Make PackDomainName return error if the root label doesn't fit

[fcelda]

At the moment, if the buffer provided to PackDomainName doesn't fit the trailing root label for the name, the function will skip writing the root label however it will return an adjusted offset and no error.

I believe this is a bug and not intentional behavior because it would be really confusing.

Here is a small reproducer:

wire := make([]byte, 4)
off, err := dns.PackDomainName("foo.", wire, 0, nil, false)

fmt.Printf("off = %d, err = %s\n", off, err) // off = 5, err = <nil>

_ = wire[:off] // panic: runtime error: slice bounds out of range [:5] with capacity 4

I'm providing a fix that will make the function return an error if the buffer doesn't fit the root label. A unit test is included as well.

[miekg]

there is no other test where a single extra case can be added?

[fcelda]

I don't think so. There are a couple tests in parse_test.go but none that could be just extended with the new case.

If you prefer that, I could move the unit to parse_test.go and/or limit it to checking just this edge case with no space for the root label.

[miekg]

then just remove it, there are already too many tests that may or may not be actually testing something sane

[fcelda]

OK. Removed.

[miekg]

why else and not return here

[fcelda]

Question of coding style. I've updated this.

[fcelda]

Thank you!