fix: Fix BlobReader handling of interleaved reads and seeks (#721)

* tests (fileio): add tarfile based test case for reader seek

Background info: #462

* fix: Patch blob reader to return correct seek/tell values (#462)

Co-authored-by: Andrew Gorcester <gorcester@google.com>

google/cloud/storage/fileio.py

@@ -123,9 +123,12 @@ def read(self, size=-1):
         # If the read request demands more bytes than are buffered, fetch more.
         remaining_size = size - len(result)
         if remaining_size > 0 or size < 0:
+            self._pos += self._buffer.tell()
+            read_size = len(result)
+
             self._buffer.seek(0)
             self._buffer.truncate(0)  # Clear the buffer to make way for new data.
-            fetch_start = self._pos + len(result)
+            fetch_start = self._pos
             if size > 0:
                 # Fetch the larger of self._chunk_size or the remaining_size.
                 fetch_end = fetch_start + max(remaining_size, self._chunk_size)
@@ -154,9 +157,8 @@ def read(self, size=-1):
                 self._buffer.write(result[size:])
                 self._buffer.seek(0)
                 result = result[:size]
-
-        self._pos += len(result)
-
+            # Increment relative offset by true amount read.
+            self._pos += len(result) - read_size
         return result
 
     def read1(self, size=-1):
@@ -174,29 +176,33 @@ def seek(self, pos, whence=0):
         if self._blob.size is None:
             self._blob.reload(**self._download_kwargs)
 
-        initial_pos = self._pos
+        initial_offset = self._pos + self._buffer.tell()
 
         if whence == 0:
-            self._pos = pos
+            target_pos = pos
         elif whence == 1:
-            self._pos += pos
+            target_pos = initial_offset + pos
         elif whence == 2:
-            self._pos = self._blob.size + pos
+            target_pos = self._blob.size + pos
         if whence not in {0, 1, 2}:
             raise ValueError("invalid whence value")
 
-        if self._pos > self._blob.size:
-            self._pos = self._blob.size
+        if target_pos > self._blob.size:
+            target_pos = self._blob.size
 
         # Seek or invalidate buffer as needed.
-        difference = self._pos - initial_pos
-        new_buffer_pos = self._buffer.seek(difference, 1)
-        if new_buffer_pos != difference:  # Buffer does not contain new pos.
-            # Invalidate buffer.
+        if target_pos < self._pos:
+            # Target position < relative offset <= true offset.
+            # As data is not in buffer, invalidate buffer.
             self._buffer.seek(0)
             self._buffer.truncate(0)
-
-        return self._pos
+            new_pos = target_pos
+            self._pos = target_pos
+        else:
+            # relative offset <= target position <= size of file.
+            difference = target_pos - initial_offset
+            new_pos = self._pos + self._buffer.seek(difference, 1)
+        return new_pos
 
     def close(self):
         self._buffer.close()

tests/unit/test_fileio.py

@@ -247,6 +247,41 @@ def initialize_size(**_):
 
         reader.close()
 
+    def test_advanced_seek(self):
+        blob = mock.Mock()
+
+        def read_from_fake_data(start=0, end=None, **_):
+            return TEST_BINARY_DATA[start:end] * 1024
+
+        blob.download_as_bytes = mock.Mock(side_effect=read_from_fake_data)
+        blob.size = None
+        download_kwargs = {"if_metageneration_match": 1}
+        reader = self._make_blob_reader(blob, chunk_size=1024, **download_kwargs)
+
+        # Seek needs the blob size to work and should call reload() if the size
+        # is not known. Set a mock to initialize the size if reload() is called.
+        def initialize_size(**_):
+            blob.size = len(TEST_BINARY_DATA) * 1024
+
+        blob.reload = mock.Mock(side_effect=initialize_size)
+
+        self.assertEqual(reader.tell(), 0)
+        # Mimic tarfile access pattern. Read tarinfo block.
+        reader.read(512)
+        self.assertEqual(reader.tell(), 512)
+        self.assertEqual(reader.seek(512), 512)
+        # Mimic read actual tar content.
+        reader.read(400)
+        self.assertEqual(reader.tell(), 912)
+        # Tarfile offsets are rounded up by block size
+        # A sanity seek/read is used to check for unexpected ends.
+        reader.seek(1023)
+        reader.read(1)
+        self.assertEqual(reader.tell(), 1024)
+        reader.read(512)
+        self.assertEqual(reader.tell(), 1536)
+        reader.close()
+
     def test_close(self):
         blob = mock.Mock()
         reader = self._make_blob_reader(blob)