sass: Breaking Change Proposal: Move compilation_id out of the protobuf to improve performance in multi-thread/multi-isolate context

• Embedded protocol (https://github.com/sass/sass/pull/3599)
• Dart Sass (https://github.com/sass/dart-sass/pull/1981)
• Node.js Embedded Host (https://github.com/sass/embedded-host-node/pull/224)

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Today we have the wire data encoded as:

[ varint proto_length ][ proto ]

This is a propose for a breaking change to remove compilation_id field inside proto, and replace it with a top level session_id like below to serve the same purpose:

[ varint session_id ][ varint proto_length ][ proto ]

The motivation is to improve I/O performance in a multi-thread context, by offloading message encoding and decoding to different CPU cores running different threads/isolates to reduce the pressure on the main isolate/thread due to potential heavy I/O multiplexing on stdio. With this change, both host, and compiler would be able to pass raw proto to a thread/isolate that is currently working on a specific compilation, and the decoding of the proto can happen on the child thread’s CPU cycle, giving the main thread more free CPU cycle to purely process multiplexed I/O.

session_id should be in the range of uint32, the reason to choose varint as its encoding instead is:

1. For most of cases it would be shorter
2. Avoid the possible confusion on endianness of uint32 on wire.