ValidateConnect built in callbacks cannot have their errors type asserted.

[adriansmares]

Is your feature request related to a problem? Please describe.

Hello, I am using pgx with a connection string which includes target_session_attrs=read-write. The built in callback correctly rejects connections to read only replicas:

pgx/pgconn/config.go

Lines 1070 to 1083 in b56d88f

	// ValidateConnectTargetSessionAttrsReadWrite is a ValidateConnectFunc that implements libpq compatible
	// target_session_attrs=read-write.
	func ValidateConnectTargetSessionAttrsReadWrite(ctx context.Context, pgConn *PgConn) error {
	result, err := pgConn.Exec(ctx, "show transaction_read_only").ReadAll()
	if err != nil {
	return err
	}
	if string(result[0].Rows[0][0]) == "on" {
	return errors.New("read only connection")
	}
	return nil
	}

However, I cannot use errors.Is with this error, and have to assert the error text.

Describe the solution you'd like

I would like for there to be dedicated errors such that I can do errors.Is(err, package.ErrConnectionReadOnly) in my transaction retry logic.

Describe alternatives you've considered

The error can either live in pgconn, or somewhere else. I am not sure if it should be a new set of errors or we should reuse existing errors in order to reduce duplication - any feedback is open.

Additional context

I can implement this change, but please let me know your thoughts on which errors we should use.