support swapping with removable storage on android-21+

This uses the new Storage Access Framework, which was required for
accessing files on the SD Card starting in android-19.  But the API
was really limited until android-21, and not really complete until
android-23 or even android-26.  So the levels of usability will vary a
lot based on how new the version of Android is.

app/src/androidTest/java/org/fdroid/fdroid/MainActivityEspressoTest.java

@@ -1,5 +1,6 @@
 package org.fdroid.fdroid;
 
+import android.Manifest;
 import android.app.Instrumentation;
 import android.os.Build;
 import android.support.test.InstrumentationRegistry;
@@ -7,6 +8,7 @@ import android.support.test.espresso.IdlingPolicies;
 import android.support.test.espresso.ViewInteraction;
 import android.support.test.filters.LargeTest;
 import android.support.test.rule.ActivityTestRule;
+import android.support.test.rule.GrantPermissionRule;
 import android.support.test.runner.AndroidJUnit4;
 import android.support.test.uiautomator.UiDevice;
 import android.support.test.uiautomator.UiObject;
@@ -120,6 +122,14 @@ public class MainActivityEspressoTest {
     public ActivityTestRule<MainActivity> activityTestRule =
             new ActivityTestRule<>(MainActivity.class);
 
+    @Rule
+    public GrantPermissionRule accessCoarseLocationPermissionRule = GrantPermissionRule.grant(
+            Manifest.permission.ACCESS_COARSE_LOCATION);
+
+    @Rule
+    public GrantPermissionRule writeExternalStoragePermissionRule = GrantPermissionRule.grant(
+            Manifest.permission.WRITE_EXTERNAL_STORAGE);
+
     @Test
     public void bottomNavFlavorCheck() {
         onView(withText(R.string.updates)).check(matches(isDisplayed()));

app/src/basic/java/org/fdroid/fdroid/views/main/NearbyViewBinder.java

@@ -0,0 +1,10 @@
+package org.fdroid.fdroid.views.main;
+
+import android.app.Activity;
+import android.content.Intent;
+
+class NearbyViewBinder {
+    static void onActivityResult(Activity activity, Intent data) {
+        throw new IllegalStateException("unimplemented");
+    }
+}

app/src/full/AndroidManifest.xml

@@ -77,6 +77,9 @@
         <service
                 android:name=".localrepo.CacheSwapAppsService"
                 android:exported="false"/>
+        <service
+                android:name=".localrepo.TreeUriScannerIntentService"
+                android:exported="false"/>
 
         <activity
                 android:name=".views.panic.PanicPreferencesActivity"

