dirstate: add a function to update tracking status while "moving" parents

The `scmutil.dirstateparent` is moving the dirstate parent without touching the
working copy. It is used by history-rewriting operations like amending of
folding.

The function was directly doing the "low level" computation and dirstate
change.  All that logic belong to the dirstate and should be moved there.

For this purpose we introduce a new function that does just that and use it.

Differential Revision: https://phab.mercurial-scm.org/D11012

mercurial/dirstate.py

@@ -72,6 +72,17 b' def _getfsnow(vfs):'
         vfs.unlink(tmpname)
 
 
+def requires_parents_change(func):
+    def wrap(self, *args, **kwargs):
+        if not self.pendingparentchange():
+            msg = 'calling `%s` outside of a parentchange context'
+            msg %= func.__name__
+            raise error.ProgrammingError(msg)
+        return func(self, *args, **kwargs)
+
+    return wrap
+
+
 @interfaceutil.implementer(intdirstate.idirstate)
 class dirstate(object):
     def __init__(
@@ -440,6 +451,44 b' class dirstate(object):'
     def copies(self):
         return self._map.copymap
 
+    @requires_parents_change
+    def update_file_reference(
+        self,
+        filename,
+        p1_tracked,
+    ):
+        """Set a file as tracked in the parent (or not)
+
+        This is to be called when adjust the dirstate to a new parent after an history
+        rewriting operation.
+
+        It should not be called during a merge (p2 != nullid) and only within
+        a `with dirstate.parentchange():` context.
+        """
+        if self.in_merge:
+            msg = b'update_file_reference should not be called when merging'
+            raise error.ProgrammingError(msg)
+        entry = self._map.get(filename)
+        if entry is None:
+            wc_tracked = False
+        else:
+            wc_tracked = entry.tracked
+        if p1_tracked and wc_tracked:
+            # the underlying reference might have changed, we will have to
+            # check it.
+            self.normallookup(filename)
+        elif not (p1_tracked or wc_tracked):
+            # the file is no longer relevant to anyone
+            self._drop(filename)
+        elif (not p1_tracked) and wc_tracked:
+            if not entry.added:
+                self._add(filename)
+        elif p1_tracked and not wc_tracked:
+            if entry is None or not entry.removed:
+                self._remove(filename)
+        else:
+            assert False, 'unreachable'
+
     def _addpath(
         self,
         f,

mercurial/scmutil.py

@@ -1485,25 +1485,15 b' def movedirstate(repo, newctx, match=Non'
     copies = dict(ds.copies())
     ds.setparents(newctx.node(), repo.nullid)
     s = newctx.status(oldctx, match=match)
+
     for f in s.modified:
-        if ds[f] == b'r':
-            # modified + removed -> removed
-            continue
-        ds.normallookup(f)
+        ds.update_file_reference(f, p1_tracked=True)
 
     for f in s.added:
-        if ds[f] == b'r':
-            # added + removed -> unknown
-            ds.drop(f)
-        elif ds[f] != b'a':
-            ds.add(f)
+        ds.update_file_reference(f, p1_tracked=False)
 
     for f in s.removed:
-        if ds[f] == b'a':
-            # removed + added -> normal
-            ds.normallookup(f)
-        elif ds[f] != b'r':
-            ds.remove(f)
+        ds.update_file_reference(f, p1_tracked=True)
 
     # Merge old parent and old working dir copies
     oldcopies = copiesmod.pathcopies(newctx, oldctx, match)