x/mobile: Build tags not being passed along properly

[adam-p]

gomobile build and bind are not passing along -tags properly.

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

go1.7.4

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

darwin/amd64

What did you do?

Modify the "basic" example (example/basic/main.go) like so:

- // +build darwin linux windows
+ // +build aaa,bbb

This will require the tags aaa and bbb to be passed in order for it to build. Confirm that it initially doesn't build like so:

$ gomobile build -v -x golang.org/x/mobile/example/basic
...resulting in something like...
gomobile: no buildable Go source files in GOPATH/src/golang.org/x/mobile/example/basic

As expected.

Now supply the required tags. It still fails, and you can see how the tags are getting mangled.

$ gomobile build -v -x -tags "aaa bbb" golang.org/x/mobile/example/basic
...resulting in something like...
can't load package: package golang.org/x/mobile/example/basic: no buildable Go source files in GOPATH/src/golang.org/x/mobile/example/basic
...
gomobile: go build -pkgdir=GOPATH/pkg/gomobile/pkg_android_arm64 -tags="aaa,bbb" -v -x -buildmode=c-shared -o /var/folders/xh/mp5b069545n32zn6ds3dbn100000gn/T/gomobile-work-217015845/lib/arm64-v8a/libtest.so golang.org/x/mobile/example/basic failed: exit status 1

Note the -tags="aaa,bbb".

Here's the fix diff:

diff --git a/cmd/gomobile/build.go b/cmd/gomobile/build.go
index 3ed4576..e4c4db6 100644
--- a/cmd/gomobile/build.go
+++ b/cmd/gomobile/build.go
@@ -15,7 +15,6 @@ import (
        "os/exec"
        "regexp"
        "runtime"
-       "strconv"
        "strings"
 )

@@ -270,7 +269,7 @@ func goCmd(subcmd string, srcs []string, env []string, args ...string) error {
                "go",
                subcmd,
                "-pkgdir="+pkgdir(env),
-               "-tags="+strconv.Quote(strings.Join(ctx.BuildTags, ",")),
+               "-tags",strings.Join(ctx.BuildTags, " "),
        )
        if buildV {
                cmd.Args = append(cmd.Args, "-v")

So, the tags should be passed as a separate argument to exec.Command, space-separated. The =, the quoting, and the comma-joining all need to go.

After applying that patch, the code builds as expected.

Impact

We discovered this the hard way and had to hack around it by patching gomobile before building it. This can be seen here. (I'll be filing another bug for the hack below it.)

[gopherbot]

CL https://golang.org/cl/34955 mentions this issue.