change abstract_replication_strategy::get_ranges to not return wrap-arounds

The main motivation behind this change is to make get_ranges() easier for
consumers to work with the returned ranges, e.g. binary search to find a
range in which a token is contained. In addition, a wrap-around range
introduces corner cases, so we should avoid it altogether.

Suppose that a node owns three tokens: -5, 6, 8

get_ranges() would return the following ranges:
(8, -5], (-5, 6], (6, 8]
get_ranges() will now return the following ranges:
(-inf, -5], (-5, 6], (6, 8], (8, +inf)

Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <4bda1428d1ebbe7c8af25aa65119edc5b97bc2eb.1453827605.git.raphaelsc@scylladb.com>

locator/abstract_replication_strategy.cc

@@ -123,6 +123,15 @@ abstract_replication_strategy::get_ranges(inet_address ep) const {
         }
         prev_tok = tok;
     }
+    if (!ret.empty()) {
+        // Make ret contain no wrap-around range by unwrapping the first element.
+        auto& r = ret.front();
+        if (r.is_wrap_around(dht::token_comparator())) {
+            auto split_ranges = r.unwrap();
+            r = split_ranges.first;
+            ret.push_back(split_ranges.second);
+        }
+    }
     return ret;
 }
 

locator/abstract_replication_strategy.hh

@@ -101,6 +101,7 @@ public:
     replication_strategy_type get_type() const { return _my_type; }
 
     // get_ranges() returns the list of ranges held by the given endpoint.
+    // The list is sorted, and its elements are non overlapping and non wrap-around.
     // It the analogue of Origin's getAddressRanges().get(endpoint).
     // This function is not efficient, and not meant for the fast path.
     std::vector<range<token>> get_ranges(inet_address ep) const;

service/storage_service.hh

@@ -1609,6 +1609,7 @@ public:
 #endif
     /**
      * Get all ranges an endpoint is responsible for (by keyspace)
+     * Replication strategy's get_ranges() guarantees that no wrap-around range is returned.
      * @param ep endpoint we are interested in.
      * @return ranges for the specified endpoint.
      */