changegroup: restore default node ordering (issue6001)

Changeset db5501d9 changed the default node ordering from "storage" to
"linearize".

While the new API is more explicit and cleaner, the "linearize" order is
problematic on certain repositories like netbeans where it makes bundling
slower the more nodes we bundle.

Pushing and pulling 100 changesets was ~20% slower and pushing and pulling
1000 changesets was ~600% slower.

A very quick analysis of profile traces showed that the pull operation was
taking more time creating the delta.

Putting back the old default order seems to be the safe option. With more time
during the next cycle, we can understand better the impact of sorting with the
DAG order by default, the source of the regression and how to mitigate it.

/!\ We are still waiting for the full performance impact but with this patch,
bundling and pulling locally (not on the performance workstation) 1000
changesets on the netbeans repository is as fast as before the regression.

Differential Revision: https://phab.mercurial-scm.org/D5196

mercurial/utils/storageutil.py

@@ -1,480 +1,480 b''
 # storageutil.py - Storage functionality agnostic of backend implementation.
 #
 # Copyright 2018 Gregory Szorc <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 from __future__ import absolute_import
 
 import hashlib
 import re
 import struct
 
 from ..i18n import _
 from ..node import (
     bin,
     nullid,
     nullrev,
 )
 from .. import (
     dagop,
     error,
     mdiff,
     pycompat,
 )
 
 _nullhash = hashlib.sha1(nullid)
 
 def hashrevisionsha1(text, p1, p2):
     """Compute the SHA-1 for revision data and its parents.
 
     This hash combines both the current file contents and its history
     in a manner that makes it easy to distinguish nodes with the same
     content in the revision graph.
     """
     # As of now, if one of the parent node is null, p2 is null
     if p2 == nullid:
         # deep copy of a hash is faster than creating one
         s = _nullhash.copy()
         s.update(p1)
     else:
         # none of the parent nodes are nullid
         if p1 < p2:
             a = p1
             b = p2
         else:
             a = p2
             b = p1
         s = hashlib.sha1(a)
         s.update(b)
     s.update(text)
     return s.digest()
 
 METADATA_RE = re.compile(b'\x01\n')
 
 def parsemeta(text):
     """Parse metadata header from revision data.
 
     Returns a 2-tuple of (metadata, offset), where both can be None if there
     is no metadata.
     """
     # text can be buffer, so we can't use .startswith or .index
     if text[:2] != b'\x01\n':
         return None, None
     s = METADATA_RE.search(text, 2).start()
     mtext = text[2:s]
     meta = {}
     for l in mtext.splitlines():
         k, v = l.split(b': ', 1)
         meta[k] = v
     return meta, s + 2
 
 def packmeta(meta, text):
     """Add metadata to fulltext to produce revision text."""
     keys = sorted(meta)
     metatext = b''.join(b'%s: %s\n' % (k, meta[k]) for k in keys)
     return b'\x01\n%s\x01\n%s' % (metatext, text)
 
 def iscensoredtext(text):
     meta = parsemeta(text)[0]
     return meta and b'censored' in meta
 
 def filtermetadata(text):
     """Extract just the revision data from source text.
 
     Returns ``text`` unless it has a metadata header, in which case we return
     a new buffer without hte metadata.
     """
     if not text.startswith(b'\x01\n'):
         return text
 
     offset = text.index(b'\x01\n', 2)
     return text[offset + 2:]
 
 def filerevisioncopied(store, node):
     """Resolve file revision copy metadata.
 
     Returns ``False`` if the file has no copy metadata. Otherwise a
     2-tuple of the source filename and node.
     """
     if store.parents(node)[0] != nullid:
         return False
 
     meta = parsemeta(store.revision(node))[0]
 
     # copy and copyrev occur in pairs. In rare cases due to old bugs,
     # one can occur without the other. So ensure both are present to flag
     # as a copy.
     if meta and b'copy' in meta and b'copyrev' in meta:
         return meta[b'copy'], bin(meta[b'copyrev'])
 
     return False
 
 def filedataequivalent(store, node, filedata):
     """Determines whether file data is equivalent to a stored node.
 
