Re: [Qemu-devel] [PATCH v2] os: truncate pidfile on creation

[Eric Blake <eblake@redhat.com>]

On 03/20/2018 11:58 AM, Florian Larysch wrote:
> qemu_create_pidfile does not truncate the pidfile when it creates it,
> but rather overwrites its contents with the new pid. This works fine as
> long as the length of the pid doesn't decrease, but this might happen in
> case of wraparounds, causing pidfiles to contain trailing garbage which
> breaks operations such as 'kill $(cat pidfile)'.
> 
> Instead, always truncate the file before writing it.
> 
> Note that the order is important here: We cannot simply use O_TRUNC in
> the open() call because another qemu process might truncate the pidfile
> of a process that is still running before reaching the lockf() barrier.
> 
> The Windows version suffers from a similar problem, but as it does not
> provide effective mutual exclusion anyway (because the file handle is
> closed immediately after writing to it), adopting this behavior still
> seems to be an improvement, as it at least prevents garbeled pidfiles.

s/garbeled/garbled/

> 
> Signed-off-by: Florian Larysch <fl@n621.de>
> ---
>   os-posix.c | 6 ++++++
>   os-win32.c | 2 +-
>   2 files changed, 7 insertions(+), 1 deletion(-)
> 

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
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