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");
}
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