bpo-19675: Terminate processes if construction of a pool is failing.

Lib/multiprocessing/pool.py

@@ -173,7 +173,15 @@ def __init__(self, processes=None, initializer=None, initargs=(),
 
         self._processes = processes
         self._pool = []
-        self._repopulate_pool()
+        try:
+            self._repopulate_pool()
+        except Exception:
+            for p in self._pool:
+                if p.exitcode is None:
+                    p.terminate()
+            for p in self._pool:
+                p.join()
+            raise
 
         self._worker_handler = threading.Thread(
             target=Pool._handle_workers,
@@ -235,10 +243,10 @@ def _repopulate_pool(self):
                                    self._initargs, self._maxtasksperchild,
                                    self._wrap_exception)
                             )
-            self._pool.append(w)
             w.name = w.name.replace('Process', 'PoolWorker')
             w.daemon = True
             w.start()
+            self._pool.append(w)
             util.debug('added worker')
 
     def _maintain_pool(self):

Misc/NEWS.d/next/Library/2018-02-10-23-41-05.bpo-19675.-dj35-.rst

@@ -0,0 +1,2 @@
+multiprocessing.Pool does no longer leak processes if the construction of
+the pool fails.