Implement checksum to check whether DIRTY contacts have "really" changed

* contact data hash code = hash code of data fields and group memberships
* Before every contact sync, all dirty contacts are checked whether they're
  "really dirty" (= data hash code has changed). If they're not, the DIRTY
  flag is reset. Works around Android 7 behavior of setting contacts to DIRTY
  even if onky meta data has been updated (for instance, lastContacted after
  a call or SMS),
* When an "upload" sync is initiated by notifyChange and there are no
  "really dirty" contacts, the sync is ignored.
* contact upload: clearDirty() saves hash code, too
* contact download: create()/update() saves hash code, too
* debugging: sync flags (extras) are now logged

app/src/main/java/at/bitfire/davdroid/resource/LocalAddressBook.java

@@ -84,6 +84,34 @@ public class LocalAddressBook extends AndroidAddressBook implements LocalCollect
         return deleted.toArray(new LocalResource[deleted.size()]);
     }
 
+    /**
+     * Queries all contacts with DIRTY flag and checks whether their data checksum has changed, i.e.
+     * if they're "really dirty" (= data has changed, not only metadata, which is not hashed).
+     * The DIRTY flag is removed from contacts which are not "really dirty", i.e. from contacts
+     * whose contact data checksum has not changed.
+     * @return number of "really dirty" contacts
+     */
+    public int verifyDirty() throws ContactsStorageException {
+        int reallyDirty = 0;
+        for (LocalContact contact : getDirtyContacts()) {
+            try {
+                if (contact.getLastHashCode() == contact.dataHashCode()) {
+                    // hash is code still the same, contact is not "really dirty" (only metadata been have changed)
+                    App.log.log(Level.FINE, "Contact data hash has not changed, resetting dirty flag", contact);
+                    contact.resetDirty();
+                } else
+                    reallyDirty++;
+            } catch(FileNotFoundException e) {
+                throw new ContactsStorageException("Couldn't calculate hash code", e);
+            }
+        }
+
+        if (includeGroups)
+            reallyDirty += getDirtyGroups().length;
+
+        return reallyDirty;
+    }
+
     /**
      * Returns an array of local contacts/groups which have been changed locally (DIRTY != 0).
      */

app/src/main/java/at/bitfire/davdroid/resource/LocalContact.java

@@ -10,6 +10,8 @@ package at.bitfire.davdroid.resource;
 
 import android.content.ContentProviderOperation;
 import android.content.ContentValues;
+import android.database.Cursor;
+import android.net.Uri;
 import android.os.RemoteException;
 import android.provider.ContactsContract;
 import android.provider.ContactsContract.CommonDataKinds.GroupMembership;
@@ -34,10 +36,13 @@ import at.bitfire.vcard4android.CachedGroupMembership;
 import at.bitfire.vcard4android.Contact;
 import at.bitfire.vcard4android.ContactsStorageException;
 import ezvcard.VCardVersion;
+import lombok.Cleanup;
 
 import static at.bitfire.vcard4android.GroupMethod.GROUP_VCARDS;
 
 public class LocalContact extends AndroidContact implements LocalResource {
+    public static final String COLUMN_HASHCODE = ContactsContract.RawContacts.SYNC3;
+
     protected final Set<Long>
             cachedGroupMemberships = new HashSet<>(),
             groupMemberships = new HashSet<>();
@@ -61,15 +66,30 @@ public class LocalContact extends AndroidContact implements LocalResource {
         return TextUtils.isEmpty(getETag());
     }
 
+    public void resetDirty() throws ContactsStorageException {
+        ContentValues values = new ContentValues(1);
+        values.put(ContactsContract.RawContacts.DIRTY, 0);
+        try {
+            addressBook.provider.update(rawContactSyncURI(), values, null, null);
+        } catch(RemoteException e) {
+            throw new ContactsStorageException("Couldn't clear dirty flag", e);
+        }
+    }
+
     public void clearDirty(String eTag) throws ContactsStorageException {
         try {
-            ContentValues values = new ContentValues(1);
-            values.put(ContactsContract.RawContacts.DIRTY, 0);
+            ContentValues values = new ContentValues(3);
             values.put(COLUMN_ETAG, eTag);
+            values.put(ContactsContract.RawContacts.DIRTY, 0);
+
+            int hashCode = dataHashCode();
+            values.put(COLUMN_HASHCODE, hashCode);
+            App.log.finer("Clearing dirty flag with eTag = " + eTag + ", contact hash = " + hashCode);
+
             addressBook.provider.update(rawContactSyncURI(), values, null, null);
 
             this.eTag = eTag;
-        } catch (RemoteException e) {
+        } catch (FileNotFoundException|RemoteException e) {
             throw new ContactsStorageException("Couldn't clear dirty flag", e);
         }
     }
@@ -138,6 +158,53 @@ public class LocalContact extends AndroidContact implements LocalResource {
     }
 
