[libcamera-devel] [PATCH] libcamera: log: Destroy LogCategory instances in a controlled way

[Kieran Bingham]

Hi Laurent,

On 23/05/2021 01:04, Laurent Pinchart wrote:
> The LogCategory instances are constructed on first use as static
> variables in accessor functions, following the Meyers singleton pattern.
> As a result, their destruction order is not guaranteed. This can cause
> issues as the global Logger object, constructed in a similar fashion, is
> accessed from the LogCategory destructor and may be destroyed first.
> 
> To fix this, keep the same singleton pattern, but allocate the
> LogCategory instances dynamically. As they get registered with the
> global Logger instance, we can destroy them in the Logger destructor.
> 
> This only avoids destruction order issues between LogCategory and
> Logger, and doesn't address yet the fact that LOG() calls from
> destructors of global objects may access an already destroyed Logger.

Are log categories now the only dynamic thing that could be referenced
after the destructor is called?

This may be well into the direction of hack-on-hack - but as the
Logger->instance() is static - we know it has a defined region of memory
- so even after the destructor is called - it is still 'there' (It can't
be freed).

So - when the destructor is called - the LogCategories are now removed -
but perhaps that's ok - at that point, we can disable all filtering -
and any log message that occurs after 'destructor' could be written to
std-out - perhaps with some 'emergency' / 'destruct' constant category.


Anyway, that's all musings on how to consider the next issues.

If this helps prevent crashes, then it's worthwhile on it's own.

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

Although - now thinking further - Is the crash that can occur because a
Log is reported with a LogCategory that has been unregistered?

And if we can detect that, can we apply the same logic and report the
log output as an always print, 'Emergency' or 'Shutdown' category?

In otherwords, if we can detect the parts we can't access - we should
still be able to print the log message without crashing ...


> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> ---
>  include/libcamera/internal/log.h |  5 ++---
>  src/libcamera/log.cpp            | 32 +++++++++++---------------------
>  2 files changed, 13 insertions(+), 24 deletions(-)
> 
> diff --git a/include/libcamera/internal/log.h b/include/libcamera/internal/log.h
> index b66bf55bc57d..1032bdd93e7c 100644
> --- a/include/libcamera/internal/log.h
> +++ b/include/libcamera/internal/log.h
> @@ -29,7 +29,6 @@ class LogCategory
>  {
>  public:
>  	explicit LogCategory(const char *name);
> -	~LogCategory();
>  
>  	const char *name() const { return name_; }
>  	LogSeverity severity() const { return severity_; }
> @@ -48,8 +47,8 @@ extern const LogCategory &_LOG_CATEGORY(name)();
>  #define LOG_DEFINE_CATEGORY(name)					\
>  const LogCategory &_LOG_CATEGORY(name)()				\
>  {									\
> -	static LogCategory category(#name);				\
> -	return category;						\
> +	static LogCategory *category = new LogCategory(#name);		\
> +	return *category;						\
>  }
>  
>  class LogMessage
> diff --git a/src/libcamera/log.cpp b/src/libcamera/log.cpp
> index dd991647b9bc..74829a56916e 100644
> --- a/src/libcamera/log.cpp
> +++ b/src/libcamera/log.cpp
> @@ -248,6 +248,8 @@ void LogOutput::writeStream(const std::string &str)
>  class Logger
>  {
>  public:
> +	~Logger();
> +
>  	static Logger *instance();
>  
>  	void write(const LogMessage &msg);
> @@ -267,7 +269,6 @@ private:
>  
>  	friend LogCategory;
>  	void registerCategory(LogCategory *category);
> -	void unregisterCategory(LogCategory *category);
>  
>  	std::unordered_set<LogCategory *> categories_;
>  	std::list<std::pair<std::string, LogSeverity>> levels_;
> @@ -369,6 +370,12 @@ void logSetLevel(const char *category, const char *level)
>  	Logger::instance()->logSetLevel(category, level);
>  }
>  
> +Logger::~Logger()
> +{
> +	for (LogCategory *category : categories_)
> +		delete category;
> +}
> +
>  /**
>   * \brief Retrieve the logger instance
>   *
> @@ -665,18 +672,6 @@ void Logger::registerCategory(LogCategory *category)
>  	}
>  }
>  
> -/**
> - * \brief Unregister a log category from the logger
> - * \param[in] category The log category
> - *
> - * If the \a category hasn't been registered with the logger this function
> - * performs no operation.
> - */
> -void Logger::unregisterCategory(LogCategory *category)
> -{
> -	categories_.erase(category);
> -}
> -
>  /**
>   * \enum LogSeverity
>   * Log message severity
> @@ -711,11 +706,6 @@ LogCategory::LogCategory(const char *name)
>  	Logger::instance()->registerCategory(this);
>  }
>  
> -LogCategory::~LogCategory()
> -{
> -	Logger::instance()->unregisterCategory(this);
> -}
> -
>  /**
>   * \fn LogCategory::name()
>   * \brief Retrieve the log category name
> @@ -746,12 +736,12 @@ void LogCategory::setSeverity(LogSeverity severity)
>   * The default log category is named "default" and is used by the LOG() macro
>   * when no log category is specified.
>   *
> - * \return A pointer to the default log category
> + * \return A reference to the default log category
>   */
>  const LogCategory &LogCategory::defaultCategory()
>  {
> -	static const LogCategory category("default");
> -	return category;
> +	static const LogCategory *category = new LogCategory("default");
> +	return *category;
>  }
>  
>  /**
> 

-- 
Regards
--
Kieran