x/mobile: iOS bind clobbers existing build tags

[adam-p]

gomobile bind for iOS adds a -tags ios argument that clobbers any user-supplied -tags.

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?

Reproduction scenario:

First, modify the bind iOS example code (golang.org/x/mobile/example/bind/hello) to require the presence of a build tag. Like so:

diff --git a/example/bind/hello/hello.go b/example/bind/hello/hello.go
index 2d98ff9..441030b 100644
--- a/example/bind/hello/hello.go
+++ b/example/bind/hello/hello.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

+// +build aaa
+
 // Package hello is a trivial package for gomobile bind example.
 package hello

Attempting to gomobile bind now without the build tag fails as expected.

$ gomobile bind -v -x -target=ios golang.org/x/mobile/example/bind/hello

gomobile: package "golang.org/x/mobile/example/bind/hello": no buildable Go source files in GOPATH/src/golang.org/x/mobile/example/bind/hello

However, providing the tag still fails, although at a later point in the build:

$ gomobile bind -tags aaa -v -x -target=ios golang.org/x/mobile/example/bind/hello

...lots of building output...
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.2.sdk -miphoneos-version-min=6.1 -arch armv7 CGO_ENABLED=1 GOPATH=$WORK/gen:$GOPATH go build -pkgdir=$GOMOBILE/pkg_darwin_arm -tags aaa -v -x -buildmode=c-archive -tags=ios -o $WORK/hello-arm.a $WORK/src/iosbin/main.go
WORK=/var/folders/m5/9r6jj7g57z5gkl_7441qzk440000gn/T/go-build842214217
/var/folders/m5/9r6jj7g57z5gkl_7441qzk440000gn/T/gomobile-work-594580178/src/gomobile_bind/go_hellomain.go:18:2: no buildable Go source files in GOPATH/src/golang.org/x/mobile/example/bind/hello
rm -r -f "$WORK"
gomobile: darwin-armv7: go build -pkgdir=GOPATH/pkg/gomobile/pkg_darwin_arm -tags aaa -v -x -buildmode=c-archive -tags=ios -o /var/folders/m5/9r6jj7g57z5gkl_7441qzk440000gn/T/gomobile-work-594580178/hello-arm.a /var/folders/m5/9r6jj7g57z5gkl_7441qzk440000gn/T/gomobile-work-594580178/src/iosbin/main.go failed: exit status 1

Notice the problem: There are two -tags arguments. The first is the one we supplied, and the second -- which clobbers the first -- is added by the iOS bind code.

The fix for the problem is this:

diff --git a/cmd/gomobile/bind_iosapp.go b/cmd/gomobile/bind_iosapp.go
index 4224b06..3d3214d 100644
--- a/cmd/gomobile/bind_iosapp.go
+++ b/cmd/gomobile/bind_iosapp.go
@@ -197,7 +197,8 @@ var iosModuleMapTmpl = template.Must(template.New("iosmmap").Parse(`framework mo
 func goIOSBindArchive(name, path string, env, fileBases []string) (string, error) {
        arch := getenv(env, "GOARCH")
        archive := filepath.Join(tmpdir, name+"-"+arch+".a")
-       err := goBuild(path, env, "-buildmode=c-archive", "-tags=ios", "-o", archive)
+       ctx.BuildTags = append(ctx.BuildTags, "ios")
+       err := goBuild(path, env, "-buildmode=c-archive", "-o", archive)
        if err != nil {
                return "", err
        }

NOTE: This is imperfect. It results in the ios tag get added multiple times for a large build. But that still functions correctly, whereas the current code does not. A proper fix should probably check for the presence of the ios tag before re-adding it.

After rebuilding gomobile, we get this:

$ gomobile bind -tags aaa -v -x -target=ios golang.org/x/mobile/example/bind/hello

...lots of output...
gomobile: darwin-armv7: go build -pkgdir=GOPATH/pkg/gomobile/pkg_darwin_arm -tags="aaa,ios" -v -x -buildmode=c-archive -o /var/folders/m5/9r6jj7g57z5gkl_7441qzk440000gn/T/gomobile-work-441301049/hello-arm.a /var/folders/m5/9r6jj7g57z5gkl_7441qzk440000gn/T/gomobile-work-441301049/src/iosbin/main.go failed: exit status 1

You can see that the -tags argument is better, but incorrect. This is due to issue #18515. If we apply the fix for that, we get:

$ gomobile bind -tags aaa -v -x -target=ios golang.org/x/mobile/example/bind/hello

...lots of output
GOPATH=$WORK/gen:$GOPATH go build -pkgdir=$GOMOBILE/pkg_darwin_arm -tags aaa ios -v -x -buildmode=c-archive -o $WORK/hello-arm.a $WORK/src/iosbin/main.go
...lots of output...
...success!

Note that the -tags argument is correct and works as expected. (However, later on in the output you'll see -tags aaa ios ios ios and so on.)

Impact

We discovered this the hard way and had to hack around it by patching gomobile before building it. This can be seen here.

[gopherbot]

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