     Returns True if the passed file data would hash to the same value
     as a stored revision and False otherwise.
 
     When a stored revision is censored, filedata must be empty to have
     equivalence.
 
     When a stored revision has copy metadata, it is ignored as part
     of the compare.
     """
 
     if filedata.startswith(b'\x01\n'):
         revisiontext = b'\x01\n\x01\n' + filedata
     else:
         revisiontext = filedata
 
     p1, p2 = store.parents(node)
 
     computednode = hashrevisionsha1(revisiontext, p1, p2)
 
     if computednode == node:
         return True
 
     # Censored files compare against the empty file.
     if store.iscensored(store.rev(node)):
         return filedata == b''
 
     # Renaming a file produces a different hash, even if the data
     # remains unchanged. Check if that's the case.
     if store.renamed(node):
         return store.read(node) == filedata
 
     return False
 
 def iterrevs(storelen, start=0, stop=None):
     """Iterate over revision numbers in a store."""
     step = 1
 
     if stop is not None:
         if start > stop:
             step = -1
         stop += step
         if stop > storelen:
             stop = storelen
     else:
         stop = storelen
 
     return pycompat.xrange(start, stop, step)
 
 def fileidlookup(store, fileid, identifier):
     """Resolve the file node for a value.
 
     ``store`` is an object implementing the ``ifileindex`` interface.
 
     ``fileid`` can be:
 
     * A 20 byte binary node.
     * An integer revision number
     * A 40 byte hex node.
     * A bytes that can be parsed as an integer representing a revision number.
 
     ``identifier`` is used to populate ``error.LookupError`` with an identifier
     for the store.
 
     Raises ``error.LookupError`` on failure.
     """
     if isinstance(fileid, int):
         try:
             return store.node(fileid)
         except IndexError:
             raise error.LookupError('%d' % fileid, identifier,
                                     _('no match found'))
 
     if len(fileid) == 20:
         try:
             store.rev(fileid)
             return fileid
         except error.LookupError:
             pass
 
     if len(fileid) == 40:
         try:
             rawnode = bin(fileid)
             store.rev(rawnode)
             return rawnode
         except TypeError:
             pass
 
     try:
         rev = int(fileid)
 
         if b'%d' % rev != fileid:
             raise ValueError
 
         try:
             return store.node(rev)
         except (IndexError, TypeError):
             pass
     except (ValueError, OverflowError):
         pass
 
     raise error.LookupError(fileid, identifier, _('no match found'))
 
 def resolvestripinfo(minlinkrev, tiprev, headrevs, linkrevfn, parentrevsfn):
     """Resolve information needed to strip revisions.
 
     Finds the minimum revision number that must be stripped in order to
     strip ``minlinkrev``.
 
     Returns a 2-tuple of the minimum revision number to do that and a set
     of all revision numbers that have linkrevs that would be broken
     by that strip.
 
     ``tiprev`` is the current tip-most revision. It is ``len(store) - 1``.
     ``headrevs`` is an iterable of head revisions.
     ``linkrevfn`` is a callable that receives a revision and returns a linked
     revision.
     ``parentrevsfn`` is a callable that receives a revision number and returns
     an iterable of its parent revision numbers.
     """
     brokenrevs = set()
     strippoint = tiprev + 1
 
     heads = {}
     futurelargelinkrevs = set()
     for head in headrevs:
         headlinkrev = linkrevfn(head)
         heads[head] = headlinkrev
         if headlinkrev >= minlinkrev:
             futurelargelinkrevs.add(headlinkrev)
 
