diff -r 64a873ca7135 -r d29db0a0c4eb mercurial/worker.py
--- a/mercurial/worker.py	Thu Jun 27 11:09:09 2019 +0200
+++ b/mercurial/worker.py	Thu Jun 27 11:39:35 2019 +0200
@@ -100,8 +100,9 @@
     workers
 
     hasretval - when True, func and the current function return an progress
-    iterator then a list (encoded as an iterator that yield many (False, ..)
-    then a (True, list)). The resulting list is in the natural order.
+    iterator then a dict (encoded as an iterator that yield many (False, ..)
+    then a (True, dict)). The dicts are joined in some arbitrary order, so
+    overlapping keys are a bad idea.
 
     threadsafe - whether work items are thread safe and can be executed using
     a thread-based worker. Should be disabled for CPU heavy tasks that don't
@@ -162,8 +163,8 @@
     ui.flush()
     parentpid = os.getpid()
     pipes = []
-    retvals = []
-    for i, pargs in enumerate(partition(args, workers)):
+    retval = {}
+    for pargs in partition(args, workers):
         # Every worker gets its own pipe to send results on, so we don't have to
         # implement atomic writes larger than PIPE_BUF. Each forked process has
         # its own pipe's descriptors in the local variables, and the parent
@@ -171,7 +172,6 @@
         # care what order they're in).
         rfd, wfd = os.pipe()
         pipes.append((rfd, wfd))
-        retvals.append(None)
         # make sure we use os._exit in all worker code paths. otherwise the
         # worker may do some clean-ups which could cause surprises like
         # deadlock. see sshpeer.cleanup for example.
@@ -192,7 +192,7 @@
                         os.close(w)
                     os.close(rfd)
                     for result in func(*(staticargs + (pargs,))):
-                        os.write(wfd, util.pickle.dumps((i, result)))
+                        os.write(wfd, util.pickle.dumps(result))
                     return 0
 
                 ret = scmutil.callcatch(ui, workerfunc)
@@ -226,9 +226,9 @@
         while openpipes > 0:
             for key, events in selector.select():
                 try:
-                    i, res = util.pickle.load(key.fileobj)
+                    res = util.pickle.load(key.fileobj)
                     if hasretval and res[0]:
-                        retvals[i] = res[1]
+                        retval.update(res[1])
                     else:
                         yield res
                 except EOFError:
@@ -249,7 +249,7 @@
             os.kill(os.getpid(), -status)
         sys.exit(status)
     if hasretval:
-        yield True, sum(retvals, [])
+        yield True, retval
 
 def _posixexitstatus(code):
     '''convert a posix exit status into the same form returned by
@@ -281,9 +281,9 @@
             try:
                 while not self._taskqueue.empty():
                     try:
-                        i, args = self._taskqueue.get_nowait()
+                        args = self._taskqueue.get_nowait()
                         for res in self._func(*self._staticargs + (args,)):
-                            self._resultqueue.put((i, res))
+                            self._resultqueue.put(res)
                             # threading doesn't provide a native way to
                             # interrupt execution. handle it manually at every
                             # iteration.
@@ -318,11 +318,10 @@
     workers = _numworkers(ui)
     resultqueue = pycompat.queue.Queue()
     taskqueue = pycompat.queue.Queue()
-    retvals = []
+    retval = {}
     # partition work to more pieces than workers to minimize the chance
     # of uneven distribution of large tasks between the workers
-    for pargs in enumerate(partition(args, workers * 20)):
-        retvals.append(None)
+    for pargs in partition(args, workers * 20):
         taskqueue.put(pargs)
     for _i in range(workers):
         t = Worker(taskqueue, resultqueue, func, staticargs)
@@ -331,9 +330,9 @@
     try:
         while len(threads) > 0:
             while not resultqueue.empty():
-                (i, res) = resultqueue.get()
+                res = resultqueue.get()
                 if hasretval and res[0]:
-                    retvals[i] = res[1]
+                    retval.update(res[1])
                 else:
                     yield res
             threads[0].join(0.05)
@@ -346,13 +345,13 @@
         trykillworkers()
         raise
     while not resultqueue.empty():
-        (i, res) = resultqueue.get()
+        res = resultqueue.get()
         if hasretval and res[0]:
-            retvals[i] = res[1]
+            retval.update(res[1])
         else:
             yield res
     if hasretval:
-        yield True, sum(retvals, [])
+        yield True, retval
 
 if pycompat.iswindows:
     _platformworker = _windowsworker