refactor: move Go backend code from tun2socks to app (#496)

- **Relocated Go backend code**: I moved all the Go backend code from the separate `tun2socks` module (`tun2socks/intra`) into the main `app` module (`app/src/main/go/intra`), because a separate `tun2socks` module wasn't providing any benefits and was making dependency management more complex.
- **Upgraded Gradle binary from `7.0.2` to `7.2`**: This upgrade might fix a NPE bug that was found in the DEX compiler.
- **Used a fixed `GOTOOLCHAIN` version**: This will avoid a `cgo` bug that was introduced in `go1.22`.

Android/app/build.gradle

@@ -15,6 +15,31 @@ try {
     keystoreProperties['storePassword'] = ''
 }
 
+// Go backend build constants
+def goSourceDir = "${projectDir}/src/go"
+def goSourcePackages = ["${goSourceDir}/intra",
+                        "${goSourceDir}/intra/android",
+                        "${goSourceDir}/intra/doh",
+                        "${goSourceDir}/intra/split",
+                        "${goSourceDir}/intra/protect"]
+def goBuildDir = file("${buildDir}/go")
+def goBackendAAR = file("${goBuildDir}/backend.aar")
+
+// gomobile won't use the Go version in go.mod. So we need to manually read it from go.mod and
+// explicitly set the GOTOOLCHAIN environment variable later.
+def goModLines = new File("${rootDir}/../go.mod").readLines()
+def goToolchain = goModLines.find { it.startsWith('toolchain ') }
+if (goToolchain == null) {
+    goToolchain = goModLines.find { it.startsWith('go ') }
+    if (goToolchain == null) {
+        throw new GradleException('cannot locate Go toolchain version in go.mod')
+    }
+}
+// goToolchain: ['toolchain go1.x.y' or 'go 1.x.y'] -> 'go1.x.y'
+goToolchain = goToolchain.replace("toolchain", "").replace(" ", "")
+println "using Go toolchain ${goToolchain}"
+
+// Standard Android build definition
 android {
     signingConfigs {
         config {
@@ -106,10 +131,51 @@ dependencies {
     implementation 'com.google.firebase:firebase-crashlytics:18.2.6'
     implementation 'com.google.firebase:firebase-crashlytics-ndk:18.2.6'
     implementation 'com.google.firebase:firebase-config:21.0.1'
-    // For go-tun2socks
-    implementation project(path: ':tun2socks', configuration: 'aarBinary')
+
+    // Go backend (use fileTree instead of files to prevent Android Studio sync errors)
+    implementation fileTree(goBuildDir) {
+        include '*.aar'
+        builtBy 'compileGoBackend'
+    }
 }
 
 // For Firebase Analytics
 apply plugin: 'com.google.gms.google-services'
 apply plugin: 'com.google.firebase.firebase-perf'
+
+// Go backend build tasks
+tasks.register('compileGoBackend', Exec) {
+    dependsOn 'ensureGoMobile'
+
+    // invoke gomobile to compile backend code
+    inputs.dir(goSourceDir)
+    outputs.file(goBackendAAR)
+
+    environment 'GOTOOLCHAIN', goToolchain
+    environment 'ANDROID_HOME', android.sdkDirectory
+    environment 'PATH', goBuildDir.getPath() +
+                        System.getProperty('path.separator') +
+                        System.getenv('PATH')
+
+    commandLine("${goBuildDir}/gomobile", 'bind',
+                '-ldflags=-s -w',
+                '-target=android',
+                "-androidapi=${android.defaultConfig.minSdk}",
+                '-o', goBackendAAR,
+                *goSourcePackages)
+}
+
+tasks.register('ensureGoMobile', Exec) {
+    // install gomobile and gobind into the build folder
+    outputs.file("${goBuildDir}/gomobile")
+    outputs.file("${goBuildDir}/gobind")
+
+    doFirst { goBuildDir.mkdirs() }
+
+    environment 'GOTOOLCHAIN', goToolchain
+
+    commandLine('go', 'build',
+                '-o', goBuildDir,
+                'golang.org/x/mobile/cmd/gomobile',
+                'golang.org/x/mobile/cmd/gobind')
+}

