commit aa072db86b6b06b54d7f5846231bdf4a73b0cd84 (HEAD, refs/remotes/origin/master)
Author: Michael Albinus <michael.albinus@gmx.de>
Date:   Sat Apr 11 09:08:54 2020 +0200

    * lisp/net/tramp.el (tramp-action-process-alive): Read pending output.

diff --git a/lisp/net/tramp.el b/lisp/net/tramp.el
index e30f27fd33..a16085d7fb 100644
--- a/lisp/net/tramp.el
+++ b/lisp/net/tramp.el
@@ -4074,6 +4074,8 @@ The terminal type can be configured with `tramp-terminal-type'."
 (defun tramp-action-process-alive (proc _vec)
   "Check, whether a process has finished."
   (unless (process-live-p proc)
+    ;; There might be pending output.
+    (while (tramp-accept-process-output proc 0))
     (throw 'tramp-action 'process-died)))
 
 (defun tramp-action-out-of-band (proc vec)