Re: [boost] [log][regression] Error with Boost.Lob build jamfile breaks MinGW regression tester in develop

Subject: Re: [boost] [log][regression] Error with Boost.Lob build jamfile breaks MinGW regression tester in develop
From: Vladimir Prus (vladimir.prus_at_[hidden])
Date: 2016-07-12 15:38:19

On 09-Jul-16 7:21 PM, Andrey Semashev wrote:
> There are multiple message compilers on Windows. MinGW ships windmc,
> IIRC. It can also work with mc from MSVC if one is found. So disabling
> it based on the C++ compiler is not quite correct.
>
> The whole point of this <conditional> stuff is to detect if there's any
> way to build .mc files, and I would really prefer if Boost.Build handled
> that. I think, unless I'm doing something terribly wrong, Boost.Build
> should be fixed.

Although Boost.Build can possible tell whether .mc files can be handled 
- without writing a full-blown configuration test - I think the 
immediately faulty code is check-message-compiler. What is has is:

    if ! [ has-config-flag BOOST_LOG_WITHOUT_EVENT_LOG : $(properties) ]
    {
        local has_mc = ...
        if ! $(has_mc)
        {
            result += <define>BOOST_LOG_WITHOUT_EVENT_LOG ;

The algorithm Boost.Build uses to determine final properties for a 
metatarget is iterative - all conditionals and indirect conditionals
are applied until we reach a fixed point.

Say, the function above determines there's no mc, and sets 
<define>BOOST_LOG_WITHOUT_EVENT_LOG. On the next iteration of the loop,
the function is called again and has-config-flag does this:

        if ( <define>$(flag) in $(properties) ...
        {
             return 1 ;
         }

and sees <define>BOOST_LOG_WITHOUT_EVENT_LOG, therefore 
check-message-compiler returns nothing, and on next iteration it is 
called without
the define, and add it again. After a few iteration, Boost.Build decides 
that the process does not converge, and gives up.

I would recommend that check-message-compiler be modified to ignore 
current defines, and always return the same value.

In the meantime, I'll see whether a more convenient way of implemeting
'has_mc' above can be added - but check-message-compiler still will
have to be fixed.

Thanks,
Volodya