Add some comments about safety of calling maxVersion

ConflictSet.cpp

@@ -1682,7 +1682,8 @@ void mergeWithChild(Node *&self, WriteContext *tls, ConflictSet::Impl *impl,
   child->parentsIndex = self->parentsIndex;
 
   // Max versions are stored in the parent, so we need to update it now
-  // that we have a new parent.
+  // that we have a new parent. Safe we call since the root never has a partial
+  // key.
   setMaxVersion(child, std::max(childMaxVersion, tls->zero));
 
   self = child;
@@ -1957,6 +1958,7 @@ bool checkPrefixRead(Node *n, const std::span<const uint8_t> key,
   auto remaining = key;
   for (;; ++tls->prefix_read_iterations_accum) {
     if (remaining.size() == 0) {
+      // There's no way to encode a prefix read of "", so n is not the root
       return maxVersion(n) <= readVersion;
     }
 
@@ -2754,6 +2756,7 @@ struct CheckRangeRightSide {
 
   bool backtrack() {
     for (;;) {
+      // searchPathLen > prefixLen implies n is not the root
       if (searchPathLen > prefixLen && maxVersion(n) > readVersion) {
         ok = false;
         return true;
@@ -2855,6 +2858,9 @@ bool checkRangeRead(Node *n, std::span<const uint8_t> begin,
     return false;
   }
 
+  // This makes it safe to check maxVersion within CheckRangeLeftSide. If this
+  // were false, then we would have returned above since lcp == begin.size().
+  assert(!(n->parent == nullptr && begin.size() == 0));
   CheckRangeLeftSide checkRangeLeftSide{n, begin, lcp + 1, readVersion, tls};
   CheckRangeRightSide checkRangeRightSide{n, end, lcp + 1, readVersion, tls};
 
@@ -3069,7 +3075,8 @@ void addWriteRange(Node *&root, std::span<const uint8_t> begin,
        beginNode = erase(beginNode, tls, impl, /*logical*/ true, endNode)) {
   }
 
-  // Inserting end trashed endNode's maxVersion. Fix that
+  // Inserting end trashed endNode's maxVersion. Fix that. Safe to call since
+  // the end key always has non-zero size.
   fixupMaxVersion(endNode, tls);
 }