hash_table: return TooOld outside the two-billion-version window

addWrites now advances newestVersion (even when count == 0), and check
treats a readVersion more than two billion versions behind newestVersion as
TooOld, matching the documented postcondition and the radix tree and skip
list implementations. Previously hash_table discarded writeVersion entirely,
so callers that switched implementations silently lost the "this read version
is too old, retry" signal.

HashTable.cpp

@@ -18,15 +18,21 @@ struct string_hash {
   }
 };
 
+// Reads more than this many versions behind `newestVersion` are `TooOld`, so
+// that a caller whose read version fell out of the version window is told to
+// retry. See the `addWrites` postcondition in ConflictSet.h.
+constexpr int64_t kVersionWindow = 2'000'000'000;
+
 struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
-  Impl(int64_t oldestVersion) : oldestVersion(oldestVersion) {}
+  Impl(int64_t oldestVersion)
+      : oldestVersion(oldestVersion), newestVersion(oldestVersion) {}
   void check(const ConflictSet::ReadRange *reads, ConflictSet::Result *results,
              int count) const {
     for (int i = 0; i < count; ++i) {
       auto key =
           std::string_view((const char *)reads[i].begin.p, reads[i].begin.len);
       auto version = reads[i].readVersion;
-      if (version < oldestVersion) {
+      if (version < oldestVersion || version < newestVersion - kVersionWindow) {
         results[i] = TooOld;
         continue;
       }
@@ -38,6 +44,10 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
 
   void addWrites(const ConflictSet::WriteRange *writes, int count,
                  int64_t writeVersion) {
+    // Advance newestVersion before the loop, so that it also advances when
+    // count == 0, as documented in ConflictSet.h.
+    assert(writeVersion >= newestVersion);
+    newestVersion = writeVersion;
     for (int i = 0; i < count; ++i) {
       auto &max = map[std::string((const char *)writes[i].begin.p,
                                   writes[i].begin.len)];
@@ -84,6 +94,7 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
 private:
   int64_t keyUpdates = 0;
   int64_t oldestVersion;
+  int64_t newestVersion;
   std::unordered_map<std::string, int64_t, string_hash, std::equal_to<>> map;
   std::string removalKey;
 };