Improve building wrapper and native binaries on Windows and Linux

* Do not ask for root if root is disabled in settings
* Show error in UI when libSyncthing.so is missing
* build-syncthing - Install Go on demand on windows
* build-syncthing - Install Android NDK on demand on windows
* Update README.md
* Update APK version to 0.14.51.rc3.6 / 4162

.gitignore

@@ -32,6 +32,9 @@ gradle/wrapper/gradlew*
 # Prebuilt-go
 syncthing/go
 syncthing/go.tgz
+syncthing/go.zip
+syncthing/android-ndk-r16b
+syncthing/ndk.zip
 
 # External build artifacts
 ext/

README.md

@@ -14,7 +14,7 @@
 
 A wrapper of [Syncthing](https://github.com/syncthing/syncthing) for Android. Head to the "releases" section for builds. Please open an issue under this fork if you need help. Important: Please don't file bugs at the upstream repository "syncthing-android" if you are using this fork.
 
-<img src="app/src/main/play/en-GB/listing/phoneScreenshots/screenshot_phone_1.png" alt="screenshot 1" width="200" /> <img src="app/src/main/play/en-GB/listing/phoneScreenshots/screenshot_phone_2.png" alt="screenshot 2" width="200" /> <img src="app/src/main/play/en-GB/listing/phoneScreenshots/screenshot_phone_3.png" alt="screenshot 3" width="200" />
+<img src="app/src/main/play/en-GB/listing/phoneScreenshots/screenshot_phone_12.png" alt="screenshot 1" width="200" /> <img src="app/src/main/play/en-GB/listing/phoneScreenshots/screenshot_phone_11.png" alt="screenshot 2" width="200" /> <img src="app/src/main/play/en-GB/listing/phoneScreenshots/screenshot_phone_09.png" alt="screenshot 3" width="200" />
 
 # Translations
 
@@ -22,21 +22,33 @@ The project is translated on [Transifex](https://www.transifex.com/projects/p/sy
 
 # Building
 
-### Dependencies
+### Prerequisites
-- Android SDK (you can skip this if you are using Android Studio)
+- Android SDK
-- Android NDK (`$ANDROID_NDK_HOME` should point at the root directory of your NDK)
+`You can skip this if you are using Android Studio.`
-- Go (see [here](https://docs.syncthing.net/dev/building.html#prerequisites) for the required version)
+- Android NDK r16b
+`$ANDROID_NDK_HOME environment variable should point at the root directory of your NDK. If the variable is not set, build-syncthing.py will automatically try to download and setup the NDK.`
+- Go 1.9.7
+`Make sure, Go is installed and available on the PATH environment variable. If Go is not found on the PATH environment variable, build-syncthing.py will automatically try to download and setup GO on the PATH.`
+- Python 2.7
+`Make sure, Python is installed and available on the PATH environment variable.`
 
 ### Build instructions
 
 Make sure you clone the project with
-`git clone https://github.com/Catfriend1/syncthing-android.git --recursive`. Alternatively, run
+`git clone https://github.com/Catfriend1/syncthing-android.git --recursive`.
-`git submodule init && git submodule update` in the project folder.
+Alternatively, run `git submodule init && git submodule update` in the project folder.
 
 A Linux VM, for example running Debian, is recommended to build this.
 
-Build Syncthing using `./gradlew cleanNative buildNative`. Then use `./gradlew assembleDebug` or
+Build Syncthing and the Syncthing-Android wrapper using the following commands:
-Android Studio to build the apk.
+`./gradlew buildNative`
+`./gradlew lint assembleDebug`
+
+You can also use Android Studio to build the apk after you manually ran the `./gradlew buildNative` command in the repository root.
+
+To clean up all files generated during build, use the following commands:
+`./gradlew cleanNative`
+`./gradlew clean`
 
 # License
 

app/build.gradle

@@ -35,8 +35,8 @@ android {
         applicationId "com.github.catfriend1.syncthingandroid"
         minSdkVersion 16
         targetSdkVersion 26
-        versionCode 4161
+        versionCode 4162
-        versionName "0.14.51.rc3.5"
+        versionName "0.14.51.rc3.6"
         testApplicationId 'com.github.catfriend1.syncthingandroid.test'
         testInstrumentationRunner 'android.support.test.runner.AndroidJUnitRunner'
         playAccountConfig = playAccountConfigs.defaultAccountConfig

app/src/main/java/com/nutomic/syncthingandroid/activities/FirstStartActivity.java

@@ -39,9 +39,11 @@ import android.widget.Toast;
 import com.nutomic.syncthingandroid.R;
 import com.nutomic.syncthingandroid.SyncthingApp;
 import com.nutomic.syncthingandroid.service.Constants;
+import com.nutomic.syncthingandroid.service.SyncthingRunnable.ExecutableNotFoundException;
 import com.nutomic.syncthingandroid.util.ConfigXml;
 
 import java.lang.ref.WeakReference;
+
 import javax.inject.Inject;
 
 public class FirstStartActivity extends Activity {
@@ -79,7 +81,8 @@ public class FirstStartActivity extends Activity {
     private Button mBackButton;
     private Button mNextButton;
 
-    @Inject SharedPreferences mPreferences;
+    @Inject
+    SharedPreferences mPreferences;
 
     /**
      * Handles activity behaviour depending on prerequisites.
@@ -479,6 +482,9 @@ public class FirstStartActivity extends Activity {
             }
             try {
                 configXml = new ConfigXml(firstStartActivity);
+            } catch (ExecutableNotFoundException e) {
+                publishProgress(firstStartActivity.getString(R.string.executable_not_found, e.getMessage()));
+                cancel(true);
             } catch (ConfigXml.OpenConfigException e) {
                 publishProgress(firstStartActivity.getString(R.string.config_create_failed));
                 cancel(true);

app/src/main/java/com/nutomic/syncthingandroid/activities/WebGuiActivity.java

@@ -123,7 +123,6 @@ public class WebGuiActivity extends SyncthingActivity
 
     /**
      * Initialize WebView.
-     *
      * Ignore lint javascript warning as js is loaded only from our known, local service.
      */
     @Override
@@ -134,7 +133,11 @@ public class WebGuiActivity extends SyncthingActivity
         setContentView(R.layout.activity_web_gui);
 
         mLoadingView = findViewById(R.id.loading);
+        try {
             mConfig = new ConfigXml(this);
+        } catch (Exception e) {
+            throw new RuntimeException(e.getMessage());
+        }
         loadCaCert();
 
         mWebView = findViewById(R.id.webview);

app/src/main/java/com/nutomic/syncthingandroid/service/SyncthingRunnable.java

@@ -56,9 +56,13 @@ public class SyncthingRunnable implements Runnable {
     private final File mSyncthingBinary;
     private String[] mCommand;
     private final File mLogFile;
-    @Inject SharedPreferences mPreferences;
     private final boolean mUseRoot;
-    @Inject NotificationHandler mNotificationHandler;
+
+    @Inject
+    SharedPreferences mPreferences;
+
+    @Inject
+    NotificationHandler mNotificationHandler;
 
     public enum Command {
         deviceid,           // Output the device ID to the command line.
@@ -104,11 +108,15 @@ public class SyncthingRunnable implements Runnable {
 
     @Override
     public void run() {
+        try {
             run(false);
+        } catch (ExecutableNotFoundException e) {
+            throw new RuntimeException(e.getMessage());
+        }
     }
 
     @SuppressLint("WakelockTimeout")
-    public String run(boolean returnStdOut) {
+    public String run(boolean returnStdOut) throws ExecutableNotFoundException {
         Boolean sendStopToService = false;
         Boolean restartSyncthingNative = false;
         int exitCode;
@@ -315,7 +323,11 @@ public class SyncthingRunnable implements Runnable {
      * Manually run "sysctl fs.inotify" in a root shell terminal to check current limit.
      */
     private void increaseInotifyWatches() {
-        if (!mUseRoot || !Shell.SU.available()) {
+        if (!mUseRoot) {
+            // Settings prohibit using root privileges. Cannot increase inotify limit.
+            return;
+        }
+        if (!Shell.SU.available()) {
             Log.i(TAG, "increaseInotifyWatches: Root is not available. Cannot increase inotify limit.");
             return;
         }
@@ -327,7 +339,11 @@ public class SyncthingRunnable implements Runnable {
      * Look for a running libsyncthing.so process and nice its IO.
      */
     private void niceSyncthing() {
-        if (!mUseRoot || !Shell.SU.available()) {
+        if (!mUseRoot) {
+            // Settings prohibit using root privileges. Cannot nice syncthing.
+            return;
+        }
+        if (!Shell.SU.available()) {
             Log.i(TAG_NICE, "Root is not available. Cannot nice syncthing.");
             return;
         }
@@ -482,7 +498,16 @@ public class SyncthingRunnable implements Runnable {
         return targetEnv;
     }
 
