allow only one thread in pbar_callback

This commit adds a lock to limit the number
of threads in `pbar_callback` (a local function
in `datalad.downloaders.s3.S3DownloadSession.download`)
to one. This should prevent faulty commands in
the "datalad" git annex special remote.

datalad/downloaders/s3.py

@@ -11,6 +11,7 @@
 
 import re
 from urllib.parse import urlsplit, unquote as urlunquote
+from threading import Lock
 
 import boto3
 import botocore
@@ -159,15 +160,17 @@ def __init__(self, size=None, filename=None, url=None, headers=None,
         self.bucket = bucket
         self.key = key
         self.version_kwargs = version_kwargs
+        self.pbar_callback_lock = Lock()
 
     def download(self, f=None, pbar=None, size=None):
         # S3 specific (the rest is common with e.g. http)
         def pbar_callback(downloaded):
-            if pbar:
-                try:
-                    pbar.update(downloaded, increment=True)
-                except:  # MIH: what does it do? MemoryError?
-                    pass  # do not let pbar spoil our fun
+            with self.pbar_callback_lock:
+                if pbar:
+                    try:
+                        pbar.update(downloaded, increment=True)
+                    except:  # MIH: what does it do? MemoryError?
+                        pass  # do not let pbar spoil our fun
 
         if f:
             if size is None: