network_topology_strategy: validate integers

In order to prevent users from creating a network topology
strategy instance with invalid inputs, it's not enough to use
std::stol() on the input: a string "3abc" still returns the number '3',
but will later confuse cqlsh and other drivers, when they ask for
topology strategy details.
The error message is now more human readable, since for incorrect
numeric inputs it used to return a rather cryptic message:
    ServerError: stol()
This commit fixes the issue and comes with a simple test.

Fixes #3801
Tests: unit(dev)
Message-Id: <7aaae83d003738f047d28727430ca0a5cec6b9c6.1583478000.git.sarna@scylladb.com>

locator/network_topology_strategy.cc

@@ -85,6 +85,10 @@ network_topology_strategy::network_topology_strategy(
                 "NetworkTopologyStrategy");
         }
 
+        if (val.empty() || std::any_of(val.begin(), val.end(), [] (char c) {return !isdigit(c);})) {
+            throw exceptions::configuration_exception(
+                    format("Replication factor must be numeric and non-negative, found '{}'", val));
+        }
         _dc_rep_factor.emplace(key, std::stol(val));
         _datacenteres.push_back(key);
     }

test/boost/network_topology_strategy_test.cc

@@ -37,6 +37,7 @@
 #include <boost/range/algorithm/adjacent_find.hpp>
 #include <boost/algorithm/cxx11/iota.hpp>
 #include "test/lib/log.hh"
+#include "test/lib/cql_test_env.hh"
 
 using namespace locator;
 
@@ -597,4 +598,13 @@ SEASTAR_TEST_CASE(testCalculateEndpoints) {
     });
 }
 
+SEASTAR_TEST_CASE(test_invalid_dcs) {
+    return do_with_cql_env_thread([] (auto& e) {
+        for (auto& incorrect : std::vector<std::string>{"3\"", "", "!!!", "abcb", "!3", "-5", "0x123"}) {
+            BOOST_REQUIRE_THROW(e.execute_cql("CREATE KEYSPACE abc WITH REPLICATION "
+                    "= {'class': 'NetworkTopologyStrategy', 'dc1':'" + incorrect + "'}").get(),
+                    exceptions::configuration_exception);
+        };
+    });
+}