-    private Process setupAndLaunch(HashMap<String, String> env) throws IOException {
+    private Process setupAndLaunch(HashMap<String, String> env) throws IOException, ExecutableNotFoundException {
+        // Check if "libSyncthing.so" exists.
+        if (mCommand.length > 0) {
+            File libSyncthing = new File(mCommand[0]);
+            if (!libSyncthing.exists()) {
+                Log.e(TAG, "CRITICAL - Syncthing core binary is missing in APK package location " + mCommand[0]);
+                throw new ExecutableNotFoundException(mCommand[0]);
+            }
+        }
+
         if (mUseRoot) {
             ProcessBuilder pb = new ProcessBuilder("su");
             Process process = pb.start();
@@ -508,4 +533,16 @@ public class SyncthingRunnable implements Runnable {
             return pb.start();
         }
     }
+
+    public class ExecutableNotFoundException extends Exception {
+
+        public ExecutableNotFoundException(String message) {
+            super(message);
+        }
+
+        public ExecutableNotFoundException(String message, Throwable throwable) {
+            super(message, throwable);
+        }
+
+    }
 }

app/src/main/java/com/nutomic/syncthingandroid/service/SyncthingService.java

@@ -120,18 +120,27 @@ public class SyncthingService extends Service {
      * Indicates the current state of SyncthingService and of Syncthing itself.
      */
     public enum State {
-        /** Service is initializing, Syncthing was not started yet. */
+        /**
+         * Service is initializing, Syncthing was not started yet.
+         */
         INIT,
-        /** Syncthing binary is starting. */
+        /**
+         * Syncthing binary is starting.
+         */
         STARTING,
-        /** Syncthing binary is running,
+        /**
+         * Syncthing binary is running,
          * Rest API is available,
          * RestApi class read the config and is fully initialized.
          */
         ACTIVE,
-        /** Syncthing binary is shutting down. */
+        /**
+         * Syncthing binary is shutting down.
+         */
         DISABLED,
-        /** There is some problem that prevents Syncthing from running. */
+        /**
+         * There is some problem that prevents Syncthing from running.
+         */
         ERROR,
     }
 
@@ -141,13 +150,7 @@ public class SyncthingService extends Service {
      * {@link onStartCommand}.
      */
     private State mCurrentState = State.DISABLED;
-
     private ConfigXml mConfig;
-    private @Nullable PollWebGuiAvailableTask mPollWebGuiAvailableTask = null;
-    private @Nullable RestApi mApi = null;
-    private @Nullable EventProcessor mEventProcessor = null;
-    private @Nullable RunConditionMonitor mRunConditionMonitor = null;
-    private @Nullable SyncthingRunnable mSyncthingRunnable = null;
     private StartupTask mStartupTask = null;
     private Thread mSyncthingRunnableThread = null;
     private Handler mHandler;
@@ -155,8 +158,26 @@ public class SyncthingService extends Service {
     private final HashSet<OnServiceStateChangeListener> mOnServiceStateChangeListeners = new HashSet<>();
     private final SyncthingServiceBinder mBinder = new SyncthingServiceBinder(this);
 
-    @Inject NotificationHandler mNotificationHandler;
+    private @Nullable
-    @Inject SharedPreferences mPreferences;
+    PollWebGuiAvailableTask mPollWebGuiAvailableTask = null;
+
+    private @Nullable
+    RestApi mApi = null;
+
+    private @Nullable
+    EventProcessor mEventProcessor = null;
+
+    private @Nullable
+    RunConditionMonitor mRunConditionMonitor = null;
+
+    private @Nullable
+    SyncthingRunnable mSyncthingRunnable = null;
+
+    @Inject
+    NotificationHandler mNotificationHandler;
+
+    @Inject
+    SharedPreferences mPreferences;
 
     /**
      * Object that must be locked upon accessing mCurrentState
@@ -245,7 +266,8 @@ public class SyncthingService extends Service {
         if (ACTION_RESTART.equals(intent.getAction()) && mCurrentState == State.ACTIVE) {
             shutdown(State.INIT, () -> launchStartupTask(SyncthingRunnable.Command.main));
         } else if (ACTION_STOP.equals(intent.getAction()) && mCurrentState == State.ACTIVE) {
-            shutdown(State.DISABLED, () -> {});
+            shutdown(State.DISABLED, () -> {
+            });
         } else if (ACTION_RESET_DATABASE.equals(intent.getAction())) {
             Log.i(TAG, "Invoking reset of database");
             shutdown(State.INIT, () -> {
@@ -305,7 +327,8 @@ public class SyncthingService extends Service {
                     return;
                 }
                 Log.v(TAG, "Stopping syncthing");
-                shutdown(State.DISABLED, () -> {});
+                shutdown(State.DISABLED, () -> {
+                });
             }
         }
     }
@@ -355,6 +378,12 @@ public class SyncthingService extends Service {
             try {
                 syncthingService.mConfig = new ConfigXml(syncthingService);
                 syncthingService.mConfig.updateIfNeeded();
+            } catch (SyncthingRunnable.ExecutableNotFoundException e) {
+                syncthingService.mNotificationHandler.showCrashedNotification(R.string.config_read_failed, "SycnthingRunnable.ExecutableNotFoundException");
+                synchronized (syncthingService.mStateLock) {
+                    syncthingService.onServiceStateChange(State.ERROR);
+                }
+                cancel(true);
             } catch (ConfigXml.OpenConfigException e) {
                 syncthingService.mNotificationHandler.showCrashedNotification(R.string.config_read_failed, "ConfigXml.OpenConfigException");
                 synchronized (syncthingService.mStateLock) {
@@ -483,21 +512,22 @@ public class SyncthingService extends Service {
                     mDestroyScheduled = true;
                 } else {
                     Log.i(TAG, "Shutting down syncthing binary immediately");
-                    shutdown(State.DISABLED, () -> {});
+                    shutdown(State.DISABLED, () -> {
+                    });
                 }
             }
         } else {
             // If the storage permission got revoked, we did not start the binary and
             // are in State.INIT requiring an immediate shutdown of this service class.
             Log.i(TAG, "Shutting down syncthing binary due to missing storage permission.");
-            shutdown(State.DISABLED, () -> {});
+            shutdown(State.DISABLED, () -> {
+            });
         }
         super.onDestroy();
     }
 
     /**
      * Stop Syncthing and all helpers like event processor and api handler.
-     *
      * Sets {@link #mCurrentState} to newState, and calls onKilledListener once Syncthing is killed.
      */
     private void shutdown(State newState, OnSyncthingKilled onKilledListener) {
@@ -550,7 +580,8 @@ public class SyncthingService extends Service {
         onKilledListener.onKilled();
     }
 
-    public @Nullable RestApi getApi() {
+    public @Nullable
+    RestApi getApi() {
         return mApi;
     }
 
@@ -568,7 +599,6 @@ public class SyncthingService extends Service {
 
     /**
      * Register a listener for the syncthing API state changing.
-     *
      * The listener is called immediately with the current state, and again whenever the state
      * changes. The call is always from the GUI thread.
      *
@@ -692,7 +722,8 @@ public class SyncthingService extends Service {
         Log.v(TAG, "importConfig BEGIN");
 
         // Shutdown synchronously.
-        shutdown(State.DISABLED, () -> {});
+        shutdown(State.DISABLED, () -> {
+        });
 
         // Import config, privateKey and/or publicKey.
         try {
@@ -736,11 +767,13 @@ public class SyncthingService extends Service {
                         // Preferences that are no longer used and left-overs from previous versions of the app.
                         case "first_start":
                         case "notify_crashes":
+                            Log.v(TAG, "importConfig: Ignoring deprecated pref \"" + prefKey + "\".");
+                            break;
                         // Cached information which is not available on SettingsActivity.
                         case Constants.PREF_DEBUG_FACILITIES_AVAILABLE:
                         case Constants.PREF_EVENT_PROCESSOR_LAST_SYNC_ID:
                         case Constants.PREF_LAST_BINARY_VERSION:
-                            Log.v(TAG, "importConfig: Ignoring pref \"" + prefKey + "\".");
+                            Log.v(TAG, "importConfig: Ignoring cache pref \"" + prefKey + "\".");
                             break;
                         default:
                             Log.v(TAG, "importConfig: Adding pref \"" + prefKey + "\" to commit ...");

app/src/main/java/com/nutomic/syncthingandroid/util/ConfigXml.java

@@ -41,7 +41,6 @@ import javax.xml.transform.stream.StreamResult;
 
