Implement tell for async stream reader (#741)

Author: martindurant

s3fs/core.py

@@ -2295,13 +2295,16 @@ def __init__(self, fs, path, mode):
         self.path = path
         self.mode = mode
         self.r = None
+        self.loc = 0
 
     async def read(self, length=-1):
         if self.r is None:
             bucket, key, gen = self.fs.split_path(self.path)
             r = await self.fs._call_s3("get_object", Bucket=bucket, Key=key)
             self.r = r["Body"]
-        return await self.r.read(length)
+        out = await self.r.read(length)
+        self.loc += len(out)
+        return out
 
 
 def _fetch_range(fs, bucket, key, version_id, start, end, req_kw=None):

s3fs/tests/test_s3fs.py

@@ -2614,6 +2614,7 @@ async def read_stream():
         f = await fs.open_async(fn, mode="rb", block_seze=1000)
         while True:
             got = await f.read(1000)
+            assert f.tell()
             if not got:
                 break
             out.append(got)