--- a/mercurial/localrepo.py	Thu Jun 25 13:13:21 2020 +0530
+++ b/mercurial/localrepo.py	Fri Oct 16 18:57:55 2020 +0530
@@ -558,6 +558,16 @@
     # is present. We never write SHARESAFE_REQUIREMENT for a repo if store
     # is not present, refer checkrequirementscompat() for that
     if requirementsmod.SHARESAFE_REQUIREMENT in requirements:
+
+        if (
+            shared
+            and requirementsmod.SHARESAFE_REQUIREMENT
+            not in _readrequires(sharedvfs, True)
+        ):
+            raise error.Abort(
+                _(b"share source does not support exp-sharesafe requirement")
+            )
+
         if shared:
             # This is a shared repo
             storevfs = vfsmod.vfs(sharedvfs.join(b'store'))

--- a/mercurial/upgrade.py	Thu Jun 25 13:13:21 2020 +0530
+++ b/mercurial/upgrade.py	Fri Oct 16 18:57:55 2020 +0530
@@ -80,6 +80,7 @@
         requirements.SIDEDATA_REQUIREMENT,
         requirements.COPIESSDC_REQUIREMENT,
         requirements.NODEMAP_REQUIREMENT,
+        requirements.SHARESAFE_REQUIREMENT,
     }
     for name in compression.compengines:
         engine = compression.compengines[name]
@@ -1470,3 +1471,11 @@
                         b' New shares will be created in safe mode.\n'
                     )
                 )
+            if sharedsafe.name in removedreqs:
+                ui.warn(
+                    _(
+                        b'repository downgraded to not use share safe mode, '
+                        b'existing shares will not work and needs to'
+                        b' be reshared.\n'
+                    )
+                )

--- a/tests/test-share-safe.t	Thu Jun 25 13:13:21 2020 +0530
+++ b/tests/test-share-safe.t	Fri Oct 16 18:57:55 2020 +0530
@@ -377,4 +377,79 @@
   |
   o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
   
+
+
+Create a safe share from upgrade one
+
+  $ cd ..
+  $ hg share non-share-safe ss-share
+  updating working directory
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd ss-share
+  $ hg log -GT "{node}: {desc}\n"
+  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
+  |
+  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
+  
+  $ cd ../non-share-safe
+
+Test that downgrading works too
+
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > share =
+  > [format]
+  > exp-share-safe = False
+  > EOF
+
+  $ hg debugupgraderepo -q
+  requirements
+     preserved: dotencode, fncache, generaldelta, revlogv1, sparserevlog, store
+     removed: exp-sharesafe
+  
+  $ hg debugupgraderepo -q --run
+  upgrade will perform the following actions:
+  
+  requirements
+     preserved: dotencode, fncache, generaldelta, revlogv1, sparserevlog, store
+     removed: exp-sharesafe
+  
+  repository downgraded to not use share safe mode, existing shares will not work and needs to be reshared.
+
+  $ hg debugrequirements
+  dotencode
+  fncache
+  generaldelta
+  revlogv1
+  sparserevlog
+  store
+
+  $ cat .hg/requires
+  dotencode
+  fncache
+  generaldelta
+  revlogv1
+  sparserevlog
+  store
+
+  $ test -f .hg/store/requires
+  [1]
+
+  $ hg log -GT "{node}: {desc}\n"
+  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
+  |
+  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
+  
+
+Make sure existing shares still works
+
+  $ hg log -GT "{node}: {desc}\n" -R ../nss-share
+  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
+  |
+  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
+  
   $ hg unshare -R ../nss-share
+
+  $ hg log -GT "{node}: {desc}\n" -R ../ss-share
+  abort: share source does not support exp-sharesafe requirement
+  [255]