 /**
  * Provides direct access to the config.xml file in the file system.
- *
  * This class should only be used if the syncthing API is not available (usually during startup).
  */
 public class ConfigXml {
@@ -53,19 +52,21 @@ public class ConfigXml {
     private static final int FOLDER_ID_APPENDIX_LENGTH = 4;
 
     private final Context mContext;
-    @Inject SharedPreferences mPreferences;
+
+    @Inject
+    SharedPreferences mPreferences;
 
     private final File mConfigFile;
 
     private Document mConfig;
 
-    public ConfigXml(Context context) throws OpenConfigException {
+    public ConfigXml(Context context) throws OpenConfigException, SyncthingRunnable.ExecutableNotFoundException {
         mContext = context;
         mConfigFile = Constants.getConfigFile(mContext);
         boolean isFirstStart = !mConfigFile.exists();
         if (isFirstStart) {
             Log.i(TAG, "App started for the first time. Generating keys and config.");
-            new SyncthingRunnable(context, SyncthingRunnable.Command.generate).run();
+            new SyncthingRunnable(context, SyncthingRunnable.Command.generate).run(true);
         }
 
         readConfig();
@@ -122,7 +123,6 @@ public class ConfigXml {
 
     /**
      * Updates the config file.
-     *
      * Sets ignorePerms flag to true on every folder, force enables TLS, sets the
      * username/password, and disables weak hash checking.
      */
@@ -210,7 +210,6 @@ public class ConfigXml {
 
     /**
      * Updates syncthing options to a version specific target setting in the config file.
-     *
      * Used for one-time config migration from a lower syncthing version to the current version.
      * Enables filesystem watcher.
      * Returns if changes to the config have been made.
@@ -273,7 +272,6 @@ public class ConfigXml {
 
     /**
      * Set device model name as device name for Syncthing.
-     *
      * We need to iterate through XML nodes manually, as mConfig.getDocumentElement() will also
      * return nested elements inside folder element. We have to check that we only rename the
      * device corresponding to the local device ID.

app/src/main/res/values/strings.xml

@@ -652,6 +652,7 @@ Please report any problems you encounter via Github.</string>
     <string name="syncthing_terminated">Syncthing was terminated</string>
 
     <!-- Toast shown if syncthing failed to create or read the config -->
+    <string name="executable_not_found">Core executable \"%s\" is missing. Check build and logcat output.</string>
     <string name="config_create_failed">Failed to create configuration. Check logcat output.</string>
     <string name="config_read_failed">Failed to read configuration. Consider backing up data from your sync folders, then clear this app\'s data from Android settings and launch it again.</string>
 

syncthing/build-syncthing.py

@@ -44,17 +44,13 @@ def get_min_sdk(project_dir):
 
     fail('Failed to find minSdkVersion')
 
-
-def get_ndk_home():
-    if not os.environ.get('ANDROID_NDK_HOME', ''):
-        fail('Error: ANDROID_NDK_HOME environment variable not defined')
-    return os.environ['ANDROID_NDK_HOME']
-
 def which(program):
     import os
     def is_exe(fpath):
         return os.path.isfile(fpath) and os.access(fpath, os.X_OK)
 
+    if (sys.platform == 'win32'):
+        program += ".exe"
     fpath, fname = os.path.split(program)
     if fpath:
         if is_exe(program):
@@ -67,26 +63,40 @@ def which(program):
 
     return None
 
+def change_permissions_recursive(path, mode):
+    import os
+    for root, dirs, files in os.walk(path, topdown=False):
+        for dir in [os.path.join(root,d) for d in dirs]:
+            os.chmod(dir, mode)
+        for file in [os.path.join(root, f) for f in files]:
+            os.chmod(file, mode)
+
 def install_go():
     import os
     import tarfile
+    import zipfile
     import urllib
     import hashlib
 
