# HG changeset patch
# User Simon Sapin <simon.sapin@octobus.net>
# Date 2022-01-06 11:46:10
# Node ID 5026a0d37526e9f5539c3aeda28974b04ebc81b5
# Parent  faa243f345cc19a432eb0890ab738d7775532234

filelog: add a comment explaining a fast path in filerevisioncopied()

Differential Revision: https://phab.mercurial-scm.org/D11964

diff --git a/mercurial/utils/storageutil.py b/mercurial/utils/storageutil.py
--- a/mercurial/utils/storageutil.py
+++ b/mercurial/utils/storageutil.py
@@ -112,6 +112,13 @@ def filerevisioncopied(store, node):
     2-tuple of the source filename and node.
     """
     if store.parents(node)[0] != sha1nodeconstants.nullid:
+        # When creating a copy or move we set filelog parents to null,
+        # because contents are probably unrelated and making a delta
+        # would not be useful.
+        # Conversely, if filelog p1 is non-null we know
+        # there is no copy metadata.
+        # In the presence of merges, this reasoning becomes invalid
+        # if we reorder parents. See tests/test-issue6528.t.
         return False
 
     meta = parsemeta(store.revision(node))[0]