cmd/go: downloads follow plain-HTTP redirects even when the -insecure flag is not set [1.11 backport]

[gopherbot]

@FiloSottile requested issue #29591 to be considered for backport to the next 1.11 minor release.

Agreed, we should not. Depending on how many packages this will break, we should consider backporting it, too. It's true that it requires a server misconfiguration, but it breaks user expectations about not having to trust their network.

@gopherbot please open two backport issues.

[julieqiu]

Approving since this is a security issue. Please follow the instructions at https://github.com/golang/go/wiki/MinorReleases to create the cherrypick CL.

[dmitshur]

The upstream issue #29591 has not been resolved yet, and it has been bumped up to Go 1.13 milestone, so no progress can be made here at this time.

Edit: That said, CL 156838 which resolves the upstream issue has a +2, just a R=Go1.13 label. Perhaps progress can be made here. I'll let people who worked on the fix decide. /cc @bcmills @rsc

[bcmills]

I think we want to land the CL and give it a bit of soak time, then backport it.

[andybons]

It's been a while. How's the soaking going?

[bcmills]

Haven't heard any complaints so far, but it's only been a month. I'm still feeling a bit uneasy about it. (Maybe that implies that we shouldn't backport it at all?)

[andybons]

OK. Pushing to 1.11.11 for now.

[bcmills]

I've been thinking about this some more. I don't think we'll have adequate soak time on the change until after Go 1.13 is released: I suspect that the problems will be in the long tail of packages, and users on the long tail by and large are not testing against HEAD on a nightly basis.

While this is a security issue, I don't think it's major enough to risk breaking long-tail packages at a point release.

I'm going to tentatively schedule the 1.12 backport for a couple of weeks after the 1.13 release, when we'll have a clearer view of just how broken the long tail is. At that point, Go 1.11 will no longer be supported anyway, so closing the 1.11 backport issue.