diff -r aab70f05d6ec -r 705c37f22859 mercurial/upgrade.py
--- a/mercurial/upgrade.py	Mon Nov 30 14:11:03 2020 +0530
+++ b/mercurial/upgrade.py	Thu Jun 25 13:13:21 2020 +0530
@@ -137,6 +137,7 @@
         requirements.SIDEDATA_REQUIREMENT,
         requirements.COPIESSDC_REQUIREMENT,
         requirements.NODEMAP_REQUIREMENT,
+        requirements.SHARESAFE_REQUIREMENT,
     }
     for name in compression.compengines:
         engine = compression.compengines[name]
@@ -337,6 +338,26 @@
 
 
 @registerformatvariant
+class sharedsafe(requirementformatvariant):
+    name = b'exp-sharesafe'
+    _requirement = requirements.SHARESAFE_REQUIREMENT
+
+    default = False
+
+    description = _(
+        b'old shared repositories do not share source repository '
+        b'requirements and config. This leads to various problems '
+        b'when the source repository format is upgraded or some new '
+        b'extensions are enabled.'
+    )
+
+    upgrademessage = _(
+        b'Upgrades a repository to share-safe format so that future '
+        b'shares of this repository share its requirements and configs.'
+    )
+
+
+@registerformatvariant
 class sparserevlog(requirementformatvariant):
     name = b'sparserevlog'
 
@@ -1439,3 +1460,13 @@
                         b'repository is verified\n'
                     )
                 )
+
+            if sharedsafe.name in addedreqs:
+                ui.warn(
+                    _(
+                        b'repository upgraded to share safe mode, existing'
+                        b' shares will still work in old non-safe mode. '
+                        b'Re-share existing shares to use them in safe mode'
+                        b' New shares will be created in safe mode.\n'
+                    )
+                )