util: make ProgressStream seekable

botocore wants a seekable stream apparently.

rohmu/util.py

@@ -123,7 +123,8 @@ def __init__(self, raw_stream: BinaryIO) -> None:
         self.bytes_read = 0
 
     def seekable(self) -> bool:
-        return False
+        """A progress stream is seekable if the underlying stream is."""
+        return self.raw_stream.seekable()
 
     def writable(self) -> bool:
         return False
@@ -191,7 +192,15 @@ def tell(self) -> int:
         return self.raw_stream.tell()
 
     def seek(self, offset: int, whence: int = 0) -> int:
-        raise UnsupportedOperation("seek")
+        """Seek the underlying file if this operation is supported.
+
+        NOTE: Calling this method will reset the bytes_read field!
+
+        """
+        result = self.raw_stream.seek(offset, whence)
+
+        self.bytes_read = 0
+        return result
 
     def truncate(self, size: Optional[int] = None) -> int:
         raise UnsupportedOperation("truncate")

test/test_util.py

@@ -48,15 +48,14 @@ def test_progress_stream() -> None:
     progress_stream = ProgressStream(stream)
     assert progress_stream.readable()
     assert not progress_stream.writable()
-    assert not progress_stream.seekable()
+    # stream is seekable if underlying stream is
+    assert progress_stream.seekable()
 
     assert progress_stream.read(14) == b"Hello, World!\n"
     assert progress_stream.bytes_read == 14
     assert progress_stream.readlines() == [b"Second line\n", b"This is a longer third line\n"]
     assert progress_stream.bytes_read == 54
 
-    with pytest.raises(UnsupportedOperation):
-        progress_stream.seek(0)
     with pytest.raises(UnsupportedOperation):
         progress_stream.truncate(0)
     with pytest.raises(UnsupportedOperation):
@@ -66,6 +65,13 @@ def test_progress_stream() -> None:
     with pytest.raises(UnsupportedOperation):
         progress_stream.fileno()
 
+    # seeking the stream, in any position, resets the bytes_read counter
+    progress_stream.seek(10)
+    assert progress_stream.bytes_read == 0
+    # the seek works as expected on the stream
+    assert progress_stream.read(10) == b"ld!\nSecond"
+    assert progress_stream.bytes_read == 10
+
     assert not progress_stream.closed
     with progress_stream:
         # check that __exit__ closes the file