environment: solve one spec per child process (#40876)

Looking at the memory profiles of concurrent solves
for environment with unify:false, it seems memory
is only ramping up.

This exchange in the potassco mailing list:
 https://sourceforge.net/p/potassco/mailman/potassco-users/thread/b55b5b8c2e8945409abb3fa3c935c27e%40lohn.at/#msg36517698

Seems to suggest that clingo doesn't release memory
until end of the application.

Since when unify:false we distribute work to processes,
here we give a maxtaskperchild=1, so we clean memory
after each solve.

lib/spack/spack/environment/environment.py

@@ -1525,7 +1525,11 @@ def _concretize_separately(self, tests=False):
         batch = []
         for j, (i, concrete, duration) in enumerate(
             spack.util.parallel.imap_unordered(
-                _concretize_task, args, processes=num_procs, debug=tty.is_debug()
+                _concretize_task,
+                args,
+                processes=num_procs,
+                debug=tty.is_debug(),
+                maxtaskperchild=1,
             )
         ):
             batch.append((i, concrete))

lib/spack/spack/util/parallel.py

@@ -6,6 +6,7 @@
 import os
 import sys
 import traceback
+from typing import Optional
 
 
 class ErrorFromWorker:
@@ -53,7 +54,9 @@ def __call__(self, *args, **kwargs):
         return value
 
 
-def imap_unordered(f, list_of_args, *, processes: int, debug=False):
+def imap_unordered(
+    f, list_of_args, *, processes: int, maxtaskperchild: Optional[int] = None, debug=False
+):
     """Wrapper around multiprocessing.Pool.imap_unordered.
 
     Args:
@@ -62,6 +65,8 @@ def imap_unordered(f, list_of_args, *, processes: int, debug=False):
         processes: maximum number of processes allowed
         debug: if False, raise an exception containing just the error messages
             from workers, if True an exception with complete stacktraces
+        maxtaskperchild: number of tasks to be executed by a child before being
+            killed and substituted
 
     Raises:
         RuntimeError: if any error occurred in the worker processes
@@ -70,7 +75,7 @@ def imap_unordered(f, list_of_args, *, processes: int, debug=False):
         yield from map(f, list_of_args)
         return
 
-    with multiprocessing.Pool(processes) as p:
+    with multiprocessing.Pool(processes, maxtasksperchild=maxtaskperchild) as p:
         for result in p.imap_unordered(Task(f), list_of_args):
             if isinstance(result, ErrorFromWorker):
                 raise RuntimeError(result.stacktrace if debug else str(result))