setdiscovery: don't use dagutil for parent resolution

_updatesample()'s one remaining use of revlogdag is for resolving
the parents of a revision.

In 2 cases, we actually resolve parents. In 1, we operate on the
inverted DAG and resolve children.

This commit teaches _updatesample() to receive an argument defining
the function to resolve "parent" revisions. Call sites pass in
changelog.parentrevs() or a wrapper around changelog.children()
accordingly.

The use of children() is semantically correct. But it is quadratic,
since revlog.children() does a range scan over all revisions starting
at its input and effectively calls parentrevs() to build up the list
of children. So calling it repeatedly in a loop is a recipe for
bad performance. I will be implementing something better in a
subsequent commit. I wanted to get the porting off of dagutil done
in a way that was simple and correct.

Like other patches in this series, this change is potentially impacted
but revlogdag's ignorance of filtered revisions. The new code is
filtering aware, since changelog's revs() (used by children() will
skip filtered revisions and therefore hidden children won't appear.
This is potentially backwards incompatible. But no tests fail and
I think this code should respect visibility.

Differential Revision: https://phab.mercurial-scm.org/D4322

mercurial/setdiscovery.py

@@ -56,7 +56,7 b' from . import ('
     util,
 )
 
-def _updatesample(dag, revs, heads, sample, quicksamplesize=0):
+def _updatesample(revs, heads, sample, parentfn, quicksamplesize=0):
     """update an existing sample to match the expected size
 
     The sample is updated with revs exponentially distant from each head of the
@@ -66,10 +66,10 b' def _updatesample(dag, revs, heads, samp'
     reached. Otherwise sampling will happen until roots of the <revs> set are
     reached.
 
-    :dag: a dag object from dagutil
     :revs:  set of revs we want to discover (if None, assume the whole dag)
     :heads: set of DAG head revs
     :sample: a sample to update
+    :parentfn: a callable to resolve parents for a revision
     :quicksamplesize: optional target size of the sample"""
     dist = {}
     visit = collections.deque(heads)
@@ -87,12 +87,13 b' def _updatesample(dag, revs, heads, samp'
             if quicksamplesize and (len(sample) >= quicksamplesize):
                 return
         seen.add(curr)
-        for p in dag.parents(curr):
-            if not revs or p in revs:
+
+        for p in parentfn(curr):
+            if p != nullrev and (not revs or p in revs):
                 dist.setdefault(p, d + 1)
                 visit.append(p)
 
-def _takequicksample(repo, dag, headrevs, revs, size):
+def _takequicksample(repo, headrevs, revs, size):
     """takes a quick sample of size <size>
 
     It is meant for initial sampling and focuses on querying heads and close
@@ -107,18 +108,23 b' def _takequicksample(repo, dag, headrevs'
     if len(sample) >= size:
         return _limitsample(sample, size)
 
-    _updatesample(dag, None, headrevs, sample, quicksamplesize=size)
+    _updatesample(None, headrevs, sample, repo.changelog.parentrevs,
+                  quicksamplesize=size)
     return sample
 
-def _takefullsample(repo, dag, headrevs, revs, size):
+def _takefullsample(repo, headrevs, revs, size):
     sample = set(repo.revs('heads(%ld)', revs))
 
     # update from heads
     revsheads = set(repo.revs('heads(%ld)', revs))
-    _updatesample(dag, revs, revsheads, sample)
+    _updatesample(revs, revsheads, sample, repo.changelog.parentrevs)
     # update from roots
     revsroots = set(repo.revs('roots(%ld)', revs))
-    _updatesample(dag.inverse(), revs, revsroots, sample)
+
+    # TODO this is quadratic
+    parentfn = lambda rev: repo.changelog.children(repo.changelog.node(rev))
+
+    _updatesample(revs, revsroots, sample, parentfn)
     assert sample
     sample = _limitsample(sample, size)
     if len(sample) < size:
@@ -244,7 +250,7 b' def findcommonheads(ui, local, remote,'
         if len(undecided) < targetsize:
             sample = list(undecided)
         else:
-            sample = samplefunc(local, dag, ownheads, undecided, targetsize)
+            sample = samplefunc(local, ownheads, undecided, targetsize)
 
         roundtrips += 1
         progress.update(roundtrips)