jni,cmd/tailscale: replace jni.JVMFor with direct cast

The JVMFor function converted an uintptr to a pointer, which is not
guaranteed to work in general. This change removes JVMFor, forcing the
unsafe conversion to the user of the jni packge.

Updates tailscale/tailscale#1195

Signed-off-by: Elias Naur <mail@eliasnaur.com>

cmd/tailscale/backend.go

@@ -38,7 +38,7 @@ type backend struct {
 	// when no nameservers are provided by Tailscale.
 	avoidEmptyDNS bool
 
-	jvm jni.JVM
+	jvm *jni.JVM
 }
 
 type androidRouter struct {
@@ -64,7 +64,7 @@ var fallbackNameservers = []netaddr.IP{netaddr.IPv4(8, 8, 8, 8), netaddr.IPv4(8,
 // VPN status was revoked.
 var errVPNNotPrepared = errors.New("VPN service not prepared or was revoked")
 
-func newBackend(dataDir string, jvm jni.JVM, store *stateStore, settings func(*router.Config) error) (*backend, error) {
+func newBackend(dataDir string, jvm *jni.JVM, store *stateStore, settings func(*router.Config) error) (*backend, error) {
 	logf := logger.RusagePrefixLog(log.Printf)
 	b := &backend{
 		jvm:      jvm,

cmd/tailscale/main.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 	"sync"
 	"time"
+	"unsafe"
 
 	"gioui.org/app"
 	"gioui.org/io/system"
@@ -29,7 +30,7 @@ import (
 //go:generate go run github.com/go-bindata/go-bindata/go-bindata -nocompress -o logo.go tailscale.png google.png
 
 type App struct {
-	jvm jni.JVM
+	jvm *jni.JVM
 	// appCtx is a global reference to the com.tailscale.ipn.App instance.
 	appCtx jni.Object
 
@@ -116,7 +117,7 @@ var backendEvents = make(chan UIEvent)
 
 func main() {
 	a := &App{
-		jvm:     jni.JVMFor(app.JavaVM()),
+		jvm:     (*jni.JVM)(unsafe.Pointer(app.JavaVM())),
 		appCtx:  jni.Object(app.AppContext()),
 		updates: make(chan struct{}, 1),
 	}

cmd/tailscale/store.go

@@ -16,7 +16,7 @@ import (
 // backend by androidx.security.crypto.EncryptedSharedPreferences (see
 // App.java).
 type stateStore struct {
-	jvm jni.JVM
+	jvm *jni.JVM
 	// appCtx is the global Android app context.
 	appCtx jni.Object
 
@@ -25,7 +25,7 @@ type stateStore struct {
 	decrypt jni.MethodID
 }
 
-func newStateStore(jvm jni.JVM, appCtx jni.Object) *stateStore {
+func newStateStore(jvm *jni.JVM, appCtx jni.Object) *stateStore {
 	s := &stateStore{
 		jvm:    jvm,
 		appCtx: appCtx,

jni/jni.go

@@ -50,9 +50,7 @@ __attribute__ ((visibility ("hidden"))) jsize _jni_GetArrayLength(JNIEnv *env, j
 */
 import "C"
 
-type JVM struct {
-	jvm *C.JavaVM
-}
+type JVM C.JavaVM
 
 type Env struct {
 	env *C.JNIEnv
@@ -73,32 +71,30 @@ const (
 	False Boolean = C.JNI_FALSE
 )
 
-func JVMFor(jvmPtr uintptr) JVM {
-	return JVM{
-		jvm: (*C.JavaVM)(unsafe.Pointer(jvmPtr)),
-	}
-}
-
 func EnvFor(envPtr uintptr) Env {
 	return Env{
 		env: (*C.JNIEnv)(unsafe.Pointer(envPtr)),
 	}
 }
 
+func javavm(vm *JVM) *C.JavaVM {
+	return (*C.JavaVM)(unsafe.Pointer(vm))
+}
+
 // Do invokes a function with a temporary JVM environment. The
 // environment is not valid after the function returns.
-func Do(vm JVM, f func(env Env) error) error {
+func Do(vm *JVM, f func(env Env) error) error {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 	var env *C.JNIEnv
-	if res := C._jni_GetEnv(vm.jvm, &env, C.JNI_VERSION_1_6); res != C.JNI_OK {
+	if res := C._jni_GetEnv(javavm(vm), &env, C.JNI_VERSION_1_6); res != C.JNI_OK {
 		if res != C.JNI_EDETACHED {
 			panic(fmt.Errorf("JNI GetEnv failed with error %d", res))
 		}
-		if C._jni_AttachCurrentThread(vm.jvm, &env, nil) != C.JNI_OK {
+		if C._jni_AttachCurrentThread(javavm(vm), &env, nil) != C.JNI_OK {
 			panic(errors.New("runInJVM: AttachCurrentThread failed"))
 		}
-		defer C._jni_DetachCurrentThread(vm.jvm)
+		defer C._jni_DetachCurrentThread(javavm(vm))
 	}
 
 	return f(Env{env})