Avoid re-inserting begin if begin is not a prefix of end

ConflictSet.cpp

@@ -1577,7 +1577,7 @@ bool checkRangeRead(Node *n, std::span<const uint8_t> begin,
 // Returns a pointer to the newly inserted node.  Caller must set
 // `entryPresent`, `entry` fields and `maxVersion` on the result.  The search
 // path of the result's parent will have `maxVersion` at least `writeVersion` as
-// a postcondition.
+// a postcondition. Nodes along the search path may be invalidated.
 template <bool kBegin>
 [[nodiscard]] Node *insert(Node **self, std::span<const uint8_t> key,
                            int64_t writeVersion, NodeAllocators *allocators,
@@ -1704,6 +1704,7 @@ void addWriteRange(Node *&root, int64_t oldestVersion,
     return addPointWrite(root, oldestVersion, begin, writeVersion, allocators,
                          impl);
   }
+  const bool beginIsPrefix = lcp == int(begin.size());
   auto remaining = begin.subspan(0, lcp);
 
   auto *n = root;
@@ -1772,8 +1773,9 @@ void addWriteRange(Node *&root, int64_t oldestVersion,
   }
   endNode->entry.rangeVersion = writeVersion;
 
-  if (insertedEnd) {
-    // beginNode may have been invalidated
+  if (beginIsPrefix && insertedEnd) {
+    // beginNode may have been invalidated when inserting end. TODO can we do
+    // better?
     beginNode = insert<true>(useAsRoot, begin, writeVersion, allocators, impl);
     assert(beginNode->entryPresent);
   }