Android/tun2socks/intra/android/tun2socks.go → Android/app/src/go/intra/android/tun2socks.go

@@ -22,9 +22,9 @@ import (
 	"os"
 	"strings"
 
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra"
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/doh"
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/protect"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/doh"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/protect"
 	"github.com/Jigsaw-Code/outline-sdk/network"
 )
 

Android/tun2socks/intra/doh/doh.go → Android/app/src/go/intra/doh/doh.go

@@ -32,8 +32,8 @@ import (
 	"sync"
 	"time"
 
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/doh/ipmap"
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/split"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/doh/ipmap"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/split"
 	"github.com/eycorsican/go-tun2socks/common/log"
 	"golang.org/x/net/dns/dnsmessage"
 )

Android/tun2socks/intra/packet_proxy.go → Android/app/src/go/intra/packet_proxy.go

@@ -22,8 +22,8 @@ import (
 	"sync/atomic"
 	"time"
 
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/doh"
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/protect"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/doh"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/protect"
 	"github.com/Jigsaw-Code/outline-sdk/network"
 	"github.com/Jigsaw-Code/outline-sdk/transport"
 )

Android/tun2socks/intra/sni_reporter.go → Android/app/src/go/intra/sni_reporter.go

@@ -19,7 +19,7 @@ import (
 	"sync"
 	"time"
 
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/doh"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/doh"
 	"github.com/Jigsaw-Code/choir"
 	"github.com/eycorsican/go-tun2socks/common/log"
 )

Android/tun2socks/intra/sni_reporter_test.go → Android/app/src/go/intra/sni_reporter_test.go

@@ -22,8 +22,8 @@ import (
 
 	"golang.org/x/net/dns/dnsmessage"
 
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/doh"
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/split"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/doh"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/split"
 )
 
 type qfunc func(q []byte) ([]byte, error)

Android/tun2socks/intra/split/example/main.go → Android/app/src/go/intra/split/example/main.go

@@ -22,7 +22,7 @@ import (
 	"net"
 	"os"
 
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/split"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/split"
 )
 
 func main() {

Android/tun2socks/intra/stream_dialer.go → Android/app/src/go/intra/stream_dialer.go

@@ -23,9 +23,9 @@ import (
 	"sync/atomic"
 	"time"
 
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/doh"
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/protect"
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/split"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/doh"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/protect"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/split"
 	"github.com/Jigsaw-Code/outline-sdk/transport"
 )
 

Android/tun2socks/intra/tcp.go → Android/app/src/go/intra/tcp.go

@@ -23,7 +23,7 @@ import (
 	"sync/atomic"
 	"time"
 
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/split"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/split"
 	"github.com/Jigsaw-Code/outline-sdk/transport"
 )
 

Android/tun2socks/intra/tunnel.go → Android/app/src/go/intra/tunnel.go

@@ -22,8 +22,8 @@ import (
 	"os"
 	"strings"
 
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/doh"
-	"github.com/Jigsaw-Code/Intra/Android/tun2socks/intra/protect"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/doh"
+	"github.com/Jigsaw-Code/Intra/Android/app/src/go/intra/protect"
 	"github.com/Jigsaw-Code/outline-sdk/network"
 	"github.com/Jigsaw-Code/outline-sdk/network/lwip2transport"
 )

Android/gradle.properties

@@ -10,7 +10,10 @@
 # Specifies the JVM arguments used for the daemon process.
 # The setting is particularly useful for tweaking memory settings.
 # Default value: -Xmx10248m -XX:MaxPermSize=256m
-# org.gradle.jvmargs=-Xmx2048m -XX:MaxPermSize=512m -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8
+# Increase gradle heap size to 2G to prevent OutOfMemoryError
+org.gradle.jvmargs=-Xmx2048m
+# Other values include:
+# -XX:MaxPermSize=512m -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8
 
 # When configured, Gradle will run in incubating parallel mode.
 # This option should only be used with decoupled projects. More details, visit

Android/gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,5 @@
-#Fri Jan 29 10:30:35 EST 2021
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.2-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-7.0.2-all.zip