     # This algorithm involves walking down the rev graph, starting at the
     # heads. Since the revs are topologically sorted according to linkrev,
     # once all head linkrevs are below the minlink, we know there are
     # no more revs that could have a linkrev greater than minlink.
     # So we can stop walking.
     while futurelargelinkrevs:
         strippoint -= 1
         linkrev = heads.pop(strippoint)
 
         if linkrev < minlinkrev:
             brokenrevs.add(strippoint)
         else:
             futurelargelinkrevs.remove(linkrev)
 
         for p in parentrevsfn(strippoint):
             if p != nullrev:
                 plinkrev = linkrevfn(p)
                 heads[p] = plinkrev
                 if plinkrev >= minlinkrev:
                     futurelargelinkrevs.add(plinkrev)
 
     return strippoint, brokenrevs
 
 def emitrevisions(store, nodes, nodesorder, resultcls, deltaparentfn=None,
                   candeltafn=None, rawsizefn=None, revdifffn=None, flagsfn=None,
                   sendfulltext=False,
                   revisiondata=False, assumehaveparentrevisions=False,
                   deltaprevious=False):
     """Generic implementation of ifiledata.emitrevisions().
 
     Emitting revision data is subtly complex. This function attempts to
     encapsulate all the logic for doing so in a backend-agnostic way.
 
     ``store``
        Object conforming to ``ifilestorage`` interface.
 
     ``nodes``
        List of revision nodes whose data to emit.
 
     ``resultcls``
        A type implementing the ``irevisiondelta`` interface that will be
        constructed and returned.
 
     ``deltaparentfn`` (optional)
        Callable receiving a revision number and returning the revision number
        of a revision that the internal delta is stored against. This delta
        will be preferred over computing a new arbitrary delta.
 
        If not defined, a delta will always be computed from raw revision
        data.
 
     ``candeltafn`` (optional)
        Callable receiving a pair of revision numbers that returns a bool
        indicating whether a delta between them can be produced.
 
        If not defined, it is assumed that any two revisions can delta with
        each other.
 
     ``rawsizefn`` (optional)
        Callable receiving a revision number and returning the length of the
        ``store.revision(rev, raw=True)``.
 
        If not defined, ``len(store.revision(rev, raw=True))`` will be called.
 
     ``revdifffn`` (optional)
        Callable receiving a pair of revision numbers that returns a delta
        between them.
 
        If not defined, a delta will be computed by invoking mdiff code
        on ``store.revision()`` results.
 
        Defining this function allows a precomputed or stored delta to be
        used without having to compute on.
 
     ``flagsfn`` (optional)
        Callable receiving a revision number and returns the integer flags
        value for it. If not defined, flags value will be 0.
 
     ``sendfulltext``
        Whether to send fulltext revisions instead of deltas, if allowed.
 
     ``nodesorder``
     ``revisiondata``
     ``assumehaveparentrevisions``
     ``deltaprevious``
        See ``ifiledata.emitrevisions()`` interface documentation.
     """
 
     fnode = store.node
     frev = store.rev
 
     if nodesorder == 'nodes':
         revs = [frev(n) for n in nodes]
-    elif nodesorder == 'storage':
-        revs = sorted(frev(n) for n in nodes)
-    else:
+    elif nodesorder == 'linear':
         revs = set(frev(n) for n in nodes)
         revs = dagop.linearize(revs, store.parentrevs)
+    else: # storage and default
+        revs = sorted(frev(n) for n in nodes)
 
     prevrev = None
 
     if deltaprevious or assumehaveparentrevisions:
         prevrev = store.parentrevs(revs[0])[0]
 
     # Set of revs available to delta against.
     available = set()
 
     for rev in revs:
         if rev == nullrev:
             continue
 
         node = fnode(rev)
         p1rev, p2rev = store.parentrevs(rev)
 
         if deltaparentfn:
             deltaparentrev = deltaparentfn(rev)
         else:
             deltaparentrev = nullrev
 
         # Forced delta against previous mode.
         if deltaprevious:
             baserev = prevrev
 
         # We're instructed to send fulltext. Honor that.
         elif sendfulltext:
             baserev = nullrev
 
         # There is a delta in storage. We try to use that because it
         # amounts to effectively copying data from storage and is
         # therefore the fastest.
         elif deltaparentrev != nullrev:
             # Base revision was already emitted in this group. We can
             # always safely use the delta.
             if deltaparentrev in available:
                 baserev = deltaparentrev
 
