diff --git a/tests/bundles/test-revlog-diff-relative-to-nullrev.sh b/tests/bundles/test-revlog-diff-relative-to-nullrev.sh
new file mode 100755
--- /dev/null
+++ b/tests/bundles/test-revlog-diff-relative-to-nullrev.sh
@@ -0,0 +1,23 @@
+#!/bin/bash
+#
+# Make sure to patch mercurial to create the delta against nullrev
+#          if deltainfo is None:
+#-             deltainfo = self._fullsnapshotinfo(fh, revinfo, target_rev)
+#+             deltainfo = self._builddeltainfo(revinfo, nullrev, fh)
+
+cd "`dirname \"$0\"`"
+export HGRCPATH=
+export HGMODULEPOLICY=py
+
+rm -rf nullrev-diff
+../../hg init nullrev-diff  --config format.revlog-compression=zlib
+cd nullrev-diff
+echo hi > a
+../../../hg commit -Am root-B
+../../../hg debugdeltachain a
+rm -rf .hg/cache/ .hg/wcache/
+cd ..
+
+tar cf test-revlog-diff-relative-to-nullrev.tar nullrev-diff
+
+rm -rf nullrev-diff
diff --git a/tests/bundles/test-revlog-diff-relative-to-nullrev.tar b/tests/bundles/test-revlog-diff-relative-to-nullrev.tar
new file mode 100644
index 0000000000000000000000000000000000000000..28d0754dc9d18310896fae6d75a4c00634f07c59
GIT binary patch
literal 30720
zc%1E=U5MON6vwmFg*HlC5p|{7M)BLun0u3(>{=^Z3+j{AClOrmPI5C7CzDL`QM-y1
z#g)GJAi`p?R4w=<qAe9cC|HZWNa=%H@dFhK3ieIJexSG}`!UXDnhnEd?sj+14`ydF
zyE8YFbN}bubIzT0BggUCo|0)>*8M_nkYjX0l6#~Rx&3|~0jXHU1Y@kJf{YX$V?iQ4
zI-dnZK}dZ`5*pNRcu~l*{x0wLD-^fg{^eS=7jvL{`>Tqsf&F>VVgDfXd{*o%=-&Q>
zD4>5XWZ2(lyCd6YK_O!#2G&$H+y1(evcIk%RgmODM$e(o?QeP^a}CdAqU9RYs4=n1
zT;@~9WKKxMcxpIabq^AQCiMe0dnnG#5plUs9Vmbd{hKtTg$04`{eSE&6!8B%Sj_&k
zY!@;{p8rQkPy2sa0sqg3HNwE$qYcT?bWCzEJ;$wMW7j`&@!s*rHXr@v$p>CMx#RdB
z`?B7tW<yL^X=LbMmK)TyEf$0Y_<&sfYoty8B+<VXhYQd@5B}GG5^nEH>;KpdA3y!~
z^S|Br*Jr11n$pe<z4hJX*X=LgdvN;+G$2T8uNt*J7~cL$?dFZYZ#i*l+u@J4A(gy`
z>qmZ^oY-{m{ULYvYtqqY&uqRldi4bF&j@6aq5o!$#;bkL3ya7AbMrsak^gBb2L1B@
zNrr+HJX}GlMGTD^SW)B=OEZW$f_0sk6q6CHLabqk?W>W@_;1vxTV-*!zw`w_p8m1g
zvHsVS`Y-680|Tx2UyW~-bo@Vg-Q7oaJ@ecb%DVGket%c=!yS{KTx@@N690Fse^t6h
z7(M;c#Fo#G-8Sfc_0ALbY<lzcA@$CGe|}pTyKwD+{exp$&mPB@&ffmvMCH)d`r(Z`
z*T3=9%f>g~PCfYPp`DLkn5_P^>(71X9-2D8rc!;hTB>;e>|eX?-7_y7_;v79^Og;~
zuS(yLq5rvJU@>Fl#(#t$|6eBN3q26s`;!d)N3Q9WE7Yh*P0Mzc*aynZ|K-&BAIq@=
z0R8g-i$Z&@CyPSM^~++@p|&fSwjW=k_>;FyigN2o89c{IW)b}tHb#E_k30H5NC*A%
zpw;M+#RWCr<gh&j1evc+7K{Hy6#;YgpYH!+nLz%}hYtG}TKSnJfB#Q9|I;)D`ahhQ
zo|eq0(HNInc9%j=YQ~d-xuFyVwi`cDW0L2XlFyo+<k0c>LM-=b1BK9`|3Y{E&5~dL
zN%wy=M4<n}g`h@#Rtl&E`}z8k4*fec2ulqX1hmS^;oc(reE#}ROV@u1mO=kKNS61d
zhnM>-faRh?|E<DaapbT6+v`8tO#D|7=${99{oQ%o<i~tdIDR-lxt=D_BRced*&_ea
z=uZDL^&drr`tKFRjNC#0tB%X-zt}}U*ZM~~g8uK71?Ycuk`e!X>Ke78#>mhA)B8U)
z0{ve;n9QP)1J(R~B_sZ`z$k8v?)QIcNQd~(1z9w_M#B!F`rn^q#DCAL#}mHJ>HmLp
zeE(m^D(IgJ3+&&cHGRJQ)BRsnCb0j93-FzN000000000000000000000000000000
M000000DzUnf3O+GNdN!<

diff --git a/tests/test-revlog.t b/tests/test-revlog.t
--- a/tests/test-revlog.t
+++ b/tests/test-revlog.t
@@ -32,6 +32,7 @@ Unknown version is rejected
 
 Test for CVE-2016-3630
 
+  $ mkdir test2; cd test2
   $ hg init
 
   >>> import codecs
@@ -52,3 +53,37 @@ Test for CVE-2016-3630
   >>> rl = revlog.revlog(tvfs, target=(KIND_OTHER, b'test'), radix=b'a')
   >>> rl.revision(1)
   mpatchError(*'patch cannot be decoded'*) (glob)
+
+  $ cd ..
+
+
+Regression test for support for the old repos with strange diff encoding.
+Apparently it used to be possible (maybe it's still possible, but we don't know how)
+to create commits whose diffs are encoded relative to a nullrev.
+This test checks that a repo with that encoding can still be read.
+
+This is what we did to produce the repo in test-revlog-diff-relative-to-nullrev.tar:
+
+- tweak the code in mercurial/revlogutils/deltas.py to produce such "trivial" deltas:
+>          if deltainfo is None:
+> -            deltainfo = self._fullsnapshotinfo(fh, revinfo, target_rev)
+> +            deltainfo = self._builddeltainfo(revinfo, nullrev, fh)
+- hg init
+- echo hi > a
+- hg commit -Am_
+- remove some cache files
+
+  $ tar --force-local -xf "$TESTDIR"/bundles/test-revlog-diff-relative-to-nullrev.tar
+  $ cd nullrev-diff
+  $ hg debugdeltachain a
+      rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio   readsize largestblk rddensity srchunks
+        0       1        2       -1      p1         15          3         15   5.00000        15         0    0.00000         15         15   1.00000        1
+#if rhg
+  $ hg cat --config rhg.cat=true -r 0 a
+  abort: corrupted revlog
+  [255]
+#else
+  $ hg cat --config rhg.cat=true -r 0 a
+  hi
+#endif
+  $ cd ..