Fix SendDatagram

[nekohasekai]

SendDatagram returned wrong PeerMaxDatagramFrameSize value

[marten-seemann]

Code looks correct to me, but can we have a test?

[nekohasekai]

I found that there is already a test:

quic-go/integrationtests/self/datagram_test.go

Lines 66 to 68 in 7b9d21f

	Expect(conn.SendDatagram(b)).To(MatchError(&quic.DatagramTooLargeError{
	PeerMaxDatagramFrameSize: int64(maxDatagramMessageSize),
	}))

.

But MatchError always returns true because DatagramTooLargeError implements Is(error) incorrectly

func (matcher *MatchErrorMatcher) Match(actual any) (success bool, err error) {
                ...

		// first try the built-in errors.Is
		if errors.Is(actualErr, expected.(error)) {
			return true, nil
		}

quic-go/errors.go

Lines 70 to 73 in 7b9d21f

	func (e *DatagramTooLargeError) Is(target error) bool {
	_, ok := target.(*DatagramTooLargeError)
	return ok
	}

[nekohasekai]

It almost passed. There is a race, but it doesn't seem to be related to my changes.

[marten-seemann]

I don’t understand your changes regarding the errors. I also don’t see any test that’s testing your change.

[nekohasekai]

These lines should test the PeerMaxDatagramFrameSize returned by SendDatagram but actually always pass because error has a wrong implementation of Is(error). According to the implementation of errors.Is , Is(error) should check for equality, not the type (that's what errors.As implements).

quic-go/integrationtests/self/datagram_test.go

Lines 66 to 68 in 7b9d21f

	Expect(conn.SendDatagram(b)).To(MatchError(&quic.DatagramTooLargeError{
	PeerMaxDatagramFrameSize: int64(maxDatagramMessageSize),
	}))

[marten-seemann]

That might be a separate issue. Please don't change the error implementation in this PR.

[marten-seemann]

@nekohasekai Are you still working on this PR? Would be nice to get the fix into the v0.43 release.