             # Base revision is a parent that hasn't been emitted already.
             # Use it if we can assume the receiver has the parent revision.
             elif (assumehaveparentrevisions
                   and deltaparentrev in (p1rev, p2rev)):
                 baserev = deltaparentrev
 
             # No guarantee the receiver has the delta parent. Send delta
             # against last revision (if possible), which in the common case
             # should be similar enough to this revision that the delta is
             # reasonable.
             elif prevrev is not None:
                 baserev = prevrev
             else:
                 baserev = nullrev
 
         # Storage has a fulltext revision.
 
         # Let's use the previous revision, which is as good a guess as any.
         # There is definitely room to improve this logic.
         elif prevrev is not None:
             baserev = prevrev
         else:
             baserev = nullrev
 
         # But we can't actually use our chosen delta base for whatever
         # reason. Reset to fulltext.
         if baserev != nullrev and (candeltafn and not candeltafn(baserev, rev)):
             baserev = nullrev
 
         revision = None
         delta = None
         baserevisionsize = None
 
         if revisiondata:
             if store.iscensored(baserev) or store.iscensored(rev):
                 try:
                     revision = store.revision(node, raw=True)
                 except error.CensoredNodeError as e:
                     revision = e.tombstone
 
                 if baserev != nullrev:
                     if rawsizefn:
                         baserevisionsize = rawsizefn(baserev)
                     else:
                         baserevisionsize = len(store.revision(baserev,
                                                               raw=True))
 
             elif baserev == nullrev and not deltaprevious:
                 revision = store.revision(node, raw=True)
                 available.add(rev)
             else:
                 if revdifffn:
                     delta = revdifffn(baserev, rev)
                 else:
                     delta = mdiff.textdiff(store.revision(baserev, raw=True),
                                            store.revision(rev, raw=True))
 
                 available.add(rev)
 
         yield resultcls(
             node=node,
             p1node=fnode(p1rev),
             p2node=fnode(p2rev),
             basenode=fnode(baserev),
             flags=flagsfn(rev) if flagsfn else 0,
             baserevisionsize=baserevisionsize,
             revision=revision,
             delta=delta)
 
         prevrev = rev
 
 def deltaiscensored(delta, baserev, baselenfn):
     """Determine if a delta represents censored revision data.
 
     ``baserev`` is the base revision this delta is encoded against.
     ``baselenfn`` is a callable receiving a revision number that resolves the
     length of the revision fulltext.
 
     Returns a bool indicating if the result of the delta represents a censored
     revision.
     """
     # Fragile heuristic: unless new file meta keys are added alphabetically
     # preceding "censored", all censored revisions are prefixed by
     # "\1\ncensored:". A delta producing such a censored revision must be a
     # full-replacement delta, so we inspect the first and only patch in the
     # delta for this prefix.
     hlen = struct.calcsize(">lll")
     if len(delta) <= hlen:
         return False
 
     oldlen = baselenfn(baserev)
     newlen = len(delta) - hlen
     if delta[:hlen] != mdiff.replacediffheader(oldlen, newlen):
         return False
 
     add = "\1\ncensored:"
     addlen = len(add)
     return newlen >= addlen and delta[hlen:hlen + addlen] == add

tests/test-generaldelta.t

@@ -1,402 +1,402 b''
 #require no-reposimplestore
 
 Check whether size of generaldelta revlog is not bigger than its
 regular equivalent. Test would fail if generaldelta was naive
 implementation of parentdelta: third manifest revision would be fully
 inserted due to big distance from its paren revision (zero).
 
