Improve exception handling (#230)

This improves exception chaining and fixes a case where we'd get a
BufferError after an exception. The bad case is if you have a finally
block that writes to the file after an exception, e.g. if you use a gzip
file in a context manager it will write a trailer to the underlying file
on close.

blobfile/_azure.py

@@ -1193,7 +1193,12 @@ def _upload_chunk(self, chunk: memoryview, finalize: bool) -> None:
                 # from the 400 status code)
                 retry_codes=(400,) + DEFAULT_RETRY_CODES,
             )
-            execute_api_request(self._conf, req)
+            try:
+                execute_api_request(self._conf, req)
+            except Exception:
+                del chunk, data, req.data, req
+                raise
+
             self._block_index += 1
             if self._block_index >= BLOCK_COUNT_LIMIT:
                 raise Error(

blobfile/_common.py

@@ -599,7 +599,7 @@ def execute_request(conf: Config, build_req: Callable[[], Request]) -> "urllib3.
                     else:
                         raise RequestFailure.create_from_request_response(
                             "host does not exist", request=req, response=fake_resp
-                        )
+                        ) from e
 
             err = RequestFailure.create_from_request_response(
                 message=f"request failed with exception {e}",
@@ -730,9 +730,12 @@ def _upload_buf(self, buf: memoryview, finalize: bool = False) -> int:
             size = (len(buf) // self._chunk_size) * self._chunk_size
             assert size > 0
 
-        chunk = buf[:size]
-        self._upload_chunk(chunk, finalize)
-        self._offset += len(chunk)
+        try:
+            chunk = buf[:size]
+            self._upload_chunk(chunk, finalize)
+            self._offset += len(chunk)
+        finally:
+            del chunk, buf  # pyright: ignore[reportUnboundVariable]
         return size
 
     def close(self) -> None:
@@ -761,9 +764,12 @@ def write(self, b: bytes) -> int:  # type: ignore
         else:
             self._buf += b
             if len(self._buf) >= self._chunk_size:
-                mv = memoryview(self._buf)
-                size = self._upload_buf(mv)
-                self._buf = bytearray(mv[size:])
+                try:
+                    mv = memoryview(self._buf)
+                    size = self._upload_buf(mv)
+                    self._buf = bytearray(mv[size:])
+                finally:
+                    del mv  # pyright: ignore[reportUnboundVariable]
         assert len(self._buf) < self._chunk_size
         return len(b)