From f4022ba872346278c9327c064b67c5308062767d Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Sat, 20 Jan 2018 01:28:07 +0000
Subject: [PATCH] remoting: avoid a race condition.

---
 artiq/remoting.py | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/artiq/remoting.py b/artiq/remoting.py
index 0c6a99899..aa3c27f3f 100644
--- a/artiq/remoting.py
+++ b/artiq/remoting.py
@@ -83,8 +83,16 @@ class SSHClient(Client):
                 logger.debug("Using cached {}".format(filename))
             else:
                 logger.debug("Transferring {}".format(filename))
-                with sftp.open(remote_filename, 'wb') as remote:
+                # Avoid a race condition by writing into a temporary file
+                # and atomically replacing
+                with sftp.open(remote_filename + ".~", "wb") as remote:
                     remote.write(rewritten)
+                try:
+                    sftp.rename(remote_filename + ".~", remote_filename)
+                except IOError:
+                    # Either it already exists (this is OK) or something else
+                    # happened (this isn't) and we need to re-raise
+                    sftp.stat(remote_filename)
         return remote_filename
 
     def spawn_command(self, cmd, get_pty=False, **kws):