Enforce that root does not have a partial key

ConflictSet.cpp

@@ -902,8 +902,7 @@ Node *&getChildExists(Node *self, uint8_t index) {
 }
 
 InternalVersionT maxVersion(Node *n, ConflictSet::Impl *);
-InternalVersionT exchangeMaxVersion(Node *n, InternalVersionT newMax,
-                                    ConflictSet::Impl *);
+InternalVersionT exchangeMaxVersion(Node *n, InternalVersionT newMax);
 
 void setMaxVersion(Node *n, ConflictSet::Impl *, InternalVersionT maxVersion);
 
@@ -2925,8 +2924,9 @@ void consumePartialKey(Node *&self, std::span<const uint8_t> &key,
       longestCommonPrefix(self->partialKey(), key.data(), commonLen);
   if (partialKeyIndex < self->partialKeyLen) {
     auto *old = self;
-    InternalVersionT oldMaxVersion =
-        exchangeMaxVersion(old, writeVersion, impl);
+    // Since root cannot have a partial key
+    assert(old->parent != nullptr);
+    InternalVersionT oldMaxVersion = exchangeMaxVersion(old, writeVersion);
 
     // *self will have one child (old)
     auto *newSelf = tls->allocate<Node3>(partialKeyIndex);
@@ -3596,13 +3596,11 @@ InternalVersionT maxVersion(Node *n, ConflictSet::Impl *impl) {
   }
 }
 
-InternalVersionT exchangeMaxVersion(Node *n, InternalVersionT newMax,
-                                    ConflictSet::Impl *impl) {
+// Precondition `n` is not the root
+InternalVersionT exchangeMaxVersion(Node *n, InternalVersionT newMax) {
   int index = n->parentsIndex;
   n = n->parent;
-  if (n == nullptr) {
-    return std::exchange(impl->rootMaxVersion, newMax);
-  }
+  assert(n != nullptr);
   switch (n->getType()) {
   case Type_Node0:           // GCOVR_EXCL_LINE
     __builtin_unreachable(); // GCOVR_EXCL_LINE
@@ -4141,6 +4139,10 @@ checkMaxVersion(Node *root, Node *node, InternalVersionT oldestVersion,
                                        ConflictSet::Impl *impl) {
   bool success = true;
 
+  if (node->partialKeyLen > 0) {
+    fprintf(stderr, "Root cannot have a partial key");
+    success = false;
+  }
   checkParentPointers(node, success);
   checkMaxVersion(node, node, oldestVersion, success, impl);
   checkEntriesExist(node, success);