thrift: Fix crash on unsorted column names in SlicePredicate

The column names in SlicePredicate can be passed in arbitrary order.
We converted them to clustering ranges in read_command preserving the
original order. As a result, the clustering ranges in read command may
appear out of order. This violates storage engine's assumptions and
lead to undefined behavior.

It was seen manifesting as a SIGSEGV or an abort in sstable reader
when executing a get_slice() thrift verb:

scylla: sstables/consumer.hh:476: seastar::future<> data_consumer::continuous_data_consumer<StateProcessor>::fast_forward_to(size_t, size_t) [with StateProcessor = sstables::data_consume_rows_context_m; size_t = long unsigned int]: Assertion `end >= _stream_position.position' failed.

Fixes #6486.

Tests:

   - added a new dtest to thrift_tests.py which reproduces the problem

Message-Id: <1596725657-15802-1-git-send-email-tgrabiec@scylladb.com>
(cherry picked from commit bfd129c)

thrift/handler.cc

@@ -1466,6 +1466,15 @@ class thrift_handler : public CassandraCobSvIf {
         opts.set(query::partition_slice::option::send_partition_key);
         return opts;
     }
+    static void sort_ranges(const schema& s, std::vector<query::clustering_range>& ranges) {
+        position_in_partition::less_compare less(s);
+        std::sort(ranges.begin(), ranges.end(),
+            [&less] (const query::clustering_range& r1, const query::clustering_range& r2) {
+                return less(
+                    position_in_partition_view::for_range_start(r1),
+                    position_in_partition_view::for_range_start(r2));
+            });
+    }
     static lw_shared_ptr<query::read_command> slice_pred_to_read_cmd(const schema& s, const SlicePredicate& predicate) {
         auto opts = query_opts(s);
         std::vector<query::clustering_range> clustering_ranges;
@@ -1479,6 +1488,7 @@ class thrift_handler : public CassandraCobSvIf {
                     auto ckey = make_clustering_prefix(s, to_bytes(name));
                     clustering_ranges.emplace_back(query::clustering_range::make_singular(std::move(ckey)));
                 }
+                sort_ranges(s, clustering_ranges);
                 regular_columns.emplace_back(s.regular_begin()->id);
             } else {
                 clustering_ranges.emplace_back(query::clustering_range::make_open_ended_both_sides());