maintain one ContactsDatabase instance per Loader

Fixes #3004
Closes #3041
Closes #3034

// FREEBIE

src/org/thoughtcrime/securesms/PushContactSelectionListFragment.java

@@ -86,12 +86,6 @@ public class PushContactSelectionListFragment extends    Fragment
     super.onPause();
   }
 
-  @Override
-  public void onDestroyView() {
-    super.onDestroyView();
-    ContactsDatabase.destroyInstance();
-  }
-
   @Override
   public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
     return inflater.inflate(R.layout.push_contact_selection_list_activity, container, false);

src/org/thoughtcrime/securesms/contacts/ContactsCursorLoader.java

@@ -19,6 +19,11 @@ package org.thoughtcrime.securesms.contacts;
 import android.content.Context;
 import android.database.Cursor;
 import android.support.v4.content.CursorLoader;
+import android.util.Log;
+
+import junit.framework.Assert;
+
+import java.util.concurrent.Semaphore;
 
 /**
  * CursorLoader that initializes a ContactsDatabase instance
@@ -26,10 +31,13 @@ import android.support.v4.content.CursorLoader;
  * @author Jake McGinty
  */
 public class ContactsCursorLoader extends CursorLoader {
+  private static final String TAG        = ContactsCursorLoader.class.getSimpleName();
+  private static final int    DB_PERMITS = 100;
 
   private final Context          context;
   private final String           filter;
   private final boolean          pushOnly;
+  private final Semaphore        dbSemaphore = new Semaphore(DB_PERMITS);
   private       ContactsDatabase db;
 
   public ContactsCursorLoader(Context context, String filter, boolean pushOnly) {
@@ -37,17 +45,33 @@ public class ContactsCursorLoader extends CursorLoader {
     this.context  = context;
     this.filter   = filter;
     this.pushOnly = pushOnly;
+    this.db       = new ContactsDatabase(context);
   }
 
   @Override
   public Cursor loadInBackground() {
-    ContactsDatabase.destroyInstance();
-    db = ContactsDatabase.getInstance(context);
+    try {
+      dbSemaphore.acquire();
       return db.query(filter, pushOnly);
+    } catch (InterruptedException ie) {
+      throw new AssertionError(ie);
+    } finally {
+      dbSemaphore.release();
+    }
   }
 
   @Override
   public void onReset() {
+    Log.w(TAG, "onReset()");
+    try {
+      dbSemaphore.acquire(DB_PERMITS);
+      db.close();
+      db = new ContactsDatabase(context);
+    } catch (InterruptedException ie) {
+      throw new AssertionError(ie);
+    } finally {
+      dbSemaphore.release(DB_PERMITS);
+    }
     super.onReset();
   }
 }

src/org/thoughtcrime/securesms/contacts/ContactsDatabase.java

@@ -75,19 +75,7 @@ public class ContactsDatabase {
   public static final int PUSH_TYPE   = 1;
   public static final int GROUP_TYPE  = 2;
 
-  private static ContactsDatabase instance = null;
-
-  public synchronized static ContactsDatabase getInstance(Context context) {
-    if (instance == null) instance = new ContactsDatabase(context);
-    return instance;
-  }
-
-  public synchronized static void destroyInstance() {
-    if (instance != null) instance.close();
-    instance = null;
-  }
-
-  private ContactsDatabase(Context context) {
+  public ContactsDatabase(Context context) {
     this.dbHelper = new DatabaseOpenHelper(context);
     this.context  = context;
   }