Clean up system USB storage feature a bit

app/src/main/java/com/stevesoltys/seedvault/App.kt

@@ -144,6 +144,7 @@ fun <T> permitDiskReads(func: () -> T): T {
 }
 
 fun Context.getSystemContext(isUsbStorage: () -> Boolean): Context {
-    return if (checkSelfPermission(INTERACT_ACROSS_USERS_FULL) == PERMISSION_GRANTED
+    return if (checkSelfPermission(INTERACT_ACROSS_USERS_FULL) == PERMISSION_GRANTED &&
-        && isUsbStorage()) createContextAsUser(UserHandle.SYSTEM, 0) else this
+        isUsbStorage()
+    ) createContextAsUser(UserHandle.SYSTEM, 0) else this
 }

app/src/main/java/com/stevesoltys/seedvault/plugins/saf/DocumentsProviderStoragePlugin.kt

@@ -21,7 +21,11 @@ internal class DocumentsProviderStoragePlugin(
     private val storage: DocumentsStorage,
 ) : StoragePlugin {
 
-    private val context: Context get() = appContext.getSystemContext {
+    /**
+     * Attention: This context might be from a different user. Use with care.
+     */
+    private val context: Context
+        get() = appContext.getSystemContext {
             storage.storage?.isUsb == true
         }
 

app/src/main/java/com/stevesoltys/seedvault/plugins/saf/DocumentsStorage.kt

@@ -45,19 +45,21 @@ internal class DocumentsStorage(
     private val appContext: Context,
     private val settingsManager: SettingsManager,
 ) {
-
-    private val context: Context get() = appContext.getSystemContext {
-        storage?.isUsb ?: false
-    }
-
-    private val contentResolver: ContentResolver get() = context.contentResolver
-
     internal var storage: Storage? = null
         get() {
             if (field == null) field = settingsManager.getStorage()
             return field
         }
 
+    /**
+     * Attention: This context might be from a different user. Use with care.
+     */
+    private val context: Context
+        get() = appContext.getSystemContext {
+            storage?.isUsb == true
+        }
+    private val contentResolver: ContentResolver get() = context.contentResolver
+
     internal var rootBackupDir: DocumentFile? = null
         get() = runBlocking {
             if (field == null) {

app/src/main/java/com/stevesoltys/seedvault/settings/SettingsManager.kt

@@ -5,11 +5,11 @@ import android.hardware.usb.UsbDevice
 import android.net.ConnectivityManager
 import android.net.NetworkCapabilities
 import android.net.Uri
-import android.os.UserHandle
 import androidx.annotation.UiThread
 import androidx.annotation.WorkerThread
 import androidx.documentfile.provider.DocumentFile
 import androidx.preference.PreferenceManager
+import com.stevesoltys.seedvault.getSystemContext
 import com.stevesoltys.seedvault.permitDiskReads
 import com.stevesoltys.seedvault.transport.backup.BackupCoordinator
 import java.util.concurrent.ConcurrentSkipListSet
@@ -122,8 +122,8 @@ class SettingsManager(private val context: Context) {
     @WorkerThread
     fun canDoBackupNow(): Boolean {
         val storage = getStorage() ?: return false
-        return !storage.isUnavailableUsb(context.createContextAsUser(UserHandle.SYSTEM, 0))
+        val systemContext = context.getSystemContext { storage.isUsb }
-            && !storage.isUnavailableNetwork(context)
+        return !storage.isUnavailableUsb(systemContext) && !storage.isUnavailableNetwork(context)
     }
 
     fun backupApks(): Boolean {

app/src/main/java/com/stevesoltys/seedvault/storage/SeedvaultStoragePlugin.kt

@@ -13,6 +13,9 @@ internal class SeedvaultStoragePlugin(
     private val storage: DocumentsStorage,
     private val keyManager: KeyManager,
 ) : SafStoragePlugin(appContext) {
+    /**
+     * Attention: This context might be from a different user. Use with care.
+     */
     override val context: Context
         get() = appContext.getSystemContext {
             storage.storage?.isUsb == true

app/src/main/java/com/stevesoltys/seedvault/ui/storage/StorageRootResolver.kt

@@ -18,14 +18,13 @@ import android.provider.DocumentsContract.Root.FLAG_SUPPORTS_CREATE
 import android.provider.DocumentsContract.Root.FLAG_SUPPORTS_IS_CHILD
 import android.util.Log
 import com.stevesoltys.seedvault.R
+import com.stevesoltys.seedvault.getSystemContext
 import com.stevesoltys.seedvault.ui.storage.StorageOption.SafOption
 
