rcbridge: Set cache dir via XDG_CACHE_HOME

Due to how golang calls each packages' init() functions, it's impossible
to have a shared library function that runs before any of the init()s.
Instead, we'll set the XDG_CACHE_HOME environment variable. This has its
own problems because calling setenv() from the Kotlin side only affects
libc's global environ variable, which golang does not use (it maintains
its own copy of envp). This commit works around that by importing a new
envhack package that explicitly copies libc's environ to golang via CGO.

Fixes: #11

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

app/build.gradle.kts

@@ -232,6 +232,7 @@ val rcbridge = tasks.register<Exec>("rcbridge") {
         File(rcbridgeSrcDir, "go.mod"),
         File(rcbridgeSrcDir, "go.sum"),
         File(rcbridgeSrcDir, "rcbridge.go"),
+        File(File(rcbridgeSrcDir, "envhack"), "envhack.go"),
     )
     inputs.properties(
         "android.defaultConfig.minSdk" to android.defaultConfig.minSdk,
@@ -341,7 +342,7 @@ fun checkBrackets(line: String) {
 
 fun updateChangelogLinks(baseUrl: String) {
     val file = File(rootDir, "CHANGELOG.md")
-    var regexStandaloneLink = Regex("\\[([^\\]]+)\\](?![\\(\\[])")
+    val regexStandaloneLink = Regex("\\[([^\\]]+)\\](?![\\(\\[])")
     val regexAutoLink = Regex("(Issue|PR) #(\\d+)(?: @([\\w-]+))?")
     val links = hashMapOf<LinkRef, String>()
     var skipRemaining = false

app/src/main/java/com/chiller3/rsaf/MainApplication.kt

@@ -3,6 +3,7 @@ package com.chiller3.rsaf
 import android.app.Application
 import android.app.backup.BackupManager
 import android.content.SharedPreferences
+import android.system.Os
 import android.util.Log
 import com.chiller3.rsaf.binding.rcbridge.Rcbridge
 import com.google.android.material.color.DynamicColors
@@ -45,7 +46,15 @@ class MainApplication : Application(), SharedPreferences.OnSharedPreferenceChang
     }
 
     private fun initRclone() {
-        Rcbridge.rbInit(File(cacheDir, "rclone").toString())
+        // Some of the rclone backend packages' init() functions set default values based on the
+        // value of config.GetCacheDir(). However, there's no way to call config.SetCacheDir() early
+        // enough that it'll be set before those init() functions. Instead, we'll set the
+        // XDG_CACHE_HOME environment variable to achieve the same effect. Environment variables set
+        // in libc's global environ variable are never read by golang, so rcbridge has an envhack
+        // package to explicitly copy env vars from C land to Go land.
+        Os.setenv("XDG_CACHE_HOME", cacheDir.path, true)
+
+        Rcbridge.rbInit()
         RcloneConfig.init(this)
         updateRcloneVerbosity()
     }

rcbridge/envhack/envhack.go

@@ -0,0 +1,37 @@
+package envhack
+
+/*
+#include <unistd.h>
+*/
+import "C"
+import (
+	"os"
+	"strings"
+	"unsafe"
+)
+
+func init() {
+	// Golang has its own internal copy of the environment variables. When using
+	// go as a shared library, the internal map never gets populated from
+	// _start()'s envp, so no environment variables are accessible. This
+	// terrible hack allows us to explicitly copy the environment variables from
+	// libc.
+
+	ptr := C.environ
+
+	for {
+		if *ptr == nil {
+			break
+		}
+
+		key_value := C.GoString(*ptr)
+		pieces := strings.SplitN(key_value, "=", 2)
+		if len(pieces) != 2 {
+			continue
+		}
+
+		os.Setenv(pieces[0], pieces[1])
+
+		ptr = (**C.char)(unsafe.Add(unsafe.Pointer(ptr), unsafe.Sizeof(ptr)))
+	}
+}

rcbridge/rcbridge.go

@@ -13,6 +13,9 @@
 package rcbridge
 
 import (
+	// This package's init() MUST run first
+	_ "rcbridge/envhack"
+
 	"context"
 	"io"
 	ioFs "io/fs"
@@ -65,9 +68,8 @@ var (
 )
 
 // Initialize global aspects of the library.
-func RbInit(cacheDir string) {
+func RbInit() {
 	librclone.Initialize()
-	config.SetCacheDir(cacheDir)
 
 	// Don't allow interactive password prompts
 	ci := fs.GetConfig(context.Background())