app/src/full/java/org/fdroid/fdroid/localrepo/TreeUriScannerIntentService.java

@@ -0,0 +1,171 @@
+/*
+ * Copyright (C) 2018 Hans-Christoph Steiner <hans@eds.org>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 3
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+ * MA 02110-1301, USA.
+ */
+
+package org.fdroid.fdroid.localrepo;
+
+import android.annotation.TargetApi;
+import android.app.IntentService;
+import android.content.Context;
+import android.content.Intent;
+import android.net.Uri;
+import android.os.Process;
+import android.support.v4.provider.DocumentFile;
+import android.util.Log;
+import org.apache.commons.io.FileUtils;
+import org.apache.commons.io.IOUtils;
+import org.fdroid.fdroid.IndexV1Updater;
+import org.fdroid.fdroid.IndexUpdater;
+import org.fdroid.fdroid.Utils;
+import org.fdroid.fdroid.data.Repo;
+import org.fdroid.fdroid.data.RepoProvider;
+import org.fdroid.fdroid.views.main.MainActivity;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
+import java.security.cert.Certificate;
+import java.util.jar.JarEntry;
+import java.util.jar.JarFile;
+import java.util.jar.JarInputStream;
+
+/**
+ * An {@link IntentService} subclass for handling asynchronous scanning of a
+ * removable storage device like an SD Card or USB OTG thumb drive using the
+ * Storage Access Framework.  Permission must first be granted by the user
+ * {@link android.content.Intent#ACTION_OPEN_DOCUMENT_TREE} or
+ * {@link android.os.storage.StorageVolume#createAccessIntent(String)}request,
+ * then F-Droid will have permanent access to that{@link Uri}.
+ * <p>
+ * Even though the Storage Access Framework was introduced in
+ * {@link android.os.Build.VERSION_CODES#KITKAT android-19}, this approach is only
+ * workable if {@link android.content.Intent#ACTION_OPEN_DOCUMENT_TREE} is available.
+ * It was added in {@link android.os.Build.VERSION_CODES#LOLLIPOP android-21}.
+ *
+ * @see <a href="https://commonsware.com/blog/2017/11/15/storage-situation-removable-storage.html"> The Storage Situation: Removable Storage </a>
+ * @see <a href="https://developer.android.com/training/articles/scoped-directory-access.html">Using Scoped Directory Access</a>
+ * @see <a href="https://developer.android.com/guide/topics/providers/document-provider.html">Open Files using Storage Access Framework</a>
+ */
+@TargetApi(21)
+public class TreeUriScannerIntentService extends IntentService {
+    public static final String TAG = "TreeUriScannerIntentSer";
+
+    private static final String ACTION_SCAN_TREE_URI = "org.fdroid.fdroid.localrepo.action.SCAN_TREE_URI";
+
+    public TreeUriScannerIntentService() {
+        super("TreeUriScannerIntentService");
+    }
+
+    public static void scan(Context context, Uri data) {
+        Intent intent = new Intent(context, TreeUriScannerIntentService.class);
+        intent.setAction(ACTION_SCAN_TREE_URI);
+        intent.setData(data);
+        context.startService(intent);
+    }
+
+    @Override
+    protected void onHandleIntent(Intent intent) {
+        if (intent == null || !ACTION_SCAN_TREE_URI.equals(intent.getAction())) {
+            return;
+        }
+        Uri treeUri = intent.getData();
+        if (treeUri == null) {
+            return;
+        }
+        Process.setThreadPriority(Process.THREAD_PRIORITY_LOWEST);
+        DocumentFile treeFile = DocumentFile.fromTreeUri(this, treeUri);
+        searchDirectory(treeFile);
+    }
+
+    private void searchDirectory(DocumentFile documentFileDir) {
+        DocumentFile[] documentFiles = documentFileDir.listFiles();
+        if (documentFiles == null) {
+            return;
+        }
+        for (DocumentFile documentFile : documentFiles) {
+            if (documentFile.isDirectory()) {
+                searchDirectory(documentFile);
+            } else {
+                if (IndexV1Updater.SIGNED_FILE_NAME.equals(documentFile.getName())) {
+                    registerRepo(documentFile);
+                }
+            }
+        }
+    }
+
+    /**
+     * For all files called {@link IndexV1Updater#SIGNED_FILE_NAME} found, check
+     * the JAR signature and read the fingerprint of the signing certificate.
+     * The fingerprint is then used to find whether this local repo is a mirror
+     * of an existing repo, or a totally new repo.  In order to verify the
+     * signatures in the JAR, the whole file needs to be read in first.
+     *
+     * @see JarInputStream#JarInputStream(InputStream, boolean)
+     */
+    private void registerRepo(DocumentFile index) {
+        InputStream inputStream = null;
+        try {
+            Log.i(TAG, "FOUND: " + index.getUri());
+            inputStream = getContentResolver().openInputStream(index.getUri());
+            Log.i(TAG, "repo URL: " + index.getParentFile().getUri());
+            registerRepo(this, inputStream, index.getParentFile().getUri());
+        } catch (IOException | IndexUpdater.SigningException e) {
+            e.printStackTrace();
+        } finally {
+            Utils.closeQuietly(inputStream);
+        }
+    }
+
+    public static void registerRepo(Context context, InputStream inputStream, Uri repoUri)
+            throws IOException, IndexUpdater.SigningException {
+        if (inputStream == null) {
+            return;
+        }
+        File destFile = File.createTempFile("dl-", IndexV1Updater.SIGNED_FILE_NAME, context.getCacheDir());
+        FileUtils.copyInputStreamToFile(inputStream, destFile);
+        JarFile jarFile = new JarFile(destFile, true);
+        JarEntry indexEntry = (JarEntry) jarFile.getEntry(IndexV1Updater.DATA_FILE_NAME);
+        IOUtils.readLines(jarFile.getInputStream(indexEntry));
+        Certificate certificate = IndexUpdater.getSigningCertFromJar(indexEntry);
+        Log.i(TAG, "Got certificate: " + certificate);
+        String fingerprint = Utils.calcFingerprint(certificate);
+        Log.i(TAG, "Got fingerprint: " + fingerprint);
+        destFile.delete();
+
+        Log.i(TAG, "Found a valid, signed index-v1.json");
+        for (Repo repo : RepoProvider.Helper.all(context)) {
+            if (fingerprint.equals(repo.fingerprint)) {
+                Log.i(TAG, repo.address + " has the SAME fingerprint: " + fingerprint);
+            } else {
+                Log.i(TAG, repo.address + " different fingerprint");
+            }
+        }
+
+        Intent intent = new Intent(context, MainActivity.class);
+        intent.setAction(Intent.ACTION_VIEW);
+        intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
+        intent.setData(repoUri.buildUpon()
+                .appendQueryParameter("fingerprint", fingerprint)
+                .build());
+        context.startActivity(intent);
+        // TODO parse repo URL/mirrors/fingerprint using Jackson
+        //      https://stackoverflow.com/questions/24835431/use-jackson-to-stream-parse-an-array-of-json-objects#
+        // TODO rework IndexUpdater.getSigningCertFromJar to work for here
+        // TODO check whether fingerprint is already in the database
+    }
+}

app/src/full/java/org/fdroid/fdroid/views/main/MainViewController.java

@@ -1,17 +1,12 @@
 package org.fdroid.fdroid.views.main;
 
-import android.content.Intent;
 import android.support.annotation.Nullable;
 import android.support.v4.app.Fragment;
 import android.support.v7.app.AppCompatActivity;
 import android.support.v7.widget.RecyclerView;
-import android.view.View;
-import android.widget.Button;
 import android.widget.FrameLayout;
-import android.widget.TextView;
 import org.fdroid.fdroid.R;
 import org.fdroid.fdroid.views.PreferencesFragment;
-import org.fdroid.fdroid.views.swap.SwapWorkflowActivity;
 import org.fdroid.fdroid.views.updates.UpdatesViewBinder;
 
 /**
@@ -65,29 +60,8 @@ class MainViewController extends RecyclerView.ViewHolder {
         new CategoriesViewBinder(activity, frame);
     }
 
-    /**
-     * A splash screen encouraging people to start the swap process.
-     * The swap process is quite heavy duty in that it fires up Bluetooth and/or WiFi in
-     * order to scan for peers. As such, it is quite convenient to have a more lightweight view to show
-     * in the main navigation that doesn't automatically start doing things when the user touches the
-     * navigation menu in the bottom navigation.
-     */
     public void bindSwapView() {
-        View swapView = activity.getLayoutInflater().inflate(R.layout.main_tab_swap, frame, true);
+        new NearbyViewBinder(activity, frame);
-
-        // To allow for whitelabel versions of F-Droid, make sure not to hardcode "F-Droid" into our
-        // translation here.
-        TextView subtext = (TextView) swapView.findViewById(R.id.text2);
-        subtext.setText(activity.getString(R.string.nearby_splash__both_parties_need_fdroid,
-                activity.getString(R.string.app_name)));
-
-        Button startButton = (Button) swapView.findViewById(R.id.button);
-        startButton.setOnClickListener(new View.OnClickListener() {
-            @Override
-            public void onClick(View v) {
-                activity.startActivity(new Intent(activity, SwapWorkflowActivity.class));
-            }
-        });
     }
 
