diff hgext/rebase.py @ 44341:77bb38be00ea

rebase: always be graft-like, not merge-like, also for merges

Rebase works by updating to a commit and then grafting changes on
top. However, before this patch, it would actually merge in changes
instead of grafting them in in some cases. That is, it would use the
common ancestor as base instead of using one of the parents. That
seems wrong to me, so I'm changing it so `defineparents()` always
returns a value for `base`.

This fixes the bad behavior in test-rebase-newancestor.t, which was
introduced in 65f215ea3e8e (tests: add test for rebasing merges with
ancestors of the rebase destination, 2014-11-30).

The difference in test-rebase-dest.t is because the files in the tip
revision were A, D, E, F before this patch and A, D, F, G after it. I
think both files should ideally be there.

Differential Revision: https://phab.mercurial-scm.org/D7907

--- a/hgext/rebase.py	Wed Jan 15 15:51:01 2020 +0100
+++ b/hgext/rebase.py	Thu Jan 16 00:03:19 2020 -0800
@@ -1678,22 +1678,6 @@
             elif p in state and state[p] > 0:
                 np = state[p]
 
-            # "bases" only record "special" merge bases that cannot be
-            # calculated from changelog DAG (i.e. isancestor(p, np) is False).
-            # For example:
-            #
-            #   B'   # rebase -s B -d D, when B was rebased to B'. dest for C
-            #   | C  # is B', but merge base for C is B, instead of
-            #   D |  # changelog.ancestor(C, B') == A. If changelog DAG and
-            #   | B  # "state" edges are merged (so there will be an edge from
-            #   |/   # B to B'), the merge base is still ancestor(C, B') in
-            #   A    # the merged graph.
-            #
-            # Also see https://bz.mercurial-scm.org/show_bug.cgi?id=1950#c8
-            # which uses "virtual null merge" to explain this situation.
-            if isancestor(p, np):
-                bases[i] = nullrev
-
             # If one parent becomes an ancestor of the other, drop the ancestor
             for j, x in enumerate(newps[:i]):
                 if x == nullrev:
@@ -1754,10 +1738,10 @@
     # But our merge base candidates (D and E in above case) could still be
     # better than the default (ancestor(F, Z) == null). Therefore still
     # pick one (so choose p1 above).
-    if sum(1 for b in set(bases) if b != nullrev) > 1:
+    if sum(1 for b in set(bases) if b != nullrev and b not in newps) > 1:
         unwanted = [None, None]  # unwanted[i]: unwanted revs if choose bases[i]
         for i, base in enumerate(bases):
-            if base == nullrev:
+            if base == nullrev or base in newps:
                 continue
             # Revisions in the side (not chosen as merge base) branch that
             # might contain "surprising" contents
@@ -1781,42 +1765,40 @@
                     )
                 )
 
-        # Choose a merge base that has a minimal number of unwanted revs.
-        l, i = min(
-            (len(revs), i)
-            for i, revs in enumerate(unwanted)
-            if revs is not None
-        )
-
-        # The merge will include unwanted revisions. Abort now. Revisit this if
-        # we have a more advanced merge algorithm that handles multiple bases.
-        if l > 0:
-            unwanteddesc = _(b' or ').join(
-                (
-                    b', '.join(b'%d:%s' % (r, repo[r]) for r in revs)
-                    for revs in unwanted
-                    if revs is not None
-                )
-            )
-            raise error.Abort(
-                _(b'rebasing %d:%s will include unwanted changes from %s')
-                % (rev, repo[rev], unwanteddesc)
+        if any(revs is not None for revs in unwanted):
+            # Choose a merge base that has a minimal number of unwanted revs.
+            l, i = min(
+                (len(revs), i)
+                for i, revs in enumerate(unwanted)
+                if revs is not None
             )
 
-        # newps[0] should match merge base if possible. Currently, if newps[i]
-        # is nullrev, the only case is newps[i] and newps[j] (j < i), one is
-        # the other's ancestor. In that case, it's fine to not swap newps here.
-        # (see CASE-1 and CASE-2 above)
-        if i != 0:
-            if newps[i] != nullrev:
-                newps[0], newps[i] = newps[i], newps[0]
-            bases[0], bases[i] = bases[i], bases[0]
+            # The merge will include unwanted revisions. Abort now. Revisit this if
+            # we have a more advanced merge algorithm that handles multiple bases.
+            if l > 0:
+                unwanteddesc = _(b' or ').join(
+                    (
+                        b', '.join(b'%d:%s' % (r, repo[r]) for r in revs)
+                        for revs in unwanted
+                        if revs is not None
+                    )
+                )
+                raise error.Abort(
+                    _(b'rebasing %d:%s will include unwanted changes from %s')
+                    % (rev, repo[rev], unwanteddesc)
+                )
+
+            # newps[0] should match merge base if possible. Currently, if newps[i]
+            # is nullrev, the only case is newps[i] and newps[j] (j < i), one is
+            # the other's ancestor. In that case, it's fine to not swap newps here.
+            # (see CASE-1 and CASE-2 above)
+            if i != 0:
+                if newps[i] != nullrev:
+                    newps[0], newps[i] = newps[i], newps[0]
+                bases[0], bases[i] = bases[i], bases[0]
 
     # "rebasenode" updates to new p1, use the corresponding merge base.
-    if bases[0] != nullrev:
-        base = bases[0]
-    else:
-        base = None
+    base = bases[0]
 
     repo.ui.debug(b" future parents are %d and %d\n" % tuple(newps))