fix: COPY sent via extended protocol with Sync can block server re-use in transaction mode

[levkk]

The following sequence of messages can make a server be stuck in "unfinished state", blocking its re-use by other clients:

Parse("COPY t FROM STDIN")
Bind
Execute
Sync # This message is ignored by Postgres but not by pgdog
CopyData
[...]
CopyDone
Sync

The Sync sent after Execute is not necessary, but we record it and expect a ReadyForQuery from the server which only arrives after the second Sync is sent. An additional ReadyForQuery message remains in our internal state, and server.done() never returns true, causing the server to be tied to the client until it disconnects.

Related #885

[codecov]

Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/backend/protocol/state.rs	0.00%	9 Missing ⚠️
pgdog/src/backend/server.rs	0.00%	1 Missing ⚠️

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[NikolayS]

is it a good idea to have a couple of tests where we know it's a problem – eg for tokio-postgres or postgres.js?