dht: Introduce get_uniform_tokens()

dht/token.cc

@@ -257,4 +257,17 @@ token last_token_of_compaction_group(unsigned most_significant_bits, size_t grou
     return bias(n);
 }
 
+utils::chunked_vector<dht::raw_token> get_uniform_tokens(size_t count) {
+    utils::chunked_vector<dht::raw_token> tokens;
+    tokens.reserve(count);
+
+    for (size_t i = 1; i <= count; ++i) {
+        uint64_t n = (uint128_t(i) * std::numeric_limits<uint64_t>::max()) / count;
+        tokens.push_back(raw_token{bias(n)});
+        assert(tokens.back().value != std::numeric_limits<int64_t>::min()); // See token::normalize()
+    }
+
+    return tokens;
+}
+
 } // namespace dht

dht/token.hh

@@ -358,6 +358,10 @@ inline constexpr token bias(uint64_t n) {
 size_t compaction_group_of(unsigned most_significant_bits, const token& t);
 token last_token_of_compaction_group(unsigned most_significant_bits, size_t group);
 
+// Generates 'count' tokens uniformly distributed in the token ring. Sorted.
+// All values are in the range [first_token(), last_token()]
+utils::chunked_vector<dht::raw_token> get_uniform_tokens(size_t count);
+
 struct token_comparator {
     // Return values are those of a trichotomic comparison.
     constexpr std::strong_ordering operator()(const token& t1, const token& t2) const noexcept {

test/boost/tablets_test.cc

@@ -4885,6 +4885,21 @@ SEASTAR_TEST_CASE(test_load_stats_migrate_tablet_size) {
     return make_ready_future<>();
 }
 
+// We want to generate the same uniform boundaries if tablet count is a power-of-two as
+// we did before implementing support for arbitrary token boundaries.
+// So that when advertising a "power of two" layout, e.g in the snapshot descriptor,
+// it means the same thing for all scylla versions.
+SEASTAR_THREAD_TEST_CASE(test_get_uniform_tokens_is_compatible_with_dht_last_token_of_compaction_group) {
+    for (auto log2count : {0ul, 1ul, 2ul, 3ul, 10ul}) {
+        auto tokens = dht::get_uniform_tokens(1ul << log2count);
+        for (size_t i = 0; i < tokens.size(); i++) {
+            testlog.debug("i {}, token {}", i, tokens[i]);
+            BOOST_REQUIRE_EQUAL(tokens[i], dht::last_token_of_compaction_group(log2count, i));
+            thread::maybe_yield();
+        }
+    }
+}
+
 SEASTAR_TEST_CASE(test_tablet_id_and_range_side) {
     static constexpr size_t tablet_count = 128;
     locator::tablet_map tmap(tablet_count);