Require that eraseBetween leave at least one child or entryPresent

ConflictSet.cpp

@@ -1922,7 +1922,9 @@ void eraseBetween(Node **inTree, Node256 *n, int begin, int end,
 }
 
 // Erase all nodes with a search path starting with n + [child],
-// where child in [begin, end).
+// where child in [begin, end). To avoid the need to propagate erases up the
+// search path, the caller must ensure that the result has at least one child or
+// has entryPresent.
 void eraseBetween(Node *&n, int begin, int end, WriteContext *tls) {
 #if DEBUG_VERBOSE && !defined(NDEBUG)
   fprintf(stderr, "eraseBetween: %s + [%d,%d)\n",
@@ -1946,6 +1948,7 @@ void eraseBetween(Node *&n, int begin, int end, WriteContext *tls) {
   default:                   // GCOVR_EXCL_LINE
     __builtin_unreachable(); // GCOVR_EXCL_LINE
   }
+  assert(n->numChildren > 0 || n->entryPresent);
 }
 
 // Precondition: self is not the root. May invalidate nodes along the search