[libvirt PATCH] tests: don't mix FILE* and UNIX FD I/O on same stream

[Eric Blake]

On 9/21/20 12:36 PM, Daniel P. Berrangé wrote:
> There is currently a hand in test27 that exhibits itself on FreeBSD 11.4

s/hand/hang/

> only. The behaviour is that virCommandProcessIO gets POLLIN on the
> FD for stdout, but read() blocks. Meanwhile commandtest also blocks
> in write for stderr because the pipe buffers are full.
> 
> This fix in commandhelper likely does not really address the root cause
> just hides it due to the buffering done by FILE *. Mixing UNIX FD I/O
> and FILE * I/O is bad practice regardles.

regardless

POSIX has rules for when it is safe (it has a notion of an active 
handle, and what must be done to a FILE* that is currently the active 
handle before doing further I/O via an fd that wants to become the 
active handle 
https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_05). 
  But you're right that not mixing is the easiest approach.

> 
> Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
> ---
>   tests/commandhelper.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)

With typos fixed,
Reviewed-by: Eric Blake <eblake at redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org