pyosys: catch boost::python::error_already_set

[georgerennie]

This catches exceptions from internal passes, printing them in a readable manner where the user would otherwise see an unspecified boost exception.

e.g. consider the following pass in pass.py:

import libyosys as ys

class MyPass(ys.Pass):
    def __init__(self):
        super().__init__("my_pass", "")

    def py_help(self):
        raise Exception("broken")

p = MyPass()

Previously, when invoked with yosys -m pass.py -p "help my_pass", yosys would error with

-- Running command `help my_pass' --
terminate called after throwing an instance of 'boost::python::error_already_set'

but with this change it instead shows

-- Running command `help my_pass' --
Traceback (most recent call last):
  File "<...>/pass.py", line 8, in py_help
    raise Exception("broken")
Exception: broken
ERROR: Python interpreter encountered an exception.

[povik]

I think we should catch it here instead:

yosys/misc/py_wrap_generator.py

Line 1573 in b2d7858

text += f"\n\t\t\t\t{return_stmt}" + call_string

For two reasons:

• the Yosys internals are not written to have exceptions propagating, so we should catch it as soon as we leave Python
• this will cover uses which don't go through main(), i.e. libyosys, or calling Python passes from Python

[povik]

To illustrate it a little better, the generated code looks like

		void py_execute(boost::python::list args, Design* design)
		{
			if (boost::python::override py_execute = this->get_override("py_execute"))
				py_execute(args, design); // xxx
			else
				Pass::py_execute(args, design);
		}

and // xxx marks the spot for catching the exception

[georgerennie]

Ahh sure, the main reason I hadn't done that was because I didn't want to get too into the mess of py_wrap_generator :P but I ended up doing that anyway so im happy to change it so its done that way

[georgerennie]

Have moved it into the generator. The generated c++ diff looks as follows with the other py_* functions taking the same form as py_help

43a44,48
> 	[[noreturn]] static void log_python_exception_as_error() {
> 		PyErr_Print();
> 		log_error("Python interpreter encountered an exception.\n");
> 	}
> 
447,449c452,458
< 			if (boost::python::override py_help = this->get_override("py_help"))
< 				py_help();
< 			else
---
> 			if (boost::python::override py_help = this->get_override("py_help")) {
> 				try {
> 					py_help();
> 				} catch (boost::python::error_already_set &) {
> 					log_python_exception_as_error();
> 				}
> 			} else {
450a460
> 			}

[povik]

Codewise looks good to me