x/mobile/cmd/gomobile: gomobile bind doesn't recognize android build tag

[hajimehoshi]

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/hajimehoshi/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/hajimehoshi/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/7t/qw3np69559591s1v0mk5_p1m0000gn/T/go-build866316470=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

$ cat $GOPATH/src/github.com/hajimehoshi/gomobilebindtest/test.go 
// +build android

package gomobilebindtest

func Foo() {}

$ gomobile bind github.com/hajimehoshi/gomobilebindtest

What did you expect to see?

Success to build

What did you see instead?

gomobile: /Users/hajimehoshi/go/bin/gobind -lang=go,java -outdir=/var/folders/7t/qw3np69559591s1v0mk5_p1m0000gn/T/gomobile-work-857454256 github.com/hajimehoshi/gomobilebindtest failed: exit status 1
2018/04/14 15:24:03 package "github.com/hajimehoshi/gomobilebindtest": no buildable Go source files in /Users/hajimehoshi/go/src/github.com/hajimehoshi/gomobilebindtest

[hajimehoshi]

Building for iOS also fails even with ios build tag.

[hajimehoshi]

Interestingly, adding !android build tag also fails:

$ cat $GOPATH/src/github.com/hajimehoshi/gomobilebindtest/test.go 
// +build !android

package gomobilebindtest

func Foo() {}

$ gomobile bind github.com/hajimehoshi/gomobilebindtest
gomobile: package "github.com/hajimehoshi/gomobilebindtest": no buildable Go source files in /Users/hajimehoshi/go/src/github.com/hajimehoshi/gomobilebindtest

[hajimehoshi]

git bisect says golang/mobile@4600df5 is the cause

[hajimehoshi]

CC @eliasnaur

[eliasnaur]

Yes, CL 99316 that you refer to changed gobind to be able to generate standalone bindings and changed gomobile to use that instead of its own complicated binding generator. That main benefit is that you can now avoid gomobile altogether for special needs. From the CL description:

"This greatly simplifies gomobile bind, but also paves the way to skip
gomobile bind entirely. For example:

$ gobind -outdir=$GOPATH golang.org/x/mobile/example/bind/hello
$ GOOS=android GOARCH=arm64 CC=<ndk-toolchain>/bin/clang go build -buildmode=c-shared -o libgobind.so gobind
$ ls libgobind.*
libgobind.h  libgobind.so

"

Another benefit is you can now run gobind -lang=objc,go,java some/pkg to generate a complete set of bindings for both platforms and check the result into version control.

To accomplish that without needing the Android and Xcode SDKs, gobind has to be platform neutral and as such will generate bindings without any particular GOOS or GOARCH set. If you need to vary the API, use the -tags flag to gomobile or gobind.

In summary, go install some/pkg will have to work before you can use gobind or gomobile bind on some/pkg.

The reason your !android example fails is that while bindings themselves are generated without GOOS/GOARCH set, they are set when gomobile builds the libraries for pkg.aar. So gobind will generate bindings for Foo, while gomobile will fail to find any Go files because GOOS=android. In summary, your exported API must be platform independent (or you can use tags to select variants), but your implementation can and will depend on GOOS/GOARCH.

[hajimehoshi]

Thank you for elaborating! I'm still not sure the implementation detail, but what I needed to do is to make the lib buildable both on mobile and non-mobile?

[eliasnaur]

In further thought, your use case might still be possible. Could you try https://golang.org/cl/99777?

[gopherbot]

Change https://golang.org/cl/99777 mentions this issue: cmd/gomobile,cmd/gobind: allow per-platform bindings again

[hajimehoshi]

Yeah, with that patch, I've succeeded the experiment in the first comment:

$ cat $GOPATH/src/github.com/hajimehoshi/gomobilebindtest/test.go 
// +build android

package gomobilebindtest

func Foo() {}

$ gomobile bind github.com/hajimehoshi/gomobilebindtest