Fix discarding of error values

[woodsbury]

Fixes #313

When discarding the responses from previous commands that weren't
successfully read (e.g. if their context had a deadline that was
exceeded), RESP error values get treated the same as connection errors
and cause any reading from the connection to stop. This can cause a
response to not be read off of the connection or marked for discarding,
and therefore leave the connection out of sync.

This fix disables error bubbling during the discarding process to
prevent these error values from interfering.

[mediocregopher]

Hi @woodsbury , thanks for the PR! The solution you've given here was used as part of unmarshalAgg, but I don't think it is applicable to this case. It happens to work for your test case because you used strings as your into variables, and so when this line:

err = resp3.Unmarshal(c.br, discardMU.unmarshalInto, o)

is called it happens to be able to unmarshal the SimpleError into the string (because they can be coerced). But if you make this change:

diff --git a/conn_test.go b/conn_test.go
index 73abce0..b65318c 100644
--- a/conn_test.go
+++ b/conn_test.go
@@ -290,7 +290,7 @@ func TestConnDeadlineExceeded(t *T) {
                                        }

                                        innerCtx, cancel := context.WithTimeout(ctx, mkTimeout(timeout))
-                                       into := ""
+                                       into := 0
                                        err := conn.Do(innerCtx, Cmd(&into, "EVAL", "return redis.error_reply('NOTOK')", "0"))
                                        cancel()

you'll see the test breaks again.

You're original solution that you posted in the issue, however, works fine even with that into := 0, and I think is the right way to solve this problem. If you could use that solution in this PR instead I think we can merge this.

Thanks again for debugging this and putting together a test case, that's made reviewing this much easier 🙏

[woodsbury]

No problem, I've updated my changes to check the error type instead.

[mediocregopher]

Looks good, thanks!