x/vgo: go list should avoid downloads when possible

[rogpeppe]

What version of Go are you using (go version)?

go version devel +104445e Wed Feb 7 19:22:09 2018 +0000 linux/amd64 vgo:2018-02-20.1

What did you do?

vgo list ./...

What did you expect to see?

I expected it to list the packages under the current path.
I would expect this to happen without any need to update or resolve dependencies.

What did you see instead?

It started to resolve imports by fetching dependencies.

% vgo list ./...
vgo: finding gopkg.in/tomb.v2 v0.0.0-20161208151619-d5d1b5820637
vgo: finding gopkg.in/retry.v1 v0.0.0-20180116153415-2d7c7c65cc71
... etc

[rsc]

Brad asked me to kill uses of NeedsFixButNotYet, so I'm going to call this NeedsDecision.

[myitcv]

This is somewhat related to #24749 (just updated that issue with a standalone example) - although I think there's another issue at play there on top of the behaviour being discussed in this issue.

Having had a go at an implementation for #24661, my gut feeling is that vgo list should not download by default. Not because of anything per se in #24661 (although that command, if implemented, should never download), but just a gut feel. Therefore vgo list and whatever is done for #24661 will need to indicate that their output is incomplete; with DepsErrors as appropriate.

That said, I think vgo list should gain a -download-esque flag that will ensure all dependencies listed in go.mod files encountered are present, even binary dependencies.

[rsc]

Having used this for a while, I think list downloading is the right default, both for list and for all the other go commands. That said, if you want to turn off downloading in the list command (or any other command that takes build flags), use -getmode=local.