   $ hg init repo --config format.generaldelta=no --config format.usegeneraldelta=no
   $ cd repo
   $ echo foo > foo
   $ echo bar > bar
   $ echo baz > baz
   $ hg commit -q -Am boo
   $ hg clone --pull . ../gdrepo -q --config format.generaldelta=yes
   $ for r in 1 2 3; do
   >   echo $r > foo
   >   hg commit -q -m $r
   >   hg up -q -r 0
   >   hg pull . -q -r $r -R ../gdrepo
   > done
 
   $ cd ..
   >>> from __future__ import print_function
   >>> import os
   >>> regsize = os.stat("repo/.hg/store/00manifest.i").st_size
   >>> gdsize = os.stat("gdrepo/.hg/store/00manifest.i").st_size
   >>> if regsize < gdsize:
   ...     print('generaldata increased size of manifest')
 
 Verify rev reordering doesnt create invalid bundles (issue4462)
 This requires a commit tree that when pulled will reorder manifest revs such
 that the second manifest to create a file rev will be ordered before the first
 manifest to create that file rev. We also need to do a partial pull to ensure
 reordering happens. At the end we verify the linkrev points at the earliest
 commit.
 
   $ hg init server --config format.generaldelta=True
   $ cd server
   $ touch a
   $ hg commit -Aqm a
   $ echo x > x
   $ echo y > y
   $ hg commit -Aqm xy
   $ hg up -q '.^'
   $ echo x > x
   $ echo z > z
   $ hg commit -Aqm xz
   $ hg up -q 1
   $ echo b > b
   $ hg commit -Aqm b
   $ hg merge -q 2
   $ hg commit -Aqm merge
   $ echo c > c
   $ hg commit -Aqm c
   $ hg log -G -T '{rev} {shortest(node)} {desc}'
   @  5 ebb8 c
   |
   o    4 baf7 merge
   |\
   | o  3 a129 b
   | |
   o |  2 958c xz
   | |
   | o  1 f00c xy
   |/
   o  0 3903 a
   
   $ cd ..
   $ hg init client --config format.generaldelta=false --config format.usegeneraldelta=false
   $ cd client
   $ hg pull -q ../server -r 4
   $ hg debugdeltachain x
       rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
         0       1        1       -1    base          3          2          3   1.50000         3         0    0.00000
 
   $ cd ..
 
 Test "usegeneraldelta" config
 (repo are general delta, but incoming bundle are not re-deltafied)
 
 delta coming from the server base delta server are not recompressed.
 (also include the aggressive version for comparison)
 
   $ hg clone repo --pull --config format.usegeneraldelta=1 usegd
   requesting all changes
   adding changesets
   adding manifests
   adding file changes
   added 4 changesets with 6 changes to 3 files (+2 heads)
   new changesets 0ea3fcf9d01d:bba78d330d9c
   updating to branch default
   3 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg clone repo --pull --config format.generaldelta=1 full
   requesting all changes
   adding changesets
   adding manifests
   adding file changes
   added 4 changesets with 6 changes to 3 files (+2 heads)
   new changesets 0ea3fcf9d01d:bba78d330d9c
   updating to branch default
   3 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg -R repo debugdeltachain -m
       rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
         0       1        1       -1    base        104        135        104   0.77037       104         0    0.00000
         1       1        2        0    prev         57        135        161   1.19259       161         0    0.00000
         2       1        3        1    prev         57        135        218   1.61481       218         0    0.00000
         3       2        1       -1    base        104        135        104   0.77037       104         0    0.00000
   $ hg -R usegd debugdeltachain -m
       rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
         0       1        1       -1    base        104        135        104   0.77037       104         0    0.00000
         1       1        2        0      p1         57        135        161   1.19259       161         0    0.00000
         2       1        3        1    prev         57        135        218   1.61481       218         0    0.00000
         3       1        2        0      p1         57        135        161   1.19259       275       114    0.70807
   $ hg -R full debugdeltachain -m
       rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
         0       1        1       -1    base        104        135        104   0.77037       104         0    0.00000
         1       1        2        0      p1         57        135        161   1.19259       161         0    0.00000
         2       1        2        0      p1         57        135        161   1.19259       218        57    0.35404
         3       1        2        0      p1         57        135        161   1.19259       275       114    0.70807
 