 internal object StorageRootResolver {
 
     private val TAG = StorageRootResolver::class.java.simpleName
 
-    private const val usbAuthority = "com.android.externalstorage.documents"
-
     fun getStorageRoots(context: Context, authority: String): List<SafOption> {
         val roots = ArrayList<SafOption>()
         val rootsUri = DocumentsContract.buildRootsUri(authority)
@@ -37,12 +36,17 @@ internal object StorageRootResolver {
                     if (root != null) roots.add(root)
                 }
             }
-            if (usbAuthority == authority && UserHandle.myUserId() != UserHandle.USER_SYSTEM) {
+            // add special system user roots for USB devices
-                val c: Context = context.createContextAsUser(UserHandle.SYSTEM, 0)
+            val c = context.getSystemContext {
+                authority == AUTHORITY_STORAGE && UserHandle.myUserId() != UserHandle.USER_SYSTEM
+            }
+            // only proceed if we really got a different [Context], e.g. had permission for it
+            if (context !== c) {
                 c.contentResolver.query(rootsUri, null, null, null, null)?.use { cursor ->
                     while (cursor.moveToNext()) {
-                        // Pass in context since it is used to query package manager for app icons
+                        // Pass in [context] since it is used to query package manager for app icons
                         val root = getStorageRoot(context, authority, cursor)
+                        // only add USB storage from system user, no others
                         if (root != null && root.isUsb) roots.add(root)
                     }
                 }

app/src/test/java/com/stevesoltys/seedvault/plugins/saf/StoragePluginTest.kt

@@ -39,6 +39,7 @@ internal class StoragePluginTest : BackupTest() {
         // get current set dir and for that the current token
         every { storage getProperty "currentToken" } returns token
         every { settingsManager.getToken() } returns token
+        every { storage getProperty "storage" } returns null // just to check if isUsb
         coEvery { storage.getSetDir(token) } returns setDir
         // delete contents of current set dir
         coEvery { setDir.listFilesBlocking(context) } returns listOf(backupFile)

storage/demo/src/main/java/de/grobox/storagebackuptester/plugin/TestSafStoragePlugin.kt

@@ -11,7 +11,7 @@ import javax.crypto.SecretKey
 
 @Suppress("BlockingMethodInNonBlockingContext")
 class TestSafStoragePlugin(
-    private val appContext: Context,
+    appContext: Context,
     private val getLocationUri: () -> Uri?,
 ) : SafStoragePlugin(appContext) {
 

storage/lib/src/main/java/org/calyxos/backup/storage/plugin/saf/SafStoragePlugin.kt

@@ -35,12 +35,15 @@ private const val TAG = "SafStoragePlugin"
 public abstract class SafStoragePlugin(
     private val appContext: Context,
 ) : StoragePlugin {
-
+    /**
-    private val cache = SafCache()
+     * Attention: This context could be unexpected. E.g. the system user's application context,
-    // In the case of USB storage, if INTERACT_ACROSS_USERS_FULL is granted, this context will match
+     * in the case of USB storage, if INTERACT_ACROSS_USERS_FULL permission is granted.
-    // the system user's application context. Otherwise, matches appContext.
+     * Use [appContext], if you need the context of the current app and user
+     * and [context] for all file access.
+     */
     protected abstract val context: Context
     protected abstract val root: DocumentFile?
+    private val cache = SafCache()
 
     private val folder: DocumentFile?
         get() {
@@ -48,8 +51,9 @@ public abstract class SafStoragePlugin(
             if (cache.currentFolder != null) return cache.currentFolder
 
             @SuppressLint("HardwareIds")
-            // this is unique to each combination of app-signing key, user, and device
+            // This is unique to each combination of app-signing key, user, and device
-            // so we don't leak anything by not hashing this and can use it as is
+            // so we don't leak anything by not hashing this and can use it as is.
+            // Note: Use [appContext] here to not get the wrong ID for a different user.
             val androidId = Settings.Secure.getString(appContext.contentResolver, ANDROID_ID)
             // the folder name is our user ID
             val folderName = "$androidId.sv"