Catching exceptions with multi-processing

[Fabien]

Folks,

I am developing a tool which works on individual entities (glaciers) and 
do a lot of operations on them. There are many tasks to do, one after 
each other, and each task follows the same interface:

def task_1(path_to_glacier_dir):
     open file1 in path_to_glacier_dir
     do stuff
     if dont_work:
         raise RuntimeError("didnt work")
     write file2 in path_to_glacier_dir

This way, the tasks can be run in parallel very easily:

import multiprocessing as mp
pool = mp.Pool(4)

dirs = [list_of_dirs]
pool.map(task1, dirs, chunksize=1)
pool.map(task2, dirs, chunksize=1)
pool.map(task3, dirs, chunksize=1)

... and so forth. I tested the tool for about a hundred glaciers but now 
it has to run for thousands of them. There are going to be errors, some 
of them are even expected for special outliers. What I would like the 
tool to do is that in case of error, it writes the identifier of the 
problematic glacier somewhere, the error encountered and more info if 
possible. Because of multiprocessing, I can't write in a shared file, so 
I thought that the individual processes should write a unique "error 
file" in a dedicated directory.

What I don't know how to, however, is how to do this at minimal cost and 
in a generic way for all tasks. Also, the task2 should not be run if 
task1 threw an error. Sometimes (for debugging), I'd rather keep the 
normal behavior of raising an error and stopping the program.

Do I have to wrap all tasks with a "try: exept:" block? How to switch 
between behaviors? All the solutions I could think about look quite ugly 
to me. And it seems that this is a general problem that someone cleverer 
than me had solved before ;-)

Thanks,

Fabien