Fix fetch/delete sql category bug.

Fixes #2868, where pressing <shift-tab> then <ctrl-d> in history
completion (with > 256 items) would cause later items to disappear (and
cause a crash if you try to delete again).

Cause:
Scrolling to the bottom would fetch an additional 256 items (in addition
to the 256 that are fetched at first). Deleting causes the query to
re-run, but it only fetches the initial 256 items, so the current index
is now invalid.

Fix:
After deleting from the history category, call fetchMore until it has
enough rows populated that the current index is valid.

qutebrowser/completion/models/histcategory.py

@@ -93,10 +93,12 @@ class HistoryCategory(QSqlQueryModel):
             self._query.run(pat=pattern)
         self.setQuery(self._query)
 
-    def removeRows(self, _row, _count, _parent=None):
+    def removeRows(self, row, _count, _parent=None):
         """Override QAbstractItemModel::removeRows to re-run sql query."""
         # re-run query to reload updated table
         with debug.log_time('sql', 'Re-running completion query post-delete'):
             self._query.run()
         self.setQuery(self._query)
+        while self.rowCount() < row:
+            self.fetchMore()
         return True

tests/unit/completion/test_histcategory.py

@@ -22,7 +22,6 @@
 import datetime
 
 import pytest
-from PyQt5.QtCore import QModelIndex
 
 from qutebrowser.misc import sql
 from qutebrowser.completion.models import histcategory
@@ -147,6 +146,22 @@ def test_remove_rows(hist, model_validator):
     model_validator.set_model(cat)
     cat.set_pattern('')
     hist.delete('url', 'foo')
-    # histcategory does not care which index was removed, it just regenerates
-    cat.removeRows(QModelIndex(), 1)
+    cat.removeRows(0, 1)
     model_validator.validate([('bar', 'Bar', '')])
+
+
+def test_remove_rows_fetch(hist):
+    """removeRows should fetch enough data to make the current index valid."""
+    # we cannot use model_validator as it will fetch everything up front
+    hist.insert_batch({'url': [str(i) for i in range(300)]})
+    cat = histcategory.HistoryCategory()
+    cat.set_pattern('')
+
+    # sanity check that we didn't fetch everything up front
+    assert cat.rowCount() < 300
+    cat.fetchMore()
+    assert cat.rowCount() == 300
+
+    hist.delete('url', '298')
+    cat.removeRows(297, 1)
+    assert cat.rowCount() == 299