     # Consts.
     pwd_path = os.path.dirname(os.path.realpath(__file__))
+    if sys.platform == 'win32':
+        url =               'https://dl.google.com/go/go1.9.7.windows-amd64.zip'
+        expected_shasum =   '8db4b21916a3bc79f48d0611202ee5814c82f671b36d5d2efcb446879456cd28'
+        tar_gz_fullfn = pwd_path + os.path.sep + 'go.zip';
+    else:
         url =               'https://dl.google.com/go/go1.9.7.linux-amd64.tar.gz'
         expected_shasum =   '88573008f4f6233b81f81d8ccf92234b4f67238df0f0ab173d75a302a1f3d6ee'
+        tar_gz_fullfn = pwd_path + os.path.sep + 'go.tgz';
 
     # Download prebuilt-go.
     url_base_name = os.path.basename(url)
-    tar_gz_fullfn = pwd_path + os.path.sep + 'go.tgz';
     if not os.path.isfile(tar_gz_fullfn):
-        print('Downloading prebuilt-go tar to:', tar_gz_fullfn)
+        print('Downloading prebuilt-go to:', tar_gz_fullfn)
         tar_gz_fullfn = urllib.urlretrieve(url, tar_gz_fullfn)[0]
-    print('Downloaded prebuilt-go tar to:', tar_gz_fullfn)
+    print('Downloaded prebuilt-go to:', tar_gz_fullfn)
 
-    # Verfiy SHA-1 checksum of downloaded files.
+    # Verfiy SHA-256 checksum of downloaded files.
     with open(tar_gz_fullfn, 'rb') as f:
         contents = f.read()
         found_shasum = hashlib.sha256(contents).hexdigest()
@@ -96,11 +106,18 @@ def install_go():
     print("[ok] Checksum of", tar_gz_fullfn, "matches expected value.")
 
     # Proceed with extraction of the prebuilt go.
-    # This will go to a subfolder "go" in the current path.
+    if not os.path.isfile(pwd_path + os.path.sep + 'go' + os.path.sep + 'LICENSE'):
         print("Extracting prebuilt-go ...")
+        # This will go to a subfolder "go" in the current path.
         file_name, file_extension = os.path.splitext(url_base_name)
+        if sys.platform == 'win32':
+            zip = zipfile.ZipFile(tar_gz_fullfn, 'r')
+            zip.extractall(pwd_path)
+            zip.close()
+        else:
             tar = tarfile.open(tar_gz_fullfn)
             tar.extractall(pwd_path)
+            tar.close()
 
     # Add (...).tar/go/bin" to the PATH.
     go_bin_path = pwd_path + os.path.sep + 'go' + os.path.sep + 'bin'
@@ -108,6 +125,65 @@ def install_go():
     os.environ["PATH"] += os.pathsep + go_bin_path
 
