Re: [Qemu-devel] [PATCH v4 21/29] util: use fcntl() for qemu_write_pidfile() locking

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Jul 13, 2018 at 03:09:08PM +0200, Marc-André Lureau wrote:
> According to Daniel Berrange, fcntl() locks have better portable
> semantics than lockf().

Specifically I was referring to this from 'man lockf':

   On Linux, lockf() is just an interface  on  top  of  fcntl(2)  locking.
   Many other systems implement lockf() in this way, but note that POSIX.1
   leaves the relationship between lockf() and fcntl(2) locks unspecified.
   A  portable  application  should  probably  avoid mixing calls to these
   interfaces.

IOW, if its just a shim around fcntl() on many systems, it is clearer
if we just use fcntl() directly, as we then know how fcntl() locks will
behave if they're on a network filesystem like NFS.

> Use an exclusive lock on the first byte with fcntl().
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  util/oslib-posix.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index da1d4a3201..26b11490b9 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -92,6 +92,11 @@ bool qemu_write_pidfile(const char *pidfile, Error **errp)
>  {
>      int pidfd;
>      char pidstr[32];
> +    struct flock lock = {
> +        .l_type = F_WRLCK,
> +        .l_whence = SEEK_SET,
> +        .l_len = 1,
> +    };

For the same semantics as lockf we should use  len == 0 (ie infinity)


Regards,
Daniel
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