     /**

app/src/full/java/org/fdroid/fdroid/views/main/NearbyViewBinder.java

@@ -0,0 +1,124 @@
+package org.fdroid.fdroid.views.main;
+
+import android.Manifest;
+import android.app.Activity;
+import android.content.Intent;
+import android.content.pm.PackageManager;
+import android.os.Build;
+import android.os.Environment;
+import android.support.annotation.RequiresApi;
+import android.support.v4.app.ActivityCompat;
+import android.support.v4.content.ContextCompat;
+import android.util.Log;
+import android.view.View;
+import android.widget.Button;
+import android.widget.FrameLayout;
+import android.widget.ImageView;
+import android.widget.TextView;
+import android.widget.Toast;
+import org.fdroid.fdroid.R;
+import org.fdroid.fdroid.localrepo.TreeUriScannerIntentService;
+import org.fdroid.fdroid.views.swap.SwapWorkflowActivity;
+
+import java.io.File;
+
+/**
+ * A splash screen encouraging people to start the swap process. The swap
+ * process is quite heavy duty in that it fires up Bluetooth and/or WiFi
+ * in  order to scan for peers. As such, it is quite convenient to have a
+ * more lightweight view to show in the main navigation that doesn't
+ * automatically start doing things when the user touches the navigation
+ * menu in the bottom navigation.
+ * <p>
+ * Lots of pieces of the nearby/swap functionality require that the user grant
+ * F-Droid permissions at runtime on {@code android-23} and higher. On devices
+ * that have a removable SD Card that is currently mounted, this will request
+ * permission to read it, so that F-Droid can look for repos on the SD Card.
+ * <p>
+ * Once {@link Manifest.permission#READ_EXTERNAL_STORAGE} or
+ * {@link Manifest.permission#WRITE_EXTERNAL_STORAGE} is granted for F-Droid,
+ * then it can read any file on an SD Card and no more prompts are needed. For
+ * USB OTG drives, the only way to get read permissions is to prompt the user
+ * via {@link Intent#ACTION_OPEN_DOCUMENT_TREE}.
+ * <p>
+ * For write permissions, {@code android-19} and {@code android-20} devices are
+ * basically screwed here.  {@link Intent#ACTION_OPEN_DOCUMENT_TREE} was added
+ * in {@code android-21}, and there does not seem to be any other way to get
+ * write access to the the removable storage.
+ *
+ * @see TreeUriScannerIntentService
+ */
+class NearbyViewBinder {
+    public static final String TAG = "NearbyViewBinder";
+
+    static File externalStorage = null;
+
+    NearbyViewBinder(final Activity activity, FrameLayout parent) {
+        View swapView = activity.getLayoutInflater().inflate(R.layout.main_tab_swap, parent, true);
+
+        TextView subtext = swapView.findViewById(R.id.text2);
+        subtext.setText(activity.getString(R.string.nearby_splash__both_parties_need_fdroid,
+                activity.getString(R.string.app_name)));
+
+        ImageView nearbySplash = swapView.findViewById(R.id.image);
+
+        Button startButton = swapView.findViewById(R.id.button);
+        startButton.setOnClickListener(new View.OnClickListener() {
+            @Override
+            public void onClick(View v) {
+                activity.startActivity(new Intent(activity, SwapWorkflowActivity.class));
+            }
+        });
+
+        if (Build.VERSION.SDK_INT >= 21) {
+            Log.i(TAG, "Environment.isExternalStorageRemovable(activity.getExternalFilesDir(\"\")) " +
+                    Environment.isExternalStorageRemovable(activity.getExternalFilesDir("")));
+            File[] dirs = activity.getExternalFilesDirs("");
+            if (dirs != null) {
+                for (File f : dirs) {
+                    if (f != null && Environment.isExternalStorageRemovable(f)) {
+                        // remove Android/data/org.fdroid.fdroid/files to get root
+                        externalStorage = f.getParentFile().getParentFile().getParentFile().getParentFile();
+                        break;
+                    }
+                }
+            }
+        } else if (Environment.isExternalStorageRemovable() &&
+                (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED_READ_ONLY)
+                        || Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED))) {
+            Log.i(TAG, "<21 isExternalStorageRemovable MEDIA_MOUNTED");
+            externalStorage = Environment.getExternalStorageDirectory();
+        }
+
+        if (externalStorage != null) {
+            nearbySplash.setVisibility(View.GONE);
+            View readExternalStorage = swapView.findViewById(R.id.readExternalStorage);
+            readExternalStorage.setVisibility(View.VISIBLE);
+            Button requestReadExternalStorage = swapView.findViewById(R.id.requestReadExternalStorage);
+            requestReadExternalStorage.setOnClickListener(new View.OnClickListener() {
+                @RequiresApi(api = 21)
+                @Override
+                public void onClick(View v) {
+                    File storage = externalStorage.getParentFile();
+                    File[] files = storage.listFiles();
+                    String msg = "";
+                    if (files != null) for (File f : files) {
+                        msg += "|" + f.getName();
+                    }
+                    Toast.makeText(activity, msg, Toast.LENGTH_LONG).show();
+                    final String writeExternalStorage = Manifest.permission.WRITE_EXTERNAL_STORAGE;
+                    if (Build.VERSION.SDK_INT >= 23
+                            && !externalStorage.canRead()
+                            && PackageManager.PERMISSION_GRANTED
+                            != ContextCompat.checkSelfPermission(activity, writeExternalStorage)) {
+                        ActivityCompat.requestPermissions(activity, new String[]{writeExternalStorage},
+                                MainActivity.REQUEST_STORAGE_PERMISSIONS);
+                    } else {
+                        // TODO do something
+                    }
+                }
+            });
+
+        }
+    }
+}