 Test revlog.optimize-delta-parent-choice
 
   $ hg init --config format.generaldelta=1 aggressive
   $ cd aggressive
   $ cat << EOF >> .hg/hgrc
   > [format]
   > generaldelta = 1
   > EOF
   $ touch a b c d e
   $ hg commit -Aqm side1
   $ hg up -q null
   $ touch x y
   $ hg commit -Aqm side2
 
 - Verify non-aggressive merge uses p1 (commit 1) as delta parent
   $ hg merge -q 0
   $ hg commit -q -m merge
   $ hg debugdeltachain -m
       rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
         0       1        1       -1    base         59        215         59   0.27442        59         0    0.00000
         1       1        2        0    prev         61         86        120   1.39535       120         0    0.00000
         2       1        2        0      p2         62        301        121   0.40199       182        61    0.50413
 
   $ hg strip -q -r . --config extensions.strip=
 
 - Verify aggressive merge uses p2 (commit 0) as delta parent
   $ hg up -q -C 1
   $ hg merge -q 0
   $ hg commit -q -m merge --config storage.revlog.optimize-delta-parent-choice=yes
   $ hg debugdeltachain -m
       rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
         0       1        1       -1    base         59        215         59   0.27442        59         0    0.00000
         1       1        2        0    prev         61         86        120   1.39535       120         0    0.00000
         2       1        2        0      p2         62        301        121   0.40199       182        61    0.50413
 
 Test that strip bundle use bundle2
   $ hg --config extensions.strip= strip .
   0 files updated, 0 files merged, 5 files removed, 0 files unresolved
   saved backup bundle to $TESTTMP/aggressive/.hg/strip-backup/1c5d4dc9a8b8-6c68e60c-backup.hg
   $ hg debugbundle .hg/strip-backup/*
   Stream params: {Compression: BZ}
   changegroup -- {nbchanges: 1, version: 02} (mandatory: True)
       1c5d4dc9a8b8d6e1750966d343e94db665e7a1e9
   cache:rev-branch-cache -- {} (mandatory: False)
   phase-heads -- {} (mandatory: True)
       1c5d4dc9a8b8d6e1750966d343e94db665e7a1e9 draft
 
   $ cd ..
 
