Use spawn start method in multiprocessing programs (#11391)

* Use `spawn` start method in multiprocessing programs

* Set `spawn` start method in doctest

* Use `with` statement for locks

* Pass multiprocessing context explicitly

sorts/odd_even_transposition_parallel.py

@@ -11,11 +11,11 @@
 synchronization could be used.
 """
 
-from multiprocessing import Lock, Pipe, Process
+import multiprocessing as mp
 
 # lock used to ensure that two processes do not access a pipe at the same time
 # NOTE This breaks testing on build runner. May work better locally
-# process_lock = Lock()
+# process_lock = mp.Lock()
 
 """
 The function run by the processes that sorts the list
@@ -29,36 +29,41 @@
 """
 
 
-def oe_process(position, value, l_send, r_send, lr_cv, rr_cv, result_pipe):
- process_lock = Lock()
+def oe_process(
+ position,
+ value,
+ l_send,
+ r_send,
+ lr_cv,
+ rr_cv,
+ result_pipe,
+ multiprocessing_context,
+):
+ process_lock = multiprocessing_context.Lock()
 
  # we perform n swaps since after n swaps we know we are sorted
  # we *could* stop early if we are sorted already, but it takes as long to
  # find out we are sorted as it does to sort the list with this algorithm
  for i in range(10):
  if (i + position) % 2 == 0 and r_send is not None:
  # send your value to your right neighbor
- process_lock.acquire()
- r_send[1].send(value)
- process_lock.release()
+ with process_lock:
+ r_send[1].send(value)
 
  # receive your right neighbor's value
- process_lock.acquire()
- temp = rr_cv[0].recv()
- process_lock.release()
+ with process_lock:
+ temp = rr_cv[0].recv()
 
  # take the lower value since you are on the left
  value = min(value, temp)
  elif (i + position) % 2 != 0 and l_send is not None:
  # send your value to your left neighbor
- process_lock.acquire()
- l_send[1].send(value)
- process_lock.release()
+ with process_lock:
+ l_send[1].send(value)
 
  # receive your left neighbor's value
- process_lock.acquire()
- temp = lr_cv[0].recv()
- process_lock.release()
+ with process_lock:
+ temp = lr_cv[0].recv()
 
  # take the higher value since you are on the right
  value = max(value, temp)
@@ -94,39 +99,60 @@ def odd_even_transposition(arr):
  >>> odd_even_transposition(unsorted_list) == sorted(unsorted_list + [1])
  False
  """
+ # spawn method is considered safer than fork
+ multiprocessing_context = mp.get_context("spawn")
+
  process_array_ = []
  result_pipe = []
  # initialize the list of pipes where the values will be retrieved
  for _ in arr:
- result_pipe.append(Pipe())
+ result_pipe.append(multiprocessing_context.Pipe())
  # creates the processes
  # the first and last process only have one neighbor so they are made outside
  # of the loop
- temp_rs = Pipe()
- temp_rr = Pipe()
+ temp_rs = multiprocessing_context.Pipe()
+ temp_rr = multiprocessing_context.Pipe()
  process_array_.append(
- Process(
+ multiprocessing_context.Process(
  target=oe_process,
- args=(0, arr[0], None, temp_rs, None, temp_rr, result_pipe[0]),
+ args=(
+ 0,
+ arr[0],
+ None,
+ temp_rs,
+ None,
+ temp_rr,
+ result_pipe[0],
+ multiprocessing_context,
+ ),
  )
  )
  temp_lr = temp_rs
  temp_ls = temp_rr
 
  for i in range(1, len(arr) - 1):
- temp_rs = Pipe()
- temp_rr = Pipe()
+ temp_rs = multiprocessing_context.Pipe()
+ temp_rr = multiprocessing_context.Pipe()
  process_array_.append(
- Process(
+ multiprocessing_context.Process(
  target=oe_process,
- args=(i, arr[i], temp_ls, temp_rs, temp_lr, temp_rr, result_pipe[i]),
+ args=(
+ i,
+ arr[i],
+ temp_ls,
+ temp_rs,
+ temp_lr,
+ temp_rr,
+ result_pipe[i],
+ multiprocessing_context,
+ ),
  )
  )
  temp_lr = temp_rs
  temp_ls = temp_rr
 
  process_array_.append(
- Process(
+ multiprocessing_context.Process(
  target=oe_process,
  args=(
  len(arr) - 1,
@@ -136,6 +162,7 @@ def odd_even_transposition(arr):
  temp_lr,
  None,
  result_pipe[len(arr) - 1],
+ multiprocessing_context,
  ),
  )
  )