# HG changeset patch
# User Gregory Szorc <gregory.szorc@gmail.com>
# Date 1604796358 28800
# Node ID 68aedad4c11c346c8da0b131a990d940fa8687e8
# Parent  5c736ba5dc27ef62d46b450c8a2c8b4d3be21cbb
pure: guard against empty blocks

If blocks is empty, we append `None` to the returned list, which is
incorrect.

This subtle issue was caught by pytype, which correctly identified
the return value as List[Optional[Tuple]] because of this possibility.

Differential Revision: https://phab.mercurial-scm.org/D9279

diff -r 5c736ba5dc27 -r 68aedad4c11c mercurial/pure/bdiff.py
--- a/mercurial/pure/bdiff.py	Mon Nov 16 16:38:57 2020 +0100
+++ b/mercurial/pure/bdiff.py	Sat Nov 07 16:45:58 2020 -0800
@@ -51,7 +51,10 @@
                 shift += 1
         r.append((a1, b1, l1 + shift))
         prev = a2 + shift, b2 + shift, l2 - shift
-    r.append(prev)
+
+    if prev is not None:
+        r.append(prev)
+
     return r