Advance pos_of_buffer_start when writing directly to a writer

[Colecf]

If a message is bigger than the buffer, it will be written directly to the writer, and there was a bug in that case where pos_of_buffer_start was not advanced. This causes later assertions to fail, such as the one in
Message::write_length_delimited_to, which asserts based on total_bytes_written(), which is in turn based on
pos_of_buffer_start.

[stepancheg]

Can you please add a test reproducing the issue?

[Colecf]

I couldn't really figure out rust-protobuf's test setup (I couldn't find any documentation either), but I did make a minimal reproduction here: https://github.com/Colecf/rust-protobuf-assertion-failure-reproduction

[stepancheg]

rust-protobuf's test setup

Well, it is not easy. Basically, you create two files in

test-crates/protobuf-codegen-protoc-test/src/common/v2/test_xxx_pb.proto
test-crates/protobuf-codegen-protoc-test/src/common/v2/test_xxx.rs

And run cargo test. That will copy these files into three places (v3; v2/v2 with pure codegen).

Alternatively, maybe you can write a simple unit test using CodedOutputStream calls?

[Colecf]

Alternatively, maybe you can write a simple unit test using CodedOutputStream calls?

Ah good point, I added a unit test.

[stepancheg]

Thanks!