feat: add Android comptible FileDataStoreFactory implementation

This is branched from the original implementation prior to switching to
NIO for Windows compatibility.

google-http-client-android/src/main/java/com/google/api/client/extensions/android/util/store/FileDataStoreFactory.java

@@ -0,0 +1,161 @@
+/*
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.api.client.extensions.android.util.store;
+
+import com.google.api.client.util.IOUtils;
+import com.google.api.client.util.Maps;
+import com.google.api.client.util.Throwables;
+import com.google.api.client.util.store.AbstractDataStoreFactory;
+import com.google.api.client.util.store.AbstractMemoryDataStore;
+import com.google.api.client.util.store.DataStore;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.Serializable;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.util.logging.Logger;
+
+/**
+ * Thread-safe file implementation of a credential store.
+ *
+ * <p>For security purposes, the file's permissions are set to be accessible only by the file's
+ * owner. Note that Java 1.5 does not support manipulating file permissions, and must be done
+ * manually or using the JNI.
+ *
+ * <p>Note: This class was branched from the primary implementation in google-http-client to allow
+ * the mainline implementation to support Windows file permissions.
+ *
+ * @since 1.36
+ * @author Yaniv Inbar
+ */
+public class FileDataStoreFactory extends AbstractDataStoreFactory {
+
+  private static final Logger LOGGER = Logger.getLogger(FileDataStoreFactory.class.getName());
+
+  /** Directory to store data. */
+  private final File dataDirectory;
+
+  /** @param dataDirectory data directory */
+  public FileDataStoreFactory(File dataDirectory) throws IOException {
+    dataDirectory = dataDirectory.getCanonicalFile();
+    this.dataDirectory = dataDirectory;
+    // error if it is a symbolic link
+    if (IOUtils.isSymbolicLink(dataDirectory)) {
+      throw new IOException("unable to use a symbolic link: " + dataDirectory);
+    }
+    // create parent directory (if necessary)
+    if (!dataDirectory.exists() && !dataDirectory.mkdirs()) {
+      throw new IOException("unable to create directory: " + dataDirectory);
+    }
+    setPermissionsToOwnerOnly(dataDirectory);
+  }
+
+  /** Returns the data directory. */
+  public final File getDataDirectory() {
+    return dataDirectory;
+  }
+
+  @Override
+  protected <V extends Serializable> DataStore<V> createDataStore(String id) throws IOException {
+    return new FileDataStore<V>(this, dataDirectory, id);
+  }
+
+  /**
+   * File data store that inherits from the abstract memory data store because the key-value pairs
+   * are stored in a memory cache, and saved in the file (see {@link #save()} when changing values.
+   *
+   * @param <V> serializable type of the mapped value
+   */
+  static class FileDataStore<V extends Serializable> extends AbstractMemoryDataStore<V> {
+
+    /** File to store data. */
+    private final File dataFile;
+
+    FileDataStore(FileDataStoreFactory dataStore, File dataDirectory, String id)
+        throws IOException {
+      super(dataStore, id);
+      this.dataFile = new File(dataDirectory, id);
+      // error if it is a symbolic link
+      if (IOUtils.isSymbolicLink(dataFile)) {
+        throw new IOException("unable to use a symbolic link: " + dataFile);
+      }
+      // create new file (if necessary)
+      if (dataFile.createNewFile()) {
+        keyValueMap = Maps.newHashMap();
+        // save the credentials to create a new file
+        save();
+      } else {
+        // load credentials from existing file
+        keyValueMap = IOUtils.deserialize(new FileInputStream(dataFile));
+      }
+    }
+
+    @Override
+    public void save() throws IOException {
+      IOUtils.serialize(keyValueMap, new FileOutputStream(dataFile));
+    }
+
+    @Override
+    public FileDataStoreFactory getDataStoreFactory() {
+      return (FileDataStoreFactory) super.getDataStoreFactory();
+    }
+  }
+
+  /**
+   * Attempts to set the given file's permissions such that it can only be read, written, and
+   * executed by the file's owner.
+   *
+   * @param file the file's permissions to modify
+   * @throws IOException
+   */
+  static void setPermissionsToOwnerOnly(File file) throws IOException {
+    // Disable access by other users if O/S allows it and set file permissions to readable and
+    // writable by user. Use reflection since JDK 1.5 will not have these methods
+    try {
+      Method setReadable = File.class.getMethod("setReadable", boolean.class, boolean.class);
+      Method setWritable = File.class.getMethod("setWritable", boolean.class, boolean.class);
+      Method setExecutable = File.class.getMethod("setExecutable", boolean.class, boolean.class);
+      if (!(Boolean) setReadable.invoke(file, false, false)
+          || !(Boolean) setWritable.invoke(file, false, false)
+          || !(Boolean) setExecutable.invoke(file, false, false)) {
+        LOGGER.warning("unable to change permissions for everybody: " + file);
+      }
+      if (!(Boolean) setReadable.invoke(file, true, true)
+          || !(Boolean) setWritable.invoke(file, true, true)
+          || !(Boolean) setExecutable.invoke(file, true, true)) {
+        LOGGER.warning("unable to change permissions for owner: " + file);
+      }
+    } catch (InvocationTargetException exception) {
+      Throwable cause = exception.getCause();
+      Throwables.propagateIfPossible(cause, IOException.class);
+      // shouldn't reach this point, but just in case...
+      throw new RuntimeException(cause);
+    } catch (NoSuchMethodException exception) {
+      LOGGER.warning(
+          "Unable to set permissions for "
+              + file
+              + ", likely because you are running a version of Java prior to 1.6");
+    } catch (SecurityException exception) {
+      // ignored
+    } catch (IllegalAccessException exception) {
+      // ignored
+    } catch (IllegalArgumentException exception) {
+      // ignored
+    }
+  }
+}

google-http-client/src/main/java/com/google/api/client/util/store/FileDataStoreFactory.java

@@ -45,6 +45,10 @@
  * <p>For security purposes, the file's permissions are set such that the file is only accessible by
  * the file's owner.
  *
+ * <p>Note: this class is not compatible with Android lower than API level 26 (Oreo). For an implementation
+ * compatible with Android < 26, please use com.google.api.client.extensions.android.util.store.FileDataStoreFactory
+ * which is provided by com.google.http-client:google-http-client-android.
+ *
  * @since 1.16
  * @author Yaniv Inbar
  */