Re: boost.log's text_multifile_backend is not performant on windows - perhaps it should be documented?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2021-03-04 22:58:27

Nicholas Neumann wrote:
> Thinking a little bit differently, the multifile backend could allow for a client-
> supplied callback, run after each record consume, which tells the backend
> whether to close the file or leave it open in an unbounded cache.

If you're going to use a client-supplied callback, the most straightforward one
would just return a shared_ptr<FILE> (or the appropriate equivalent). Then both
opening and closing are under the callback's control.

E.g. the simple one would just do

shared_ptr<FILE> callback( char const* fn )
{
    if( FILE* f = std::fopen( fn, "rb" ) )
        return shared_ptr<FILE>( f, std::fclose );
    else
        return shared_ptr<FILE>;
}

whereas a simple caching one would instead do

shared_ptr<FILE> callback( char const* fn )
{
    if( s_cache.count( fn ) )
    {
        return s_cache[ fn ];
    }
    else if( FILE* f = std::fopen( fn, "rb" ) )
    {
        shared_ptr<FILE> sf( f, std::fclose );
        s_cache[ fn ] = sf;
        return sf;
    }
    else
        return shared_ptr<FILE>;
}

(mutex locks protecting s_cache omitted for brevity)

and a more sophisticated cache would have a flush strategy.