x/vgo: load does not look at -tags build flag

[philpennock]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.10 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/pdp/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/pdp/go"
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/vs/7q9nz5955_z7b7zxfz437zrm0000gn/T/go-build533554610=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Following on from issue #24052 I tried to make sure that my app built with added dependencies enabled via a build tag. The "production" deploy of go.pennock.tech/dummyapp consists of building a scratch-based Docker image for pushing as an image to Heroku. It is built with the heroku build-tag, which adds an extra package import, of Heroku's metrics exporter library.

In the same directory and setup (same shell instance) as the previous successful build for #24052 I tried to enable this extra dependency:

% vgo build -tags heroku
build go.pennock.tech/dummyapp: cannot find module for path github.com/heroku/x/hmetrics

What does vgo think the dependencies are?

% vgo list -m
MODULE                      VERSION
go.pennock.tech/dummyapp    -
github.com/sirupsen/logrus  v1.0.4
golang.org/x/crypto         v0.0.0-20180222182404-49796115aa4b
golang.org/x/sys            v0.0.0-20180222210305-c1138c84af3a
% vgo list -tags heroku -m
MODULE                      VERSION
go.pennock.tech/dummyapp    -
github.com/sirupsen/logrus  v1.0.4
golang.org/x/crypto         v0.0.0-20180222182404-49796115aa4b
golang.org/x/sys            v0.0.0-20180222210305-c1138c84af3a

What did you expect to see?

A successful build.

What did you see instead?

What looks like vgo not using the build tags when deriving the list of dependencies to be resolved, only when later selecting the files to be compiled.

[rsc]

Yes, probably cmd/go/internal/imports needs to change var Tags to func Tags() and make it look at cfg.Tags in addition to the initial list. Oops.

[gopherbot]

Change https://golang.org/cl/113896 mentions this issue: x/vgo: fix ignoring tags on build, test, ...

[rsc]

I believe CL 113896 fixed this problem with ignoring tags.