diff --git a/mercurial/changelog.py b/mercurial/changelog.py
--- a/mercurial/changelog.py
+++ b/mercurial/changelog.py
@@ -448,6 +448,7 @@ class changelog(revlog.revlog):
 
     def delayupdate(self, tr):
         """delay visibility of index updates to other readers"""
+        assert not self._inner.is_open
         if self._docket is None and not self._delayed:
             if len(self) == 0:
                 self._divert = True
@@ -468,6 +469,7 @@ class changelog(revlog.revlog):
 
     def _finalize(self, tr):
         """finalize index updates"""
+        assert not self._inner.is_open
         self._delayed = False
         self.opener = self._realopener
         self._inner.opener = self.opener
@@ -494,6 +496,7 @@ class changelog(revlog.revlog):
     def _writepending(self, tr):
         """create a file containing the unfinalized state for
         pretxnchangegroup"""
+        assert not self._inner.is_open
         if self._docket:
             return self._docket.write(tr, pending=True)
         if self._delaybuf:
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -620,6 +620,13 @@ class _InnerRevlog:
         """True is a writing context is open"""
         return self._writinghandles is not None
 
+    @property
+    def is_open(self):
+        """True if any file handle is being held
+
+        Used for assert and debug in the python code"""
+        return self._segmentfile.is_open or self._segmentfile_sidedata.is_open
+
     @contextlib.contextmanager
     def writing(self, transaction, data_end=None, sidedata_end=None):
         """Open the revlog files for writing
diff --git a/mercurial/revlogutils/randomaccessfile.py b/mercurial/revlogutils/randomaccessfile.py
--- a/mercurial/revlogutils/randomaccessfile.py
+++ b/mercurial/revlogutils/randomaccessfile.py
@@ -50,6 +50,15 @@ class randomaccessfile:
         self._cached_chunk = b''
         self._cached_chunk_position = 0
 
+    @property
+    def is_open(self):
+        """True if any file handle is being held
+
+        Used for assert and debug in the python code"""
+        return (
+            self.reading_handle is not None or self.writing_handle is not None
+        )
+
     def _open(self, mode=b'r'):
         """Return a file object"""
         return self.opener(self.filename, mode=mode)