From f3470bce8d2097bb6b1e3a92b5e93169d97e1cad Mon Sep 17 00:00:00 2001
From: Matt Dowle <mattjdowle@gmail.com>
Date: Fri, 25 Sep 2020 01:02:30 -0600
Subject: [PATCH] attempt to fix GLCI segfault in R<3.5 by using length not
 LENGTH on NULL; #4655

---
 src/dogroups.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/dogroups.c b/src/dogroups.c
index 9b7b4cbc9..cbf10e09e 100644
--- a/src/dogroups.c
+++ b/src/dogroups.c
@@ -39,9 +39,12 @@ static bool anySpecialStatic(SEXP x) {
   // with PR#4164 started to copy input list columns too much. Hence PR#4655 in v1.13.2 moved that copy here just where it is needed.
   // Currently the marker is negative truelength. These specials are protected by us here and before we release them
   // we restore the true truelength for when R starts to use vector truelength.
+  const int n = length(x);
+  if (n==0)
+    return false;
   if (isVectorAtomic(x))
     return TRUELENGTH(x)<0;
-  if (isNewList(x)) for (int i=0; i<LENGTH(x); ++i) {
+  if (isNewList(x)) for (int i=0; i<n; ++i) {  // isNewList is true for NULL so use length() not LENGTH() above because LENGTH() on NULL is segfault in R<3.5 where we still define USE_RINTERNALS
     if (anySpecialStatic(VECTOR_ELT(x,i)))
       return true;
   }