 test maxdeltachainspan
 
   $ hg init source-repo
   $ cd source-repo
   $ hg debugbuilddag --new-file '.+5:brancha$.+11:branchb$.+30:branchc<brancha+2<branchb+2'
 # add an empty revision somewhere
   $ hg up tip
   14 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg rm .
   removing nf10
   removing nf11
   removing nf12
   removing nf13
   removing nf14
   removing nf15
   removing nf16
   removing nf17
   removing nf51
   removing nf52
   removing nf6
   removing nf7
   removing nf8
   removing nf9
   $ hg commit -m 'empty all'
   $ hg revert --all --rev 'p1(.)'
   adding nf10
   adding nf11
   adding nf12
   adding nf13
   adding nf14
   adding nf15
   adding nf16
   adding nf17
   adding nf51
   adding nf52
   adding nf6
   adding nf7
   adding nf8
   adding nf9
   $ hg commit -m 'restore all'
   $ hg up null
   0 files updated, 0 files merged, 14 files removed, 0 files unresolved
   $ 
   $ cd ..
   $ hg -R source-repo debugdeltachain -m
       rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
         0       1        1       -1    base         46         45         46   1.02222        46         0    0.00000
         1       1        2        0      p1         57         90        103   1.14444       103         0    0.00000
         2       1        3        1      p1         57        135        160   1.18519       160         0    0.00000
         3       1        4        2      p1         57        180        217   1.20556       217         0    0.00000
         4       1        5        3      p1         57        225        274   1.21778       274         0    0.00000
         5       1        6        4      p1         57        270        331   1.22593       331         0    0.00000
         6       2        1       -1    base         46         45         46   1.02222        46         0    0.00000
         7       2        2        6      p1         57         90        103   1.14444       103         0    0.00000
         8       2        3        7      p1         57        135        160   1.18519       160         0    0.00000
         9       2        4        8      p1         57        180        217   1.20556       217         0    0.00000
        10       2        5        9      p1         58        226        275   1.21681       275         0    0.00000
        11       2        6       10      p1         58        272        333   1.22426       333         0    0.00000
        12       2        7       11      p1         58        318        391   1.22956       391         0    0.00000
        13       2        8       12      p1         58        364        449   1.23352       449         0    0.00000
        14       2        9       13      p1         58        410        507   1.23659       507         0    0.00000
        15       2       10       14      p1         58        456        565   1.23904       565         0    0.00000
        16       2       11       15      p1         58        502        623   1.24104       623         0    0.00000
        17       2       12       16      p1         58        548        681   1.24270       681         0    0.00000
        18       3        1       -1    base         47         46         47   1.02174        47         0    0.00000
        19       3        2       18      p1         58         92        105   1.14130       105         0    0.00000
        20       3        3       19      p1         58        138        163   1.18116       163         0    0.00000
        21       3        4       20      p1         58        184        221   1.20109       221         0    0.00000
        22       3        5       21      p1         58        230        279   1.21304       279         0    0.00000
        23       3        6       22      p1         58        276        337   1.22101       337         0    0.00000
        24       3        7       23      p1         58        322        395   1.22671       395         0    0.00000
        25       3        8       24      p1         58        368        453   1.23098       453         0    0.00000
        26       3        9       25      p1         58        414        511   1.23430       511         0    0.00000
        27       3       10       26      p1         58        460        569   1.23696       569         0    0.00000
        28       3       11       27      p1         58        506        627   1.23913       627         0    0.00000
        29       3       12       28      p1         58        552        685   1.24094       685         0    0.00000
        30       3       13       29      p1         58        598        743   1.24247       743         0    0.00000
        31       3       14       30      p1         58        644        801   1.24379       801         0    0.00000
        32       3       15       31      p1         58        690        859   1.24493       859         0    0.00000
        33       3       16       32      p1         58        736        917   1.24592       917         0    0.00000
        34       3       17       33      p1         58        782        975   1.24680       975         0    0.00000
        35       3       18       34      p1         58        828       1033   1.24758      1033         0    0.00000
        36       3       19       35      p1         58        874       1091   1.24828      1091         0    0.00000
        37       3       20       36      p1         58        920       1149   1.24891      1149         0    0.00000
        38       3       21       37      p1         58        966       1207   1.24948      1207         0    0.00000
        39       3       22       38      p1         58       1012       1265   1.25000      1265         0    0.00000
        40       3       23       39      p1         58       1058       1323   1.25047      1323         0    0.00000
        41       3       24       40      p1         58       1104       1381   1.25091      1381         0    0.00000
        42       3       25       41      p1         58       1150       1439   1.25130      1439         0    0.00000
        43       3       26       42      p1         58       1196       1497   1.25167      1497         0    0.00000
        44       3       27       43      p1         58       1242       1555   1.25201      1555         0    0.00000
        45       3       28       44      p1         58       1288       1613   1.25233      1613         0    0.00000
