Issue #23992: multiprocessing: make MapResult not fail-fast upon exception.

1 files changed, 12 insertions, 8 deletions

diff --git a/Lib/multiprocessing/pool.py b/Lib/multiprocessing/pool.py
index 6d25469..ffdf426 100644
--- a/Lib/multiprocessing/pool.py
+++ b/Lib/multiprocessing/pool.py
@@ -638,22 +638,26 @@ class MapResult(ApplyResult):
             self._number_left = length//chunksize + bool(length % chunksize)
 
     def _set(self, i, success_result):
+        self._number_left -= 1
         success, result = success_result
-        if success:
+        if success and self._success:
             self._value[i*self._chunksize:(i+1)*self._chunksize] = result
-            self._number_left -= 1
             if self._number_left == 0:
                 if self._callback:
                     self._callback(self._value)
                 del self._cache[self._job]
                 self._event.set()
         else:
-            self._success = False
-            self._value = result
-            if self._error_callback:
-                self._error_callback(self._value)
-            del self._cache[self._job]
-            self._event.set()
+            if not success and self._success:
+                # only store first exception
+                self._success = False
+                self._value = result
+            if self._number_left == 0:
+                # only consider the result ready once all jobs are done
+                if self._error_callback:
+                    self._error_callback(self._value)
+                del self._cache[self._job]
+                self._event.set()
 
 #
 # Class whose instances are returned by `Pool.imap()`