convert: keep branch switching merges with ancestors (issue3340)

When running convert with a filemap, merge parents which are ancestors
of other parents are ignored. This is hardly a problem when parents
belong to the same branch, but the result could be confusing when named
branches are involved. With:

  -o-a1-a2-a3...     <- A
    \           \
     b1-b2-b3...-m-  <- B

If all b* revisions are discarded, it is useful to preserve 'm' even if
it is empty after filtering to record the branch switch.

This patch makes filemap preserve "ancestor parents" if there is no
"non-ancestor parent" on the same branch than the merge revision.

Remarks:
- I am not completely convinced by the reasons given above and those
  detailed by Matt in this thread:

http://selenic.com/pipermail/mercurial-devel/2012-May/040627.html

  The properties we try to preserve are not clearly defined. That said,
  I know this patch already helped someone on IRC and the tests output
  look reasonable.

- This is a new version of the original "convert: filemap must preserve
  fast-forward merges" patch. It has exactly the same output for 2
  parents merges, the additional complexity is here to handle more than
  two parents.

hgext/convert/filemap.py

@@ -294,23 +294,34 class filemap_source(converter_source):
         # A parent p is interesting if its mapped version (self.parentmap[p]):
         # - is not SKIPREV
         # - is still not in the list of parents (we don't want duplicates)
-        # - is not an ancestor of the mapped versions of the other parents
+        # - is not an ancestor of the mapped versions of the other parents or
+        #   there is no parent in the same branch than the current revision.
         mparents = []
-        wp = None
+        knownparents = set()
+        branch = self.commits[rev].branch
+        hasbranchparent = False
         for i, p1 in enumerate(parents):
             mp1 = self.parentmap[p1]
-            if mp1 == SKIPREV or mp1 in mparents:
-                continue
-            for p2 in parents:
-                if p1 == p2 or mp1 == self.parentmap[p2]:
+            if mp1 == SKIPREV or mp1 in knownparents:
                     continue
-                if mp1 in self.wantedancestors[p2]:
-                    break
-            else:
-                mparents.append(mp1)
-                wp = i
-
-        if wp is None and parents:
+            isancestor = util.any(p2 for p2 in parents
+                                  if p1 != p2 and mp1 != self.parentmap[p2]
+                                  and mp1 in self.wantedancestors[p2])
+            if not isancestor and not hasbranchparent and len(parents) > 1:
+                # This could be expensive, avoid unnecessary calls.
+                if self._cachedcommit(p1).branch == branch:
+                    hasbranchparent = True
+            mparents.append((p1, mp1, i, isancestor))
+            knownparents.add(mp1)
+        # Discard parents ancestors of other parents if there is a
+        # non-ancestor one on the same branch than current revision.
+        if hasbranchparent:
+            mparents = [p for p in mparents if not p[3]]
+        wp = None
+        if mparents:
+            wp = max(p[2] for p in mparents)
+            mparents = [p[1] for p in mparents]
+        elif parents:
             wp = 0
 
         self.origparents[rev] = parents
@@ -319,7 +330,6 class filemap_source(converter_source):
         if 'close' in self.commits[rev].extra:
             # A branch closing revision is only useful if one of its
             # parents belong to the branch being closed
-            branch = self.commits[rev].branch
             pbranches = [self._cachedcommit(p).branch for p in mparents]
             if branch in pbranches:
                 closed = True

tests/test-convert-filemap.t

@@ -375,3 +375,189 exercise incremental conversion at the s
   |
   o  0 "addb" files: b
   
