[libcamera-devel] [PATCH] libcamera: log: Document LOG() restrictions

[Kieran Bingham]

Hi Laurent,

On 21/05/2021 10:22, Laurent Pinchart wrote:
> The LOG() macro uses the global Logger and LogCategory objects
> internally. This can result in crashes or other undefined behaviour when
> logging from destructors of global objects, as the Logger and/or the
> LogCategory instances may have been destroyed.
> 
> This isn't ideal and should eventually be fixed. For the time being,
> document the restriction, and add a todo list item to fix the problem.

Is there a way to catch this in code? Presumably it's something to do
with the global logging object being destructed, and then an action
occuring on that object?

Could we have something like the canary I proposed on the requests, so
that if any action on the log happens after it's destructed it fires a
more meaningful assert rather than a less parsable segfault/random failure?


> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> ---
>  src/libcamera/log.cpp | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/src/libcamera/log.cpp b/src/libcamera/log.cpp
> index 94175ab34535..985952249289 100644
> --- a/src/libcamera/log.cpp
> +++ b/src/libcamera/log.cpp
> @@ -980,6 +980,12 @@ LogMessage _log(const LogCategory *category, LogSeverity severity,
>   *
>   * If the severity is set to Fatal, execution is aborted and the program
>   * terminates immediately after printing the message.
> + *
> + * \warning Logging from the destructor of a global object, either directly or
> + * indirectly, results in undefined behaviour.
> + *
> + * \todo Allow logging from destructors of global objects to the largest
> + * possible extent

I guess there's no way to say "Please delete me last"?

Are there any tricks that can be played by trying to ensure the log is
the first thing constructed?

Anyway, documenting the current limitation is a good thing

Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

>   */
>  
>  /**
> 

-- 
Regards
--
Kieran