Update src/pip/_internal/req/__init__.py

Refactor

Co-authored-by: Nguyễn Gia Phong

Author: bmartinn

src/pip/_internal/req/__init__.py

@@ -103,29 +103,23 @@ def __safe_pool_map(
 
     # first let's try to install in parallel,
     # if we fail we do it by order.
-    pool = None
-    results = None
     try:
+        # Pool context would have been nice, but not supported on Python 2.7
+        # Once officially dropped, switch to context to avoid close/join calls
         pool = Pool()
-        pool_result = pool.map_async(func, iterable)
-        # python 2.7 timeout=None will not catch KeyboardInterrupt
-        results = pool_result.get(timeout=999999)
-    except (KeyboardInterrupt, SystemExit):
-        if pool:
-            pool.terminate()
-        raise
-    except BaseException:
-        # This should only happen if the pool failed to initialize
-        # we fail silently and try serial installation
-        if pool:
+    except ImportError:
+        return [func(i) for i in iterable]
+    else:
+        try:
+            # python 2.7 timeout=None will not catch KeyboardInterrupt
+            results = pool.map_async(func, iterable).get(timeout=999999)
+        except (KeyboardInterrupt, SystemExit):
             pool.terminate()
-            pool = None
-    # close pool if it was active
-    if pool:
-        pool.close()
-        pool.join()
-
-    return results
+            raise
+        else:
+            pool.close()
+            pool.join()
+            return results
 
 
 def __single_install(