+
+test merge parents/empty merges pruning
+
+  $ glog()
+  > {
+  >     hg glog --template '{rev}:{node|short}@{branch} "{desc}" files: {files}\n' "$@"
+  > }
+
+test anonymous branch pruning
+
+  $ hg init anonymousbranch
+  $ cd anonymousbranch
+  $ echo a > a
+  $ echo b > b
+  $ hg ci -Am add
+  adding a
+  adding b
+  $ echo a >> a
+  $ hg ci -m changea
+  $ hg up 0
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ echo b >> b
+  $ hg ci -m changeb
+  created new head
+  $ hg up 1
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci -m merge
+  $ cd ..
+
+  $ cat > filemap <<EOF
+  > include a
+  > EOF
+  $ hg convert --filemap filemap anonymousbranch anonymousbranch-hg
+  initializing destination anonymousbranch-hg repository
+  scanning source...
+  sorting...
+  converting...
+  3 add
+  2 changea
+  1 changeb
+  0 merge
+  $ glog -R anonymousbranch
+  @    3:c71d5201a498@default "merge" files:
+  |\
+  | o  2:607eb44b17f9@default "changeb" files: b
+  | |
+  o |  1:1f60ea617824@default "changea" files: a
+  |/
+  o  0:0146e6129113@default "add" files: a b
+  
+  $ glog -R anonymousbranch-hg
+  o  1:cda818e7219b@default "changea" files: a
+  |
+  o  0:c334dc3be0da@default "add" files: a
+  
+  $ cat anonymousbranch-hg/.hg/shamap
+  0146e6129113dba9ac90207cfdf2d7ed35257ae5 c334dc3be0daa2a4e9ce4d2e2bdcba40c09d4916
+  1f60ea61782421edf8d051ff4fcb61b330f26a4a cda818e7219b5f7f3fb9f49780054ed6a1905ec3
+  607eb44b17f9348cd5cbd26e16af87ba77b0b037 c334dc3be0daa2a4e9ce4d2e2bdcba40c09d4916
+  c71d5201a498b2658d105a6bf69d7a0df2649aea cda818e7219b5f7f3fb9f49780054ed6a1905ec3
+
+  $ cat > filemap <<EOF
+  > include b
+  > EOF
+  $ hg convert --filemap filemap anonymousbranch anonymousbranch-hg2
+  initializing destination anonymousbranch-hg2 repository
+  scanning source...
+  sorting...
+  converting...
+  3 add
+  2 changea
+  1 changeb
+  0 merge
+  $ glog -R anonymousbranch
+  @    3:c71d5201a498@default "merge" files:
+  |\
+  | o  2:607eb44b17f9@default "changeb" files: b
+  | |
+  o |  1:1f60ea617824@default "changea" files: a
+  |/
+  o  0:0146e6129113@default "add" files: a b
+  
+  $ glog -R anonymousbranch-hg2
+  o  1:62dd350b0df6@default "changeb" files: b
+  |
+  o  0:4b9ced861657@default "add" files: b
+  
+  $ cat anonymousbranch-hg2/.hg/shamap
+  0146e6129113dba9ac90207cfdf2d7ed35257ae5 4b9ced86165703791653059a1db6ed864630a523
+  1f60ea61782421edf8d051ff4fcb61b330f26a4a 4b9ced86165703791653059a1db6ed864630a523
+  607eb44b17f9348cd5cbd26e16af87ba77b0b037 62dd350b0df695f7d2c82a02e0499b16fd790f22
+  c71d5201a498b2658d105a6bf69d7a0df2649aea 62dd350b0df695f7d2c82a02e0499b16fd790f22
+
+test named branch pruning
+
+  $ hg init namedbranch
+  $ cd namedbranch
+  $ echo a > a
+  $ echo b > b
+  $ hg ci -Am add
+  adding a
+  adding b
+  $ echo a >> a
+  $ hg ci -m changea
+  $ hg up 0
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg branch foo
+  marked working directory as branch foo
+  (branches are permanent and global, did you want a bookmark?)
+  $ echo b >> b
+  $ hg ci -m changeb
+  $ hg up default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge foo
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci -m merge
+  $ cd ..
+
+  $ cat > filemap <<EOF
+  > include a
+  > EOF
+  $ hg convert --filemap filemap namedbranch namedbranch-hg
+  initializing destination namedbranch-hg repository
+  scanning source...
+  sorting...
+  converting...
+  3 add
+  2 changea
+  1 changeb
+  0 merge
+  $ glog -R namedbranch
+  @    3:73899bcbe45c@default "merge" files:
+  |\
+  | o  2:8097982d19fc@foo "changeb" files: b
+  | |
+  o |  1:1f60ea617824@default "changea" files: a
+  |/
+  o  0:0146e6129113@default "add" files: a b
+  
+  $ glog -R namedbranch-hg
+  o  1:cda818e7219b@default "changea" files: a
+  |
+  o  0:c334dc3be0da@default "add" files: a
+  
+
+  $ cd namedbranch
+  $ hg --config extensions.mq= strip tip
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  saved backup bundle to $TESTTMP/namedbranch/.hg/strip-backup/73899bcbe45c-backup.hg
+  $ hg up foo
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci -m merge
+  $ cd ..
+
+  $ hg convert --filemap filemap namedbranch namedbranch-hg2
+  initializing destination namedbranch-hg2 repository
+  scanning source...
+  sorting...
+  converting...
+  3 add
+  2 changea
+  1 changeb
+  0 merge
+  $ glog -R namedbranch
+  @    3:e1959de76e1b@foo "merge" files:
+  |\
+  | o  2:8097982d19fc@foo "changeb" files: b
+  | |
+  o |  1:1f60ea617824@default "changea" files: a
+  |/
+  o  0:0146e6129113@default "add" files: a b
+  
+  $ glog -R namedbranch-hg2
+  o    2:dcf314454667@foo "merge" files:
+  |\
+  | o  1:cda818e7219b@default "changea" files: a
+  |/
+  o  0:c334dc3be0da@default "add" files: a
+