 
+
+
+def install_ndk():
+    import os
+    import zipfile
+    import urllib
+    import hashlib
+
+    # Consts.
+    pwd_path = os.path.dirname(os.path.realpath(__file__))
+    if sys.platform == 'win32':
+        url =               'https://dl.google.com/android/repository/android-ndk-r16b-windows-x86_64.zip'
+        expected_shasum =   'f3f1909ed1052e98dda2c79d11c22f3da28daf25'
+
+    else:
+        url =               'https://dl.google.com/android/repository/android-ndk-r16b-linux-x86_64.zip'
+        expected_shasum =   '42aa43aae89a50d1c66c3f9fdecd676936da6128'
+
+    zip_fullfn = pwd_path + os.path.sep + 'ndk.zip';
+    # Download NDK.
+    url_base_name = os.path.basename(url)
+    if not os.path.isfile(zip_fullfn):
+        print('Downloading NDK to:', zip_fullfn)
+        zip_fullfn = urllib.urlretrieve(url, zip_fullfn)[0]
+    print('Downloaded NDK to:', zip_fullfn)
+
+    # Verfiy SHA-1 checksum of downloaded files.
+    with open(zip_fullfn, 'rb') as f:
+        contents = f.read()
+        found_shasum = hashlib.sha1(contents).hexdigest()
+        print("SHA-1:", zip_fullfn, "%s" % found_shasum)
+    if found_shasum != expected_shasum:
+        fail('Error: SHA-1 checksum', found_shasum, 'of downloaded file does not match expected checksum', expected_shasum)
+    print("[ok] Checksum of", zip_fullfn, "matches expected value.")
+
+    # Proceed with extraction of the NDK if necessary.
+    ndk_home_path = pwd_path + os.path.sep + 'android-ndk-r16b'
+    if not os.path.isfile(ndk_home_path + os.path.sep + "sysroot" + os.path.sep + "NOTICE"):
+        print("Extracting NDK ...")
+        # This will go to a subfolder "android-ndk-r16b" in the current path.
+        file_name, file_extension = os.path.splitext(url_base_name)
+        zip = zipfile.ZipFile(zip_fullfn, 'r')
+        zip.extractall(pwd_path)
+        zip.close()
+
+    # Linux only - Set executable permission on files.
+    if platform.system() == 'Linux':
+        print("Setting permissions on NDK executables ...")
+        change_permissions_recursive(ndk_home_path, 0o755);
+
+    # Add "ANDROID_NDK_HOME" environment variable.
+    print('Adding ANDROID_NDK_HOME=\'' + ndk_home_path + '\'')
+    os.environ["ANDROID_NDK_HOME"] = ndk_home_path
+
+
+
+#
+# BUILD SCRIPT MAIN.
+#
 if platform.system() not in SUPPORTED_PYTHON_PLATFORMS:
     fail('Unsupported python platform %s. Supported platforms: %s', platform.system(),
          ', '.join(SUPPORTED_PYTHON_PLATFORMS))
@@ -125,12 +201,20 @@ go_bin = which("go");
 if not go_bin:
     print('Warning: go is not available on the PATH.')
     install_go();
-
     # Retry: Check if go is available.
     go_bin = which("go");
     if not go_bin:
         fail('Error: go is not available on the PATH.')
-print ('go_bin [', go_bin, ']')
+print ('go_bin=\'' + go_bin + '\'')
+
+# Check if ANDROID_NDK_HOME variable is set.
+if not os.environ.get('ANDROID_NDK_HOME', ''):
+    print('Warning: ANDROID_NDK_HOME environment variable not defined.')
+    install_ndk();
+    # Retry: Check if ANDROID_NDK_HOME variable is set.
+    if not os.environ.get('ANDROID_NDK_HOME', ''):
+        fail('Error: ANDROID_NDK_HOME environment variable not defined')
+print ('ANDROID_NDK_HOME=\'' + os.environ.get('ANDROID_NDK_HOME', '') + '\'')
 
 # Make sure all tags are available for git describe
 # https://github.com/syncthing/syncthing-android/issues/872
@@ -148,7 +232,7 @@ for target in BUILD_TARGETS:
 
     if os.environ.get('SYNCTHING_ANDROID_PREBUILT', ''):
         # The environment variable indicates the SDK and stdlib was prebuilt, set a custom paths.
-        standalone_ndk_dir = get_ndk_home() + os.path.sep + 'standalone-ndk' + os.path.sep + 'android-' + target_min_sdk + '-' + target['goarch']
+        standalone_ndk_dir = os.environ['ANDROID_NDK_HOME'] + os.path.sep + 'standalone-ndk' + os.path.sep + 'android-' + target_min_sdk + '-' + target['goarch']
         pkg_argument = []
     else:
         # Build standalone NDK toolchain if it doesn't exist.
@@ -160,7 +244,7 @@ for target in BUILD_TARGETS:
         print('Building standalone NDK for', target['arch'], 'API level', target_min_sdk, 'to', standalone_ndk_dir)
         subprocess.check_call([
             sys.executable,
-            os.path.join(get_ndk_home(), 'build', 'tools', 'make_standalone_toolchain.py'),
+            os.path.join(os.environ['ANDROID_NDK_HOME'], 'build', 'tools', 'make_standalone_toolchain.py'),
             '--arch',
             target['arch'],
             '--api',

syncthing/build.gradle

@@ -10,6 +10,7 @@ task buildNative(type: Exec) {
  */
 task cleanNative(type: Delete) {
     delete "$projectDir/../app/src/main/jniLibs/"
+    delete "android-ndk-r16b"
     delete "gobuild"
     delete "go"
 }