prevent errors due to the use of seekable download strategies for seekable but not readable files

Author: mjurbanski-reef

b2sdk/_internal/transfer/inbound/downloaded_file.py

@@ -204,6 +204,15 @@ def save(self, file: BinaryIO, allow_seeking: bool | None = None) -> None:
             logger.warning('File is not seekable, disabling strategies that require seeking')
             allow_seeking = False
 
+        if allow_seeking:  # check if file allows reading from arbitrary position
+            try:
+                file.read(0)
+            except io.UnsupportedOperation:
+                logger.warning(
+                    'File is seekable, but does not allow reads, disabling strategies that require seeking'
+                )
+                allow_seeking = False
+
         if self.progress_listener:
             file = WritingStreamWithProgress(file, self.progress_listener)
             if self.range_ is not None:

changelog.d/+parallel_download_seekable_no_read.fixed.md

@@ -0,0 +1 @@
+Prevent errors due to the use of "seekable" download strategies for seekable, but not readable files.

test/unit/bucket/test_bucket.py

@@ -2556,6 +2556,34 @@ def test_download_to_non_seekable_file(self):
             )
             assert output_file.getvalue() == self.DATA.encode()
 
+    @pytest.mark.apiver(from_ver=2)
+    def test_download_to_seekable_but_no_read_file(self):
+        file_version = self.bucket.upload_bytes(self.DATA.encode(), 'file1')
+
+        non_seekable_strategies = [
+            strat for strat in self.bucket.api.services.download_manager.strategies
+            if not isinstance(strat, ParallelDownloader)
+        ]
+        context = contextlib.nullcontext() if non_seekable_strategies else pytest.raises(
+            ValueError,
+            match='no strategy suitable for download was found!',
+        )
+        output_file = io.BytesIO()
+        seekable_but_not_readable = io.BufferedWriter(output_file)
+
+        # test sanity check
+        assert seekable_but_not_readable.seekable()
+        with pytest.raises(io.UnsupportedOperation):
+            seekable_but_not_readable.read(0)
+
+        with context:
+            self.download_file_by_id(
+                file_version.id_,
+                v2_file=seekable_but_not_readable,
+            )
+            seekable_but_not_readable.flush()
+            assert output_file.getvalue() == self.DATA.encode()
+
 
 # download empty file