Let async file know its size after first read (#745)

* Let async file know its size after first read

* initial size to None

s3fs/core.py

@@ -2296,11 +2296,13 @@ def __init__(self, fs, path, mode):
         self.mode = mode
         self.r = None
         self.loc = 0
+        self.size = None
 
     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.size = int(r["ResponseMetadata"]["HTTPHeaders"]["content-length"])
             self.r = r["Body"]
         out = await self.r.read(length)
         self.loc += len(out)

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.size == len(data)
             assert f.tell()
             if not got:
                 break