cmd/go: go mod -graph erroneously lists edges from the main module to indirect dependencies

[bcmills]

In modload, we intentionally treat the main module as requiring the entire build list. (I don't recall why, but we do.)

go/src/cmd/go/internal/modload/load.go

Line 742 in d365890

list = append(list, r.buildList[1:]...)

Unfortunately, that interacts badly with go mod -graph, which fails to take the extra requirements into account. We end up printing spurious edges from the main module to all of its transitive dependencies.

$ cd $(mktemp -d)

$ go version
go version devel +f8f3e3a3b4 Thu Jul 19 14:17:14 2018 -0400 linux/amd64

$ git clone https://github.com/rsc/quote.git .
Cloning into '.'...
remote: Counting objects: 80, done.
remote: Total 80 (delta 0), reused 0 (delta 0), pack-reused 80
Unpacking objects: 100% (80/80), done.

$ cat go.mod
module rsc.io/quote

require (
        rsc.io/quote/v3 v3.0.0
        rsc.io/sampler v1.3.0
)

$ go mod -graph
go: finding rsc.io/sampler v1.3.0
go: finding rsc.io/quote/v3 v3.0.0
rsc.io/quote golang.org/x/[email protected]
rsc.io/quote rsc.io/quote/[email protected]
rsc.io/quote rsc.io/[email protected]
rsc.io/[email protected] golang.org/x/[email protected]
rsc.io/quote/[email protected] rsc.io/[email protected]

Note the spurious line:

rsc.io/quote golang.org/x/[email protected]

Found while investigating #26474.

[gopherbot]

Change https://golang.org/cl/125657 mentions this issue: cmd/go: fix spurious edges in mod -graph output