Carol Nichols created ARROW-10960:
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Summary: [C++] [Flight] Missing protobuf data_body should result
in default value of empty bytes, not null
Key: ARROW-10960
URL: https://issues.apache.org/jira/browse/ARROW-10960
Project: Apache Arrow
Issue Type: Bug
Components: FlightRPC
Reporter: Carol Nichols
Attachments: cpp-client-empty-data-body.png,
rust-client-missing-data-body.png
h1. Problem
ProtoBuf {{proto3}} specifies that [if a message does not contain a particular
singular element, the field should get the default
value|https://developers.google.com/protocol-buffers/docs/proto3#default].
However, when the C++ {{flight-test-integration-server}} gets a {{DoPut}}
request with a {{FlightData}} message for a record batch containing no items,
and the {{FlightData}} is missing the {{data_body}} field, the server responds
with an error "Expected body in IPC message of type record batch".
h2. What happens
If I run the C++ {{flight-test-integration-server}} and the C++
{{flight-test-integration-client}} with the {{generated_null_trivial}} test
case, the test passes and I see the protobuf in wireshark shown in the
cpp-client-empty-data-body.png attachment.
Note the {{data_body}} field is present but has no value.
If I run the Rust {{flight-test-integration-client}} that I'm working on
developing, it does not send the {{data_body}} field at all if there are no
bytes to send. I see the protobuf in wireshark shown in the
rust-client-missing-data-body.png attachment.
Note the {{data_body}} field is not present.
The C++ server then returns the error message "Expected body in IPC message of
type record batch", which comes from [this check for message
body|https://github.com/apache/arrow/blob/519e9da4fc1698f686525f4226295f3680a3f3db/cpp/src/arrow/ipc/reader.cc#L92]
called in [{{ReadNext}} of the record batch stream
reader|https://github.com/apache/arrow/blob/519e9da4fc1698f686525f4226295f3680a3f3db/cpp/src/arrow/ipc/reader.cc#L787].
h2. What I expect to happen
Instead of returning an error message because of a null pointer, the Message
should get the default value of empty bytes.
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