Prefer forkserver over fork for multiprocessing

Use of 'fork' is deprecated for multithreaded
processes and also appears to cause issues with
CUDA startup, so use 'forkserver' instead if available.

modules/multifit/pyext/src/fit_fft.py

@@ -13,7 +13,7 @@
 
 multiproc_exception = None
 try:
-    from multiprocessing import Pool
+    import multiprocessing
     # Detect whether we are running Windows Python via Wine. Wine does not
     # currently support some named pipe functions which the multiprocessing
     # module needs: http://bugs.winehq.org/show_bug.cgi?id=17273
@@ -23,6 +23,15 @@
     multiproc_exception = str(detail)
 
 
+def _get_context():
+    # Use 'forkserver' rather than 'fork' start method if we can; 'fork' does
+    # not work well with multithreaded processes or CUDA
+    if 'forkserver' in multiprocessing.get_all_start_methods():
+        return multiprocessing.get_context('forkserver')
+    else:
+        return multiprocessing.get_context()
+
+
 class Fitter(object):
 
     def __init__(
@@ -161,8 +170,10 @@ def run(asmb_fn, options):
     if multiproc_exception is None and options.cpus > 1:
         # No point in spawning more processes than components
         nproc = min(options.cpus, asmb_input.get_number_of_component_headers())
-        p = Pool(processes=nproc)
+        ctx = _get_context()
+        p = ctx.Pool(processes=nproc)
         _ = list(p.imap_unordered(do_work, work_units))
+        p.close()
 
 
 def main():