Cursor.moveToNext error

Question

I'm seeing a crash report for this occassionally:

Fatal Exception: java.lang.IllegalStateException: Couldn't read row 1127, col 0 from CursorWindow.  Make sure the Cursor is initialized correctly before accessing data from it.
   at android.database.CursorWindow.nativeGetLong(CursorWindow.java)
   at android.database.CursorWindow.getLong(CursorWindow.java:511)
   at android.database.AbstractWindowedCursor.getLong(AbstractWindowedCursor.java:75)
   at android.database.AbstractCursor.moveToPosition(AbstractCursor.java:220)
   at android.database.AbstractCursor.moveToNext(AbstractCursor.java:245)
   at android.database.CursorWrapper.moveToNext(CursorWrapper.java:166)
   at com.anthonymandra.util.ImageUtils.cleanDatabase(SourceFile:381)

Apparently the moveToNext is failing mid-loop (note row 1127). The loop removes entries that represent files that can no longer be found.

final ArrayList<ContentProviderOperation> operations = new ArrayList<>();

try( Cursor cursor = c.getContentResolver().query(Meta.CONTENT_URI, null, null, null, null))
{
    if (cursor == null)
        return;

    final int uriColumn = cursor.getColumnIndex(Meta.URI);
    final int idColumn = cursor.getColumnIndex(BaseColumns._ID);

    while (cursor.moveToNext())
    {
        String uriString = cursor.getString(uriColumn);
        if (uriString == null)  // we've got some bogus data, just remove
        {
            operations.add(ContentProviderOperation.newDelete(
                    Uri.withAppendedPath(Meta.CONTENT_URI, cursor.getString(idColumn))).build());
            continue;
        }
        Uri uri = Uri.parse(uriString);
        UsefulDocumentFile file = UsefulDocumentFile.fromUri(c, uri);
        if (!file.exists())
        {
            operations.add(ContentProviderOperation.newDelete(Meta.CONTENT_URI)
                    .withSelection(getWhere(), new String[]{uriString}).build());
        }
    }
}

c.getContentResolver().applyBatch(Meta.AUTHORITY, operations);

Any idea how a cursor could fail mid-loop like that?

Answer 1

You appear to be making a fairly large query: at least 1127 rows, and for all possible columns (despite the fact that you are only using two of them). And, during your work with that Cursor, you are doing disk I/O and/or IPC back to the ContentProvider, assuming that UsefulDocumentFile is related to Android's DocumentFile.

As Prakash notes, the Cursor that you get back may contain only a subset of the information. As soon as you try advancing past that point, the Cursor needs to go back to the data source and get the next window of results. I can see you running into this sort of problem if there has been a substantial change in the data while this work has been going on (e.g., there are now fewer than 1127 rows).

I suggest that you:

• Constrain the columns that you get back to the subset that you need, and

• Avoid the I/O during the loop (e.g., spin through the Cursor to build up an ArrayList<Pair> or something, close the Cursor, then iterate over the list)