[bitbake-devel] [PATCH] daemonize: Flush stdio on exit

[Rasmus Villemoes]

On 2018-08-21 16:04, Joshua Watt wrote:
> In spite of a comment suggesting otherwise, os._exit() does not flush
> buffered output from file descriptors before exiting the process like
> os.exit() does. This means that any un-flushed output is lost in the
> daemon process, in particular the traceback from any thrown exceptions,
> making debugging exceptions in the daemon quite difficult.
> 
> The solution is to flush stdout and stderr before exiting.
> 
> Signed-off-by: Joshua Watt <JPEWhacker at gmail.com>
> ---
>  bitbake/lib/bb/daemonize.py | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/bitbake/lib/bb/daemonize.py b/bitbake/lib/bb/daemonize.py
> index 8300d1d0f0f..bf16793468e 100644
> --- a/bitbake/lib/bb/daemonize.py
> +++ b/bitbake/lib/bb/daemonize.py
> @@ -49,8 +49,8 @@ def createDaemon(function, logfile):
>              # exit() or _exit()?
>              # _exit is like exit(), but it doesn't call any functions registered
>              # with atexit (and on_exit) or any registered signal handlers.  It also
> -            # closes any open file descriptors.  Using exit() may cause all stdio
> -            # streams to be flushed twice and any temporary files may be unexpectedly
> +            # closes any open file descriptors, but doesn't flush any buffered output.
> +            # Using exit() may cause all any temporary files to be unexpectedly
>              # removed.  It's therefore recommended that child branches of a fork()
>              # and the parent branch(es) of a daemon use _exit().
>              os._exit(0)
> @@ -79,4 +79,9 @@ def createDaemon(function, logfile):
>          traceback.print_exc()
>      finally:
>          bb.event.print_ui_queue()
> +        # os._exit() doesn't flush open files like os.exit() does. Manually flush
> +        # stdout and stderr so that any logging output will be seen, particularly
> +        # exception tracebacks.
> +        sys.stdout.flush()
> +        sys.stderr.flush()
>          os._exit(0)
> 

Well, that's of course better than not getting the output, but there's
then the issue of double printing: The intermediate child does _exit(),
but the grandparent may have had stuff in the stdio buffers before the
very first fork(), and that content would be inherited down to the
grandchild. So I think you should (also) explicitly flush stdout and
stderr before the first fork().


-- 
Rasmus Villemoes
Software Developer
Prevas A/S
Hedeager 3
DK-8200 Aarhus N
+45 51210274
rasmus.villemoes at prevas.dk
www.prevas.dk