 
+    @Override
+    public int update(Contact contact) throws ContactsStorageException {
+        int result = super.update(contact);
+        updateHashCode();
+        return result;
+    }
+
+    @Override
+    public Uri create() throws ContactsStorageException {
+        Uri uri = super.create();
+        updateHashCode();
+        return uri;
+    }
+
+    /**
+     * Calculates a hash code from the contact's data (VCard) and group memberships.
+     * @return hash code of contact data (including group memberships)
+     */
+    public int dataHashCode() throws FileNotFoundException, ContactsStorageException {
+        // groupMemberships is filled by getContact()
+        return getContact().hashCode() ^ groupMemberships.hashCode();
+    }
+
+    protected void updateHashCode() throws ContactsStorageException {
+        ContentValues values = new ContentValues(1);
+        try {
+            int hashCode = dataHashCode();
+            App.log.fine("Storing contact hash = " + hashCode);
+            values.put(COLUMN_HASHCODE, hashCode);
+            addressBook.provider.update(rawContactSyncURI(), values, null, null);
+        } catch(FileNotFoundException|RemoteException e) {
+            throw new ContactsStorageException("Couldn't store contact checksum", e);
+        }
+    }
+
+    int getLastHashCode() throws ContactsStorageException {
+        try {
+            @Cleanup Cursor c = addressBook.provider.query(rawContactSyncURI(), new String[] { COLUMN_HASHCODE }, null, null, null);
+            if (c == null || !c.moveToNext() || c.isNull(0))
+                return 0;
+            return c.getInt(0);
+        } catch(RemoteException e) {
+            throw new ContactsStorageException("Could't read last hash code", e);
+        }
+    }
+
+
     public void addToGroup(BatchOperation batch, long groupID) {
         assertID();
         batch.enqueue(new BatchOperation.Operation(

app/src/main/java/at/bitfire/davdroid/syncadapter/ContactsSyncManager.java

@@ -10,6 +10,7 @@ package at.bitfire.davdroid.syncadapter;
 
 import android.accounts.Account;
 import android.content.ContentProviderClient;
+import android.content.ContentResolver;
 import android.content.ContentValues;
 import android.content.Context;
 import android.content.SyncResult;
@@ -81,6 +82,12 @@ public class ContactsSyncManager extends SyncManager {
         LocalAddressBook localAddressBook = localAddressBook();
         localAddressBook.setURL(info.url);
 
+        int reallyDirty = localAddressBook.verifyDirty();
+        if (extras.containsKey(ContentResolver.SYNC_EXTRAS_UPLOAD) && reallyDirty == 0) {
+            App.log.info("This sync was called to upload dirty contacts, but no contact data have been changed");
+            return false;
+        }
+
         // set up Contacts Provider Settings
         ContentValues values = new ContentValues(2);
         values.put(ContactsContract.Settings.SHOULD_SYNC, 1);

app/src/main/java/at/bitfire/davdroid/syncadapter/SyncAdapterService.java

@@ -74,7 +74,7 @@ public abstract class SyncAdapterService extends Service {
 
         @Override
         public void onPerformSync(Account account, Bundle extras, String authority, ContentProviderClient provider, SyncResult syncResult) {
-            App.log.info("Sync for " + authority + " has been initiated");
+            App.log.log(Level.INFO, "Sync for " + authority + " has been initiated.", extras.keySet().toArray());
 
             // required for dav4android (ServiceLoader)
             Thread.currentThread().setContextClassLoader(getContext().getClassLoader());

app/src/main/java/at/bitfire/davdroid/ui/AccountSettingsActivity.java

@@ -123,7 +123,6 @@ public class AccountSettingsActivity extends AppCompatActivity {
                     @Override
                     public boolean onPreferenceChange(Preference preference, Object newValue) {
                         settings.setSyncInterval(ContactsContract.AUTHORITY, Long.parseLong((String)newValue));
-                        getLoaderManager().restartLoader(0, getArguments(), AccountSettingsFragment.this);
                         return false;
                     }
                 });
@@ -144,7 +143,6 @@ public class AccountSettingsActivity extends AppCompatActivity {
                     @Override
                     public boolean onPreferenceChange(Preference preference, Object newValue) {
                         settings.setSyncInterval(CalendarContract.AUTHORITY, Long.parseLong((String)newValue));
-                        getLoaderManager().restartLoader(0, getArguments(), AccountSettingsFragment.this);
                         return false;
                     }
                 });
@@ -165,7 +163,6 @@ public class AccountSettingsActivity extends AppCompatActivity {
                     @Override
                     public boolean onPreferenceChange(Preference preference, Object newValue) {
                         settings.setSyncInterval(TaskProvider.ProviderName.OpenTasks.authority, Long.parseLong((String)newValue));
-                        getLoaderManager().restartLoader(0, getArguments(), AccountSettingsFragment.this);
                         return false;
                     }
                 });