upgrade: speed up when we have only nodemap to downgrade

Similar to what we do on upgrade, if we have only persistent-nodemap to
downgrade we will just delete the nodemap files and update repository
requirements instead of processing all the revlogs.

After downgrade, we are left with unrequired docket and transaction files which
seems fine but can work on deleting them if someone feels we should.

Differential Revision: https://phab.mercurial-scm.org/D9992

mercurial/revlogutils/nodemap.py

@@ -128,6 +128,14 b' def update_persistent_nodemap(revlog):'
         notr._postclose[k](None)
 
 
+def delete_nodemap(tr, repo, revlog):
+    """ Delete nodemap data on disk for a given revlog"""
+    if revlog.nodemap_file is None:
+        msg = "calling persist nodemap on a revlog without the feature enabled"
+        raise error.ProgrammingError(msg)
+    repo.svfs.unlink(revlog.nodemap_file)
+
+
 def persist_nodemap(tr, revlog, pending=False, force=False):
     """Write nodemap data on disk for a given revlog"""
     if getattr(revlog, 'filteredrevs', ()):

mercurial/upgrade_utils/engine.py

@@ -476,6 +476,27 b' def upgrade(ui, srcrepo, dstrepo, upgrad'
                 tr, unfi.manifestlog._rootstore._revlog, force=True
             )
         scmutil.writereporequirements(srcrepo, upgrade_op.new_requirements)
+    elif (
+        len(upgrade_op.removed_actions) == 1
+        and [
+            x
+            for x in upgrade_op.removed_actions
+            if x.name == b'persistent-nodemap'
+        ]
+        and not upgrade_op.upgrade_actions
+    ):
+        ui.status(
+            _(b'downgrading repository to not use persistent nodemap feature\n')
+        )
+        with srcrepo.transaction(b'upgrade') as tr:
+            unfi = srcrepo.unfiltered()
+            cl = unfi.changelog
+            nodemap.delete_nodemap(tr, srcrepo, cl)
+            # check comment 20 lines above for accessing private attributes
+            nodemap.delete_nodemap(
+                tr, srcrepo, unfi.manifestlog._rootstore._revlog
+            )
+        scmutil.writereporequirements(srcrepo, upgrade_op.new_requirements)
     else:
         with dstrepo.transaction(b'upgrade') as tr:
             _clonerevlogs(

tests/test-persistent-nodemap.t

@@ -581,7 +581,7 b' downgrading'
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib
   compression-level:  default default default
-  $ hg debugupgraderepo --run --no-backup --quiet
+  $ hg debugupgraderepo --run --no-backup
   upgrade will perform the following actions:
   
   requirements
@@ -593,8 +593,17 b' downgrading'
     - changelog
     - manifest
   
+  beginning upgrade...
+  repository locked and read-only
+  creating temporary repository to stage upgraded data: $TESTTMP/test-repo/.hg/upgrade.* (glob)
+  (it is safe to interrupt this process any time before data migration completes)
+  downgrading repository to not use persistent nodemap feature
+  removing temporary repository $TESTTMP/test-repo/.hg/upgrade.* (glob)
   $ ls -1 .hg/store/ | egrep '00(changelog|manifest)(\.n|-.*\.nd)'
-  [1]
+  00changelog-*.nd (glob)
+  00manifest-*.nd (glob)
+  undo.backup.00changelog.n
+  undo.backup.00manifest.n
   $ hg debugnodemap --metadata
 
 
@@ -643,6 +652,8 b' upgrading'
   00changelog.n
   00manifest-*.nd (glob)
   00manifest.n
+  undo.backup.00changelog.n
+  undo.backup.00manifest.n
 
   $ hg debugnodemap --metadata
   uid: * (glob)