[boost] [build] No output until an action completes?

Subject: [boost] [build] No output until an action completes?
From: Tom Westerhout (kot.tom97_at_[hidden])
Date: 2017-07-06 13:40:36

Greetings!

I've run into a problem that a Boost.Build action produces no output until
the command returns. For example, in my Jamfile I have

rule do-measure ( test-name : sources * : requirements * )
{
    ... some preparation work ...
}

actions do-measure
{
    $(COMMAND) $(ARGS)
}

notfile ha : @do-measure : ... : ... ;

COMMAND is a Python script that runs for a long time and produces some
debug information. But b2 does not print this output until COMMAND
returns. This is really unfortunate because Travis requires a command to
produce output at least once per 10 mins or something. Otherwise it thinks
the command has stalled and terminates it.

Is there some configuration parameter or a trick to make b2 print the
debug info COMMAND produces? or maybe some dummy output so that Travis
allows it to run longer than 10 mins?

Any help is highly appreciated!

Tom