--- a/mercurial/revlog.py	Wed Mar 10 05:50:20 2021 +0100
+++ b/mercurial/revlog.py	Wed Mar 10 18:09:21 2021 +0100
@@ -908,8 +908,10 @@
             if rev == wdirrev:
                 raise error.WdirUnsupported
             raise
-
-        return entry[5], entry[6]
+        if entry[5] == nullrev:
+            return entry[6], entry[5]
+        else:
+            return entry[5], entry[6]
 
     # fast parentrevs(rev) where rev isn't filtered
     _uncheckedparentrevs = parentrevs
@@ -930,7 +932,11 @@
     def parents(self, node):
         i = self.index
         d = i[self.rev(node)]
-        return i[d[5]][7], i[d[6]][7]  # map revisions to nodes inline
+        # inline node() to avoid function call overhead
+        if d[5] == nullid:
+            return i[d[6]][7], i[d[5]][7]
+        else:
+            return i[d[5]][7], i[d[6]][7]
 
     def chainlen(self, rev):
         return self._chaininfo(rev)[0]

--- a/relnotes/next	Wed Mar 10 05:50:20 2021 +0100
+++ b/relnotes/next	Wed Mar 10 18:09:21 2021 +0100
@@ -26,6 +26,13 @@
 
 == Backwards Compatibility Changes ==
 
+ * In normal repositories, the first parent of a changeset is not null,
+   unless both parents are null (like the first changeset). Some legacy
+   repositories violate this condition. The revlog code will now
+   silentely swap the parents if this condition is tested. This can
+   change the output of `hg log` when explicitly asking for first or
+   second parent.
+
 
 == Internal API Changes ==
 

--- a/tests/test-narrow-shallow-merges.t	Wed Mar 10 05:50:20 2021 +0100
+++ b/tests/test-narrow-shallow-merges.t	Wed Mar 10 18:09:21 2021 +0100
@@ -179,7 +179,7 @@
   
 
   $ hg log -T '{if(ellipsis,"...")}{node|short} {p1node|short} {p2node|short} {desc}\n' | sort
-  ...2a20009de83e 000000000000 3ac1f5779de3 outside 10
+  ...2a20009de83e 3ac1f5779de3 000000000000 outside 10
   ...3ac1f5779de3 bb96a08b062a 465567bdfb2d merge a/b/c/d 9
   ...8d874d57adea 7ef88b4dd4fa 000000000000 outside 12
   ...b844052e7b3b 000000000000 000000000000 outside 2c