copies: write empty entries in changeset when also writing to filelog

When writing to both changeset and filelog (during transition), we
don't want the reader to waste time by falling back to reading from
the filelog when there is no copy metadata. Let's write out empty copy
metadata instead (the read path is already prepared for this
case). Thanks to Greg for pointing this out.

Differential Revision: https://phab.mercurial-scm.org/D6306

mercurial/changelog.py

@@ -591,11 +591,11 b' class changelog(revlog.revlog):'
             elif branch in (".", "null", "tip"):
                 raise error.StorageError(_('the name \'%s\' is reserved')
                                          % branch)
-        if (p1copies or p2copies) and extra is None:
+        if (p1copies is not None or p2copies is not None) and extra is None:
             extra = {}
-        if p1copies:
+        if p1copies is not None:
             extra['p1copies'] = encodecopies(p1copies)
-        if p2copies:
+        if p2copies is not None:
             extra['p2copies'] = encodecopies(p2copies)
 
         if extra:

mercurial/localrepo.py

@@ -2650,6 +2650,14 b' class localrepository(object):'
                 mn = p1.manifestnode()
                 files = []
 
+            if writecopiesto == 'changeset-only':
+                # If writing only to changeset extras, use None to indicate that
+                # no entry should be written. If writing to both, write an empty
+                # entry to prevent the reader from falling back to reading
+                # filelogs.
+                p1copies = p1copies or None
+                p2copies = p2copies or None
+
             # update changelog
             self.ui.note(_("committing changelog\n"))
             self.changelog.delayupdate(tr)

tests/test-copies-in-changeset.t

@@ -103,6 +103,7 b' Test writing to both changeset and filel'
   $ hg changesetcopies
   files: j
   p1copies: j\x00a (esc)
+  p2copies: 
   $ hg debugdata j 0
   \x01 (esc)
   copy: a
@@ -115,6 +116,14 b' Test writing to both changeset and filel'
   a -> j
   $ hg showcopies --config experimental.copies.read-from=filelog-only
   a -> j
+The entries should be written to extras even if they're empty (so the client
+won't have to fall back to reading from filelogs)
+  $ echo x >> j
+  $ hg ci -m 'modify j' --config experimental.copies.write-to=compatibility
+  $ hg changesetcopies
+  files: j
+  p1copies: 
+  p2copies: 
 
 Test writing only to filelog