int argc, char *argv[])
{
         int fd;
         int ret;
         struct flock fl;

         fd = openat(AT_FDCWD, lfile, O_RDWR|O_CREAT|O_EXCL|O_CLOEXEC, 
0600);
         if (fd <0) {perror("");exit(1);}

         ret = fcntl(fd, F_SETFD, FD_CLOEXEC);
         if (ret <0) {perror("");exit(1);}

         ret = flock(fd, LOCK_EX|LOCK_NB);
         if (ret <0) {perror("");exit(1);}

         memset(&fl,0,sizeof(fl));
         fl.l_type = F_WRLCK;
         fl.l_whence=SEEK_SET;
         ret = fcntl(fd, F_SETLK, &fl);
         if (ret <0) {perror("");exit(1);}

         printf("done\n");
}