app/src/full/res/layout/main_tab_swap.xml

@@ -54,6 +54,37 @@
         android:layout_marginStart="48dp"
         android:layout_marginLeft="48dp" />
 
+    <LinearLayout
+            android:id="@+id/readExternalStorage"
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content"
+            android:visibility="gone"
+            android:orientation="horizontal"
+            app:layout_constraintTop_toBottomOf="@+id/text2"
+            app:layout_constraintRight_toRightOf="parent"
+            app:layout_constraintLeft_toLeftOf="parent"
+            android:layout_marginTop="48dp"
+            android:layout_marginEnd="24dp"
+            android:layout_marginRight="24dp"
+            android:layout_marginStart="24dp"
+            android:layout_marginLeft="24dp">
+        <TextView
+                android:layout_width="wrap_content"
+                android:layout_height="match_parent"
+                android:text="@string/nearby_splash__read_external_storage"
+                android:gravity="fill"
+                android:paddingRight="5dp"
+                android:paddingEnd="5dp"
+                android:textSize="17sp"
+                android:textColor="?attr/lightGrayTextColor"/>
+        <Button
+                android:id="@+id/requestReadExternalStorage"
+                android:layout_width="80dp"
+                android:layout_height="match_parent"
+                android:text="@string/nearby_splash__request_permission"
+                style="@style/SwapTheme.Wizard.OptionButton"/>
+    </LinearLayout>
+
     <ImageView
         android:id="@+id/image"
         android:layout_width="0dp"

app/src/main/java/org/fdroid/fdroid/IndexUpdater.java

@@ -43,6 +43,7 @@ import org.fdroid.fdroid.installer.InstallManagerService;
 import org.fdroid.fdroid.installer.InstallerService;
 import org.fdroid.fdroid.net.Downloader;
 import org.fdroid.fdroid.net.DownloaderFactory;
+import org.fdroid.fdroid.net.TreeUriDownloader;
 import org.xml.sax.InputSource;
 import org.xml.sax.SAXException;
 import org.xml.sax.XMLReader;
@@ -114,7 +115,11 @@ public class IndexUpdater {
     }
 
     protected String getIndexUrl(@NonNull Repo repo) {
-        return repo.address + "/index.jar";
+        if (repo.address.startsWith("content://")) {
+            return repo.address + TreeUriDownloader.ESCAPED_SLASH + SIGNED_FILE_NAME;
+        } else {
+            return repo.address + "/" + SIGNED_FILE_NAME;
+        }
     }
 
     public boolean hasChanged() {

app/src/main/java/org/fdroid/fdroid/IndexV1Updater.java

@@ -89,7 +89,7 @@ import java.util.jar.JarFile;
 public class IndexV1Updater extends IndexUpdater {
     public static final String TAG = "IndexV1Updater";
 
