diff -r 2cd00052ae4d -r 3fffb48539ee tests/test-persistent-nodemap.t
--- a/tests/test-persistent-nodemap.t	Wed Jul 28 13:45:07 2021 +0300
+++ b/tests/test-persistent-nodemap.t	Sun Aug 01 14:39:38 2021 +0200
@@ -428,6 +428,46 @@
   data-length: 121088
   data-unused: 0
   data-unused: 0.000%
+
+Sub-case: fallback for corrupted data file
+------------------------------------------
+
+Sabotaging the data file so that nodemap resolutions fail, triggering fallback to
+(non-persistent) C implementation.
+
+
+  $ UUID=`hg debugnodemap --metadata| grep 'uid:' | \
+  > sed 's/uid: //'`
+  $ FILE=.hg/store/00changelog-"${UUID}".nd
+  $ python -c "fobj = open('$FILE', 'r+b'); fobj.write(b'\xff' * 121088); fobj.close()"
+
+The nodemap data file is still considered in sync with the docket. This
+would fail without the fallback to the (non-persistent) C implementation:
+
+  $ hg log -r b355ef8adce0949b8bdf6afc72ca853740d65944 -T '{rev}\n' --traceback
+  5002
+
+The nodemap data file hasn't been fixed, more tests can be inserted:
+
+  $ hg debugnodemap --dump-disk | f --bytes=256 --hexdump --size
+  size=121088
+  0000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  0010: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  0020: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  0030: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  0040: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  0050: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  0060: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  0070: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  0080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  0090: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  00a0: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  00b0: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  00c0: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  00d0: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  00e0: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+  00f0: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
+
   $ mv ../tmp-data-file $FILE
   $ mv ../tmp-docket .hg/store/00changelog.n