phases: explicitly filter stripped revision at strip time

Explicit is better than implicit. The current logic is bit subtle and fragile.

It also get in the way of using something else than node-id as internal storage.
We replace it with a more explicit filtering while striping.

mercurial/localrepo.py

@@ -3371,17 +3371,6 b' class localrepository:'
         Intended for use by strip and rollback, so there's a common
         place for anything that has to be done after destroying history.
         """
-        # When one tries to:
-        # 1) destroy nodes thus calling this method (e.g. strip)
-        # 2) use phasecache somewhere (e.g. commit)
-        #
-        # then 2) will fail because the phasecache contains nodes that were
-        # removed. We can either remove phasecache from the filecache,
-        # causing it to reload next time it is accessed, or simply filter
-        # the removed nodes now and write the updated cache.
-        self._phasecache.filterunknown(self)
-        self._phasecache.write()
-
         # refresh all repository caches
         self.updatecaches()
 

mercurial/phases.py

@@ -702,6 +702,24 b' class phasecache:'
             return True
         return False
 
+    def register_strip(
+        self,
+        repo: "localrepo.localrepository",
+        tr,
+        strip_rev: int,
+    ):
+        """announce a strip to the phase cache
+
+        Any roots higher than the stripped revision should be dropped.
+        """
+        assert repo.filtername is None
+        to_rev = repo.changelog.index.rev
+        for targetphase, nodes in list(self.phaseroots.items()):
+            filtered = {n for n in nodes if to_rev(n) >= strip_rev}
+            if filtered:
+                self._updateroots(targetphase, nodes - filtered, tr)
+        self.invalidate()
+
     def filterunknown(self, repo: "localrepo.localrepository") -> None:
         """remove unknown nodes from the phase boundary
 

mercurial/repair.py

@@ -218,6 +218,7 b' def strip(ui, repo, nodelist, backup=Tru'
                 oldfiles = set(tr._offsetmap.keys())
                 oldfiles.update(tr._newfiles)
 
+                repo._phasecache.register_strip(repo, tr, striprev)
                 tr.startgroup()
                 cl.strip(striprev, tr)
                 stripmanifest(repo, striprev, tr, files)
@@ -239,7 +240,6 b' def strip(ui, repo, nodelist, backup=Tru'
                 deleteobsmarkers(repo.obsstore, stripobsidx)
                 del repo.obsstore
                 repo.invalidatevolatilesets()
-                repo._phasecache.filterunknown(repo)
 
             if tmpbundlefile:
                 ui.note(_(b"adding branch\n"))