-    private static final String SIGNED_FILE_NAME = "index-v1.jar";
+    public static final String SIGNED_FILE_NAME = "index-v1.jar";
     public static final String DATA_FILE_NAME = "index-v1.json";
 
     public IndexV1Updater(@NonNull Context context, @NonNull Repo repo) {
@@ -97,8 +97,15 @@ public class IndexV1Updater extends IndexUpdater {
     }
 
     @Override
+    /**
+     * Storage Access Framework URLs have a crazy encoded path within the URL path.
+     */
     protected String getIndexUrl(@NonNull Repo repo) {
-        return Uri.parse(repo.address).buildUpon().appendPath(SIGNED_FILE_NAME).build().toString();
+        if (repo.address.startsWith("content://")) {
+            return repo.address + "%2F" + SIGNED_FILE_NAME;
+        } else {
+            return Uri.parse(repo.address).buildUpon().appendPath(SIGNED_FILE_NAME).build().toString();
+        }
     }
 
     /**

app/src/main/java/org/fdroid/fdroid/data/NewRepoConfig.java

@@ -82,7 +82,7 @@ public class NewRepoConfig {
         host = host.toLowerCase(Locale.ENGLISH);
 
         if (uri.getPath() == null
-                || !Arrays.asList("https", "http", "fdroidrepos", "fdroidrepo").contains(scheme)) {
+                || !Arrays.asList("https", "http", "fdroidrepos", "fdroidrepo", "content").contains(scheme)) {
             isValidRepo = false;
             return;
         }

app/src/main/java/org/fdroid/fdroid/net/DownloaderFactory.java

@@ -31,6 +31,8 @@ public class DownloaderFactory {
         String scheme = uri.getScheme();
         if ("bluetooth".equals(scheme)) {
             downloader = new BluetoothDownloader(uri, destFile);
+        } else if ("content".equals(scheme)) {
+            downloader = new TreeUriDownloader(uri, destFile);
         } else {
             final String[] projection = {Schema.RepoTable.Cols.USERNAME, Schema.RepoTable.Cols.PASSWORD};
             Repo repo = RepoProvider.Helper.findByUrl(context, uri, projection);

app/src/main/java/org/fdroid/fdroid/net/ImageLoaderForUIL.java

@@ -1,6 +1,7 @@
 package org.fdroid.fdroid.net;
 
 import android.content.Context;
+import android.os.Build;
 import com.nostra13.universalimageloader.core.download.BaseImageDownloader;
 
 import java.io.IOException;
@@ -28,6 +29,10 @@ public class ImageLoaderForUIL implements com.nostra13.universalimageloader.core
             case HTTP:
             case HTTPS:
                 return DownloaderFactory.create(context, imageUri).getInputStream();
+            case CONTENT:
+                if (Build.VERSION.SDK_INT >= 19) {
+                    return DownloaderFactory.create(context, imageUri).getInputStream();
+                }
         }
         return new BaseImageDownloader(context).getStream(imageUri, extra);
     }

