Hi Steven,

my next question concerns bjam's execution of functions. The usual data flow consists of actions to be compiled into (sets of) commands, which are then executed to update a target.

Baseline bjam has some mechanism grafted into it to run Python functions, which are run at the place where "normally" commands are compiled from actions (https://github.com/boostorg/build/blob/develop/src/engine/make1.c#L470 calls https://github.com/boostorg/build/blob/develop/src/engine/function.c#L3830 which may invoke https://github.com/boostorg/build/blob/develop/src/engine/function.c#L5233).

I have been (ab-)using this mechanism in faber to allow Python functions to be executed instead of system commands. While this has worked very well, I now wonder how to handle errors. How does `function_run_actions()` communicate errors back to the caller ? The only callsite I can see is in the `cmd_new()` function, and I don't see anything akin to handling errors that the called functions (which in bjam are typically jam functions, I gather) may encounter.

While I can certainly set up some global variables to capture error states, I'd rather use existing channels to communicate status and error states back to the call site.

Thanks,

-- 

      ...ich hab' noch einen Koffer in Berlin...