Allow compiling with CGO_ENABLED=1 and no measurement-kit (#344)

I want to build an `.aar` file that only includes experiments written in Go such that I can link such `.aar` file into the OONI app along with measurement-kit/android-libs. To this end, I need to be able to compile this repository with CGO enabled, to cross compile for Android, but without linking with MK.

This work is part of #339.

.github/workflows/build.yml

@@ -13,7 +13,7 @@ jobs:
         with:
           go-version: ${{ matrix.go }}
       - uses: actions/checkout@v2
-      - run: go test -v -coverprofile=probe-engine.cov -coverpkg=./... ./...
+      - run: go test -tags nomk -v -coverprofile=probe-engine.cov -coverpkg=./... ./...
         env:
           CGO_ENABLED: 0
       - name: Send coverage

README.md

@@ -28,10 +28,9 @@ and requires Go v1.13+. We also depend on [Measurement Kit](
 https://github.com/measurement-kit/measurement-kit), a C++14 library
 implementing many OONI tests.
 
-Note that `export CGO_ENABLED=0` will disable C/C++ extensions and
-therefore will prevent Measurement Kit tests from being linked into
-the resulting Go binaries. You may want that in some cases, e.g. when
-you only want to use OONI tests written in Go.
+Note that passing the `-tags nomk` flag to Go will disable linking
+Measurement Kit into the resulting Go binaries. You may want that in
+cases where you only want to use experiments written in Go.
 
 We plan on gradually rewriting all OONI tests in Go, therefore the
 dependency on Measurement Kit will eventually be removed.

measurementkit/task_cgo.go → measurementkit/mkcgo/mkcgo.go

@@ -1,4 +1,7 @@
-package measurementkit
+// +build cgo
+
+// Package mkcgo contains CGO bindings to Measurement Kit.
+package mkcgo
 
 import (
 	// #include <measurement_kit/ffi.h>
@@ -75,7 +78,8 @@ func taskstart(settings []byte) *C.mk_task_t {
 	return C.mk_task_start(settingsp)
 }
 
-func start(settings []byte) (<-chan []byte, error) {
+// Start starts a Measurement Kit task.
+func Start(settings []byte) (<-chan []byte, error) {
 	taskp := taskstart(settings)
 	if taskp == nil {
 		return nil, errors.New("C.mk_task_start failed")
@@ -84,7 +88,3 @@ func start(settings []byte) (<-chan []byte, error) {
 	go taskloop(taskp, out)
 	return out, nil
 }
-
-func available() bool {
-	return true
-}

measurementkit/task_mk.go

@@ -0,0 +1,15 @@
+// +build !nomk
+
+package measurementkit
+
+import (
+	"github.com/ooni/probe-engine/measurementkit/mkcgo"
+)
+
+func start(settings []byte) (<-chan []byte, error) {
+	return mkcgo.Start(settings)
+}
+
+func available() bool {
+	return true
+}

measurementkit/task_otherwise.go → measurementkit/task_nomk.go

@@ -1,4 +1,4 @@
-// +build !cgo
+// +build nomk
 
 package measurementkit