app/src/main/java/org/fdroid/fdroid/net/TreeUriDownloader.java

@@ -0,0 +1,105 @@
+package org.fdroid.fdroid.net;
+
+import android.annotation.TargetApi;
+import android.content.Context;
+import android.net.Uri;
+import android.support.v4.provider.DocumentFile;
+import org.fdroid.fdroid.FDroidApp;
+
+import java.io.BufferedInputStream;
+import java.io.File;
+import java.io.FileNotFoundException;
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.MalformedURLException;
+import java.net.ProtocolException;
+
+/**
+ * An {@link Downloader} subclass for downloading files from a repo on a
+ * removable storage device like an SD Card or USB OTG thumb drive using the
+ * Storage Access Framework.  Permission must first be granted by the user via a
+ * {@link android.content.Intent#ACTION_OPEN_DOCUMENT_TREE} or
+ * {@link android.os.storage.StorageVolume#createAccessIntent(String)}request,
+ * then F-Droid will have permanent access to that{@link android.net.Uri}.
+ * <p>
+ * The base repo URL of such a repo looks like:
+ * {@code content://com.android.externalstorage.documents/tree/1AFB-2402%3A/document/1AFB-2402%3Atesty.at.or.at%2Ffdroid%2Frepo}
+ *
+ * @see android.support.v4.provider.DocumentFile#fromTreeUri(Context, Uri)
+ * @see <a href="https://developer.android.com/guide/topics/providers/document-provider.html">Open Files using Storage Access Framework</a>
+ * @see <a href="https://developer.android.com/training/articles/scoped-directory-access.html">Using Scoped Directory Access</a>
+ */
+@TargetApi(21)
+public class TreeUriDownloader extends Downloader {
+    public static final String TAG = "TreeUriDownloader";
+
+    /**
+     * Whoever designed this {@link android.provider.DocumentsContract#isTreeUri(Uri) URI system}
+     * was smoking crack, it escapes <b>part</b> of the URI path, but not all.
+     * So crazy tricks are required.
+     */
+    public static final String ESCAPED_SLASH = "%2F";
+
+    private final Context context;
+    private final Uri treeUri;
+    private final DocumentFile documentFile;
+
+    TreeUriDownloader(Uri uri, File destFile)
+            throws FileNotFoundException, MalformedURLException {
+        super(uri, destFile);
+        context = FDroidApp.getInstance();
+        String path = uri.getEncodedPath();
+        int lastEscapedSlash = path.lastIndexOf(ESCAPED_SLASH);
+        String pathChunkToEscape = path.substring(lastEscapedSlash + ESCAPED_SLASH.length());
+        String escapedPathChunk = Uri.encode(pathChunkToEscape);
+        treeUri = uri.buildUpon().encodedPath(path.replace(pathChunkToEscape, escapedPathChunk)).build();
+        documentFile = DocumentFile.fromTreeUri(context, treeUri);
+    }
+
+    /**
+     * This needs to convert {@link FileNotFoundException} and
+     * {@link IllegalArgumentException} to {@link ProtocolException} since the mirror
+     * failover logic expects network errors, not filesystem or other errors.
+     * In the downloading logic, filesystem errors are related to the file as it is
+     * being downloaded and written to disk.  Things can fail here if the USB stick is
+     * not longer plugged in, the files were deleted by some other process, etc.
+     * <p>
+     * Example: {@code IllegalArgumentException: Failed to determine if
+     * 6EED-6A10:guardianproject.info/wind-demo/fdroid/repo/index-v1.jar is child of
+     * 6EED-6A10:: java.io.File NotFoundException: No root for 6EED-6A10}
+     * <p>
+     * Example:
+     */
+    @Override
+    protected InputStream getDownloadersInputStream() throws IOException {
+        try {
+            InputStream inputStream = context.getContentResolver().openInputStream(treeUri);
+            if (inputStream == null) {
+                return null;
+            } else {
+                return new BufferedInputStream(inputStream);
+            }
+        } catch (FileNotFoundException | IllegalArgumentException e) {
+            throw new ProtocolException(e.getLocalizedMessage());
+        }
+    }
+
+    @Override
+    public boolean hasChanged() {
+        return true;  // TODO how should this actually be implemented?
+    }
+
+    @Override
+    protected long totalDownloadSize() {
+        return documentFile.length();
+    }
+
+    @Override
+    public void download() throws IOException, InterruptedException {
+        downloadFromStream(8192, false);
+    }
+
+    @Override
+    protected void close() {
+    }
+}

app/src/main/java/org/fdroid/fdroid/views/ManageReposActivity.java

@@ -22,6 +22,7 @@ package org.fdroid.fdroid.views;
 import android.annotation.SuppressLint;
 import android.content.ClipData;
 import android.content.ClipboardManager;
+import android.content.ContentResolver;
 import android.content.ContentValues;
 import android.content.Context;
 import android.content.DialogInterface;
@@ -555,6 +556,11 @@ public class ManageReposActivity extends AppCompatActivity
                         return originalAddress;
                     }
 
