storage_proxy: prevent short read due to buffer size limit from being…

… swallowed during range scan

mutation_result_merger::get() assumes that the merged result may be a
short read if at least one of the partial results is a short read (in
other words, if none of the partial results is a short read, then the
merged result is also not a short read). However this is not true;
because we update the memory accounter incrementally, we may stop
scanning early. All the partial results are full; but we did not scan
the entire range.

Fix by changing the short_read variable initialization from `no`
(which assumes we'll encounter a short read indication when processing
one of the batches) to `this->short_read()`, which also takes into
account the memory accounter.

Fixes #2001.
Message-Id: <20170108111315.17877-1-avi@scylladb.com>

service/storage_proxy.cc

@@ -3614,7 +3614,7 @@ class mutation_result_merger {
             stdx::optional<dht::decorated_key> last; // set if we had a short read
         };
         auto merged = std::map<unsigned, partitions_and_last_key>();
-        auto short_read = query::short_read::no;
+        auto short_read = query::short_read(this->short_read());
         // merge batches with equal keys, and note if we need to sort afterwards
         for (auto&& key_value : _partitions) {
             auto&& key = key_value.first;