(fix): Corrupt datafile cache fix (#280)

* a fix for problem where the datafile cache has been corrupted or removed.

* address feedback

* fix typo in comments

Author: thomaszurkan-optimizely

android-sdk/src/main/java/com/optimizely/ab/android/sdk/OptimizelyManager.java

@@ -277,8 +277,13 @@ public void run() {
     public String getDatafile(Context context,@RawRes Integer datafileRes){
         try {
             if (isDatafileCached(context)) {
-                return datafileHandler.loadSavedDatafile(context, datafileConfig);
-            } else if (datafileRes!=null) {
+                String datafile = datafileHandler.loadSavedDatafile(context, datafileConfig);
+                if (datafile != null) {
+                    return datafile;
+                }
+            }
+
+            if (datafileRes != null) {
                 return loadRawResource(context, datafileRes);
             }else{
                 logger.error("Invalid datafile resource ID.");

datafile-handler/src/main/java/com/optimizely/ab/android/datafile_handler/DatafileLoader.java

@@ -24,6 +24,7 @@
 import android.support.annotation.RequiresApi;
 
 import com.optimizely.ab.android.shared.DatafileConfig;
+import com.optimizely.ab.android.shared.OptlyStorage;
 
 import org.slf4j.Logger;
 
@@ -110,6 +111,14 @@ private static class RequestDatafileFromClientTask extends AsyncTask<Void, Void,
 
         @Override
         protected String doInBackground(Void... params) {
+
+            // if there is a problem with the cached datafile, set last modified to 1970
+            if (!datafileCache.exists() || (datafileCache.exists() && datafileCache.load() == null)) {
+                // create a wrapper for application context default storage.
+                OptlyStorage storage = new OptlyStorage(this.datafileService.getApplicationContext());
+                // set the last modified for this url to 1 millisecond past Jan 1, 1970.
+                storage.saveLong(datafileUrl, 1);
+            }
             String dataFile = datafileClient.request(datafileUrl);
             if (dataFile != null && !dataFile.isEmpty()) {
                 if (datafileCache.exists()) {