--- a/mercurial/configitems.py	Wed Jan 06 16:01:19 2021 +0530
+++ b/mercurial/configitems.py	Wed Jan 06 16:18:06 2021 +0530
@@ -1074,6 +1074,11 @@
 )
 coreconfigitem(
     b'experimental',
+    b'sharesafe-auto-upgrade-shares',
+    default=False,
+)
+coreconfigitem(
+    b'experimental',
     b'single-head-per-branch',
     default=False,
 )

--- a/mercurial/localrepo.py	Wed Jan 06 16:01:19 2021 +0530
+++ b/mercurial/localrepo.py	Wed Jan 06 16:18:06 2021 +0530
@@ -582,12 +582,23 @@
     elif shared:
         sourcerequires = _readrequires(sharedvfs, False)
         if requirementsmod.SHARESAFE_REQUIREMENT in sourcerequires:
-            ui.warn(
-                _(
-                    b'warning: source repository supports share-safe functionality.'
-                    b' Reshare to upgrade.\n'
+            if ui.configbool(b'experimental', b'sharesafe-auto-upgrade-shares'):
+                # prevent cyclic import localrepo -> upgrade -> localrepo
+                from . import upgrade
+
+                upgrade.upgrade_share_to_safe(
+                    ui,
+                    hgvfs,
+                    storevfs,
+                    requirements,
                 )
-            )
+            else:
+                ui.warn(
+                    _(
+                        b'warning: source repository supports share-safe functionality.'
+                        b' Reshare to upgrade.\n'
+                    )
+                )
 
     # The .hg/hgrc file may load extensions or contain config options
     # that influence repository construction. Attempt to load it and

--- a/mercurial/upgrade.py	Wed Jan 06 16:01:19 2021 +0530
+++ b/mercurial/upgrade.py	Wed Jan 06 16:18:06 2021 +0530
@@ -12,7 +12,10 @@
     error,
     hg,
     localrepo,
+    lock as lockmod,
     pycompat,
+    requirements as requirementsmod,
+    scmutil,
 )
 
 from .upgrade_utils import (
@@ -20,6 +23,10 @@
     engine as upgrade_engine,
 )
 
+from .utils import (
+    stringutil,
+)
+
 allformatvariant = upgrade_actions.allformatvariant
 
 
@@ -232,3 +239,28 @@
                 )
 
             upgrade_op.print_post_op_messages()
+
+
+def upgrade_share_to_safe(ui, hgvfs, storevfs, current_requirements):
+    """Upgrades a share to use share-safe mechanism"""
+    wlock = None
+    try:
+        wlock = lockmod.trylock(ui, hgvfs, b'wlock', 0, 0)
+        store_requirements = localrepo._readrequires(storevfs, False)
+        # after upgrade, store requires will be shared, so lets find
+        # the requirements which are not present in store and
+        # write them to share's .hg/requires
+        diffrequires = current_requirements - store_requirements
+        # add share-safe requirement as it will mark the share as share-safe
+        diffrequires.add(requirementsmod.SHARESAFE_REQUIREMENT)
+        scmutil.writerequires(hgvfs, diffrequires)
+        current_requirements.add(requirementsmod.SHARESAFE_REQUIREMENT)
+        ui.warn(_(b'repository upgraded to use share-safe mode\n'))
+    except error.LockError as e:
+        ui.warn(
+            _(b'failed to upgrade share, got error: %s\n')
+            % stringutil.forcebytestr(e.strerror)
+        )
+    finally:
+        if wlock:
+            wlock.release()

--- a/tests/test-share-safe.t	Wed Jan 06 16:01:19 2021 +0530
+++ b/tests/test-share-safe.t	Wed Jan 06 16:18:06 2021 +0530
@@ -479,8 +479,63 @@
   |
   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]
+
+
+Testing automatic upgrade of shares when config is set
+
+  $ hg debugupgraderepo -q --run --config format.exp-share-safe=True
+  upgrade will perform the following actions:
+  
+  requirements
+     preserved: dotencode, fncache, generaldelta, revlogv1, sparserevlog, store
+     added: exp-sharesafe
+  
+  processed revlogs:
+    - all-filelogs
+    - changelog
+    - manifest
+  
+  repository upgraded to share safe mode, existing shares will still work in old non-safe mode. Re-share existing shares to use them in safe mode New shares will be created in safe mode.
+  $ hg debugrequirements
+  dotencode
+  exp-sharesafe
+  fncache
+  generaldelta
+  revlogv1
+  sparserevlog
+  store
+  $ hg log -GT "{node}: {desc}\n" -R ../nss-share
+  warning: source repository supports share-safe functionality. Reshare to upgrade.
+  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
+  |
+  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
+  
+
+Check that if lock is taken, upgrade fails but read operation are successful
+  $ touch ../nss-share/.hg/wlock
+  $ hg log -GT "{node}: {desc}\n" -R ../nss-share --config experimental.sharesafe-auto-upgrade-shares=true
+  failed to upgrade share, got error: Lock held
+  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
+  |
+  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
+  
+  $ rm ../nss-share/.hg/wlock
+  $ hg log -GT "{node}: {desc}\n" -R ../nss-share --config experimental.sharesafe-auto-upgrade-shares=true
+  repository upgraded to use share-safe mode
+  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
+  |
+  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
+  
+
+Test that unshare works
+
+  $ hg unshare -R ../nss-share
+  $ hg log -GT "{node}: {desc}\n" -R ../nss-share
+  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
+  |
+  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
+