+                    if (originalAddress.startsWith(ContentResolver.SCHEME_CONTENT)) {
+                        // TODO check whether there is read access
+                        return originalAddress;
+                    }
+
                     final String[] pathsToCheck = {"", "fdroid/repo", "repo"};
                     for (final String path : pathsToCheck) {
 

app/src/main/java/org/fdroid/fdroid/views/main/MainActivity.java

@@ -41,7 +41,6 @@ import android.widget.Toast;
 import com.ashokvarma.bottomnavigation.BottomNavigationBar;
 import com.ashokvarma.bottomnavigation.BottomNavigationItem;
 import com.ashokvarma.bottomnavigation.TextBadgeItem;
-import org.fdroid.fdroid.views.AppDetailsActivity;
 import org.fdroid.fdroid.AppUpdateStatusManager;
 import org.fdroid.fdroid.AppUpdateStatusManager.AppUpdateStatus;
 import org.fdroid.fdroid.BuildConfig;
@@ -52,6 +51,7 @@ import org.fdroid.fdroid.R;
 import org.fdroid.fdroid.UpdateService;
 import org.fdroid.fdroid.Utils;
 import org.fdroid.fdroid.data.NewRepoConfig;
+import org.fdroid.fdroid.views.AppDetailsActivity;
 import org.fdroid.fdroid.views.ManageReposActivity;
 import org.fdroid.fdroid.views.apps.AppListActivity;
 import org.fdroid.fdroid.views.swap.SwapWorkflowActivity;
@@ -77,6 +77,8 @@ public class MainActivity extends AppCompatActivity implements BottomNavigationB
     public static final String EXTRA_VIEW_UPDATES = "org.fdroid.fdroid.views.main.MainActivity.VIEW_UPDATES";
     public static final String EXTRA_VIEW_SETTINGS = "org.fdroid.fdroid.views.main.MainActivity.VIEW_SETTINGS";
 
+    static final int REQUEST_STORAGE_PERMISSIONS = 0xB004;
+
     private static final String ADD_REPO_INTENT_HANDLED = "addRepoIntentHandled";
 
     private static final String ACTION_ADD_REPO = "org.fdroid.fdroid.MainActivity.ACTION_ADD_REPO";

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

@@ -406,6 +406,8 @@ This often occurs with apps installed via Google Play or other sources, if they
     <string name="nearby_splash__download_apps_from_people_nearby">No Internet? Get apps from people near you!</string>
     <string name="nearby_splash__find_people_button">Find people nearby</string>
     <string name="nearby_splash__both_parties_need_fdroid">Both parties need %1$s to use nearby.</string>
+    <string name="nearby_splash__read_external_storage">SD Cards can be used to swap!</string>
+    <string name="nearby_splash__request_permission">Try it</string>
 
     <string name="swap_nfc_title">Touch to swap</string>
     <string name="swap_nfc_description">If your friend has F-Droid and NFC turned on touch your devices together.
@@ -452,6 +454,10 @@ This often occurs with apps installed via Google Play or other sources, if they
     <string name="swap_connection_misc_error">Error occurred while connecting to device, can\'t swap with it!</string>
     <string name="swap_not_enabled">Swapping not enabled</string>
     <string name="swap_not_enabled_description">Before swapping, your device must be made visible.</string>
+    <string name="swap_toast_using_path">Using %1$s</string>
+    <string name="swap_toast_not_removable_storage">That choice did not match any removeable storage devices, try
+        again!</string>
+    <string name="swap_toast_find_removeable_storage">Choose your removeable SD Card or USB</string>
     <string name="swap_toast_invalid_url">Invalid URL for swapping: %1$s</string>
     <string name="swap_toast_hotspot_enabled">Wi-Fi Hotspot enabled</string>
     <string name="swap_toast_could_not_enable_hotspot">Could not enable Wi-Fi Hotspot!</string>

config/checkstyle/checkstyle.xml

@@ -17,7 +17,7 @@
         <module name="FileContentsHolder" />
         <module name="LineLength">
             <property name="max" value="118"/>
-            <property name="ignorePattern" value="https?://"/>
+            <property name="ignorePattern" value="[a-z]+://"/>
         </module>
 
         <module name="ConstantName">