--- a/mercurial/upgrade_utils/actions.py	Tue Feb 15 13:32:30 2022 -0500
+++ b/mercurial/upgrade_utils/actions.py	Tue Feb 15 23:09:07 2022 +0100
@@ -36,7 +36,10 @@
 
 
 def preservedrequirements(repo):
-    return set()
+    preserved = {
+        requirements.SHARED_REQUIREMENT,
+    }
+    return preserved & repo.requirements
 
 
 FORMAT_VARIANT = b'deficiency'
@@ -97,6 +100,9 @@
     # Whether this improvement touches the dirstate
     touches_dirstate = False
 
+    # Can this action be run on a share instead of its mains repository
+    compatible_with_share = False
+
 
 allformatvariant = []  # type: List[Type['formatvariant']]
 
@@ -899,8 +905,6 @@
         # This was a precursor to generaldelta and was never enabled by default.
         # It should (hopefully) not exist in the wild.
         b'parentdelta',
-        # Upgrade should operate on the actual store, not the shared link.
-        requirements.SHARED_REQUIREMENT,
     }
 
 
@@ -932,6 +936,16 @@
         m = _(b'cannot upgrade repository; unsupported source requirement: %s')
         blockingreqs = b', '.join(sorted(blockingreqs))
         raise error.Abort(m % blockingreqs)
+    # Upgrade should operate on the actual store, not the shared link.
+
+    bad_share = (
+        requirements.SHARED_REQUIREMENT in repo.requirements
+        and requirements.SHARESAFE_REQUIREMENT not in repo.requirements
+    )
+    if bad_share:
+        m = _(b'cannot upgrade repository; share repository without share-safe')
+        h = _(b'check :hg:`help config.format.use-share-safe`')
+        raise error.Abort(m, hint=h)
 
 
 ### Verify the validity of the planned requirement changes ####################
@@ -972,18 +986,19 @@
     Extensions should monkeypatch this to add their custom requirements.
     """
     supported = {
+        requirements.CHANGELOGV2_REQUIREMENT,
+        requirements.COPIESSDC_REQUIREMENT,
+        requirements.DIRSTATE_V2_REQUIREMENT,
         requirements.DOTENCODE_REQUIREMENT,
         requirements.FNCACHE_REQUIREMENT,
         requirements.GENERALDELTA_REQUIREMENT,
+        requirements.NODEMAP_REQUIREMENT,
         requirements.REVLOGV1_REQUIREMENT,  # allowed in case of downgrade
-        requirements.STORE_REQUIREMENT,
+        requirements.REVLOGV2_REQUIREMENT,
+        requirements.SHARED_REQUIREMENT,
+        requirements.SHARESAFE_REQUIREMENT,
         requirements.SPARSEREVLOG_REQUIREMENT,
-        requirements.COPIESSDC_REQUIREMENT,
-        requirements.NODEMAP_REQUIREMENT,
-        requirements.SHARESAFE_REQUIREMENT,
-        requirements.REVLOGV2_REQUIREMENT,
-        requirements.CHANGELOGV2_REQUIREMENT,
-        requirements.DIRSTATE_V2_REQUIREMENT,
+        requirements.STORE_REQUIREMENT,
     }
     for name in compression.compengines:
         engine = compression.compengines[name]