fix: silence expected errors for routine operations on BlobReader (#400)

Two fixes for BlobReader:
- Checksums are not supported for BlobReader's chunked downloads, so set checksum=None to silence log warnings (and add a note to the docstring explaining this).
- In Python, read() on files at EOF should return an empty result, but not raise an error. Stop BlobReader from emitting RequestRangeNotSatisfiable errors at EOF.

Fixes: #399

google/cloud/storage/blob.py

@@ -3434,6 +3434,11 @@ def open(
         latest generation number and set it; or, if the generation is known, set
         it manually, for instance with bucket.blob(generation=123456).
 
+        Checksumming (hashing) to verify data integrity is disabled for reads
+        using this feature because reads are implemented using request ranges,
+        which do not provide checksums to validate. See
+        https://cloud.google.com/storage/docs/hashes-etags for details.
+
         :type mode: str
         :param mode:
             (Optional) A mode string, as per standard Python `open()` semantics.The first

google/cloud/storage/fileio.py

@@ -14,6 +14,8 @@
 
 import io
 
+from google.api_core.exceptions import RequestRangeNotSatisfiable
+
 # Resumable uploads require a chunk size of precisely a multiple of 256 KiB.
 CHUNK_SIZE_MULTIPLE = 256 * 1024  # 256 KiB
 DEFAULT_CHUNK_SIZE = 40 * 1024 * 1024  # 40 MiB
@@ -92,10 +94,20 @@ def read(self, size=-1):
             else:
                 fetch_end = None
 
-            # Download the blob.
-            result += self._blob.download_as_bytes(
-                start=fetch_start, end=fetch_end, **self._download_kwargs
-            )
+            # Download the blob. Checksumming must be disabled as we are using
+            # chunked downloads, and the server only knows the checksum of the
+            # entire file.
+            try:
+                result += self._blob.download_as_bytes(
+                    start=fetch_start,
+                    end=fetch_end,
+                    checksum=None,
+                    **self._download_kwargs
+                )
+            except RequestRangeNotSatisfiable:
+                # We've reached the end of the file. Python file objects should
+                # return an empty response in this case, not raise an error.
+                pass
 
             # If more bytes were read than is immediately needed, buffer the
             # remainder and then trim the result.

tests/system/test_system.py

@@ -1134,6 +1134,9 @@ def test_blobwriter_and_blobreader(self):
                     file_obj.read(256 * 1024 * 2), reader.read(256 * 1024 * 2)
                 )
                 self.assertEqual(file_obj.read(), reader.read())
+                # End of file reached; further reads should be blank but not
+                # raise an error.
+                self.assertEqual(b"", reader.read())
 
     def test_blobwriter_and_blobreader_text_mode(self):
         blob = self.bucket.blob("MultibyteTextFile")

tests/unit/test_fileio.py

@@ -17,9 +17,11 @@
 import unittest
 import mock
 import io
-from google.cloud.storage.fileio import BlobReader, BlobWriter, SlidingBuffer
 import string
 
+from google.cloud.storage.fileio import BlobReader, BlobWriter, SlidingBuffer
+from google.api_core.exceptions import RequestRangeNotSatisfiable
+
 TEST_TEXT_DATA = string.ascii_lowercase + "\n" + string.ascii_uppercase + "\n"
 TEST_BINARY_DATA = TEST_TEXT_DATA.encode("utf-8")
 TEST_MULTIBYTE_TEXT_DATA = u"あいうえおかきくけこさしすせそたちつてと"
@@ -50,7 +52,7 @@ def read_from_fake_data(start=0, end=None, **_):
         # Read and trigger the first download of chunk_size.
         self.assertEqual(reader.read(1), TEST_BINARY_DATA[0:1])
         blob.download_as_bytes.assert_called_once_with(
-            start=0, end=8, **download_kwargs
+            start=0, end=8, checksum=None, **download_kwargs
         )
 
         # Read from buffered data only.
@@ -61,21 +63,36 @@ def read_from_fake_data(start=0, end=None, **_):
         self.assertEqual(reader.read(8), TEST_BINARY_DATA[4:12])
         self.assertEqual(reader._pos, 12)
         self.assertEqual(blob.download_as_bytes.call_count, 2)
-        blob.download_as_bytes.assert_called_with(start=8, end=16, **download_kwargs)
+        blob.download_as_bytes.assert_called_with(
+            start=8, end=16, checksum=None, **download_kwargs
+        )
 
         # Read a larger amount, requiring a download larger than chunk_size.
         self.assertEqual(reader.read(16), TEST_BINARY_DATA[12:28])
         self.assertEqual(reader._pos, 28)
         self.assertEqual(blob.download_as_bytes.call_count, 3)
-        blob.download_as_bytes.assert_called_with(start=16, end=28, **download_kwargs)
+        blob.download_as_bytes.assert_called_with(
+            start=16, end=28, checksum=None, **download_kwargs
+        )
 
         # Read all remaining data.
         self.assertEqual(reader.read(), TEST_BINARY_DATA[28:])
         self.assertEqual(blob.download_as_bytes.call_count, 4)
-        blob.download_as_bytes.assert_called_with(start=28, end=None, **download_kwargs)
+        blob.download_as_bytes.assert_called_with(
+            start=28, end=None, checksum=None, **download_kwargs
+        )
 
         reader.close()
 
+    def test_416_error_handled(self):
+        blob = mock.Mock()
+        blob.download_as_bytes = mock.Mock(
+            side_effect=RequestRangeNotSatisfiable("message")
+        )
+
+        reader = BlobReader(blob)
+        self.assertEqual(reader.read(), b"")
+
     def test_readline(self):
         blob = mock.Mock()
 
@@ -87,12 +104,12 @@ def read_from_fake_data(start=0, end=None, **_):
 
         # Read a line. With chunk_size=10, expect three chunks downloaded.
         self.assertEqual(reader.readline(), TEST_BINARY_DATA[:27])
-        blob.download_as_bytes.assert_called_with(start=20, end=30)
+        blob.download_as_bytes.assert_called_with(start=20, end=30, checksum=None)
         self.assertEqual(blob.download_as_bytes.call_count, 3)
 
         # Read another line.
         self.assertEqual(reader.readline(), TEST_BINARY_DATA[27:])
-        blob.download_as_bytes.assert_called_with(start=50, end=60)
+        blob.download_as_bytes.assert_called_with(start=50, end=60, checksum=None)
         self.assertEqual(blob.download_as_bytes.call_count, 6)
 
         blob.size = len(TEST_BINARY_DATA)
@@ -101,7 +118,7 @@ def read_from_fake_data(start=0, end=None, **_):
         # Read all lines. The readlines algorithm will attempt to read past the end of the last line once to verify there is no more to read.
         self.assertEqual(b"".join(reader.readlines()), TEST_BINARY_DATA)
         blob.download_as_bytes.assert_called_with(
-            start=len(TEST_BINARY_DATA), end=len(TEST_BINARY_DATA) + 10
+            start=len(TEST_BINARY_DATA), end=len(TEST_BINARY_DATA) + 10, checksum=None
         )
         self.assertEqual(blob.download_as_bytes.call_count, 13)