        46       3       29       45      p1         58       1334       1671   1.25262      1671         0    0.00000
        47       3       30       46      p1         58       1380       1729   1.25290      1729         0    0.00000
        48       3       31       47      p1         58       1426       1787   1.25316      1787         0    0.00000
        49       4        1       -1    base        197        316        197   0.62342       197         0    0.00000
        50       4        2       49      p1         58        362        255   0.70442       255         0    0.00000
        51       4        3       50    prev        356        594        611   1.02862       611         0    0.00000
        52       4        4       51      p1         58        640        669   1.04531       669         0    0.00000
        53       5        1       -1    base          0          0          0   0.00000         0         0    0.00000
        54       6        1       -1    base        369        640        369   0.57656       369         0    0.00000
   $ hg clone --pull source-repo --config experimental.maxdeltachainspan=2800 relax-chain --config format.generaldelta=yes
   requesting all changes
   adding changesets
   adding manifests
   adding file changes
   added 55 changesets with 53 changes to 53 files (+2 heads)
   new changesets 61246295ee1e:c930ac4a5b32
   updating to branch default
   14 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg -R relax-chain debugdeltachain -m
       rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
-        0       1        1       -1    base         47         46         47   1.02174        47         0    0.00000
-        1       1        2        0      p1         58         92        105   1.14130       105         0    0.00000
-        2       1        3        1      p1         58        138        163   1.18116       163         0    0.00000
-        3       1        4        2      p1         58        184        221   1.20109       221         0    0.00000
-        4       1        5        3      p1         58        230        279   1.21304       279         0    0.00000
-        5       1        6        4      p1         58        276        337   1.22101       337         0    0.00000
-        6       1        7        5      p1         58        322        395   1.22671       395         0    0.00000
-        7       1        8        6      p1         58        368        453   1.23098       453         0    0.00000
-        8       1        9        7      p1         58        414        511   1.23430       511         0    0.00000
-        9       1       10        8      p1         58        460        569   1.23696       569         0    0.00000
-       10       1       11        9      p1         58        506        627   1.23913       627         0    0.00000
-       11       1       12       10      p1         58        552        685   1.24094       685         0    0.00000
-       12       1       13       11      p1         58        598        743   1.24247       743         0    0.00000
-       13       1       14       12      p1         58        644        801   1.24379       801         0    0.00000
-       14       1       15       13      p1         58        690        859   1.24493       859         0    0.00000
-       15       1       16       14      p1         58        736        917   1.24592       917         0    0.00000
-       16       1       17       15      p1         58        782        975   1.24680       975         0    0.00000
-       17       1       18       16      p1         58        828       1033   1.24758      1033         0    0.00000
-       18       1       19       17      p1         58        874       1091   1.24828      1091         0    0.00000
-       19       1       20       18      p1         58        920       1149   1.24891      1149         0    0.00000
-       20       1       21       19      p1         58        966       1207   1.24948      1207         0    0.00000
-       21       1       22       20      p1         58       1012       1265   1.25000      1265         0    0.00000
-       22       1       23       21      p1         58       1058       1323   1.25047      1323         0    0.00000
-       23       1       24       22      p1         58       1104       1381   1.25091      1381         0    0.00000
-       24       1       25       23      p1         58       1150       1439   1.25130      1439         0    0.00000
-       25       1       26       24      p1         58       1196       1497   1.25167      1497         0    0.00000
-       26       1       27       25      p1         58       1242       1555   1.25201      1555         0    0.00000
-       27       1       28       26      p1         58       1288       1613   1.25233      1613         0    0.00000
-       28       1       29       27      p1         58       1334       1671   1.25262      1671         0    0.00000
-       29       1       30       28      p1         58       1380       1729   1.25290      1729         0    0.00000
-       30       1       31       29      p1         58       1426       1787   1.25316      1787         0    0.00000
-       31       2        1       -1    base         46         45         46   1.02222        46         0    0.00000
-       32       2        2       31      p1         57         90        103   1.14444       103         0    0.00000
-       33       2        3       32      p1         57        135        160   1.18519       160         0    0.00000
-       34       2        4       33      p1         57        180        217   1.20556       217         0    0.00000
-       35       2        5       34      p1         57        225        274   1.21778       274         0    0.00000
-       36       2        6       35      p1         57        270        331   1.22593       331         0    0.00000
-       37       2        7       36      p1         58        316        389   1.23101       389         0    0.00000
-       38       2        8       37      p1         58        362        447   1.23481       447         0    0.00000
-       39       3        1       -1    base         46         45         46   1.02222        46         0    0.00000
-       40       3        2       39      p1         57         90        103   1.14444       103         0    0.00000
-       41       3        3       40      p1         57        135        160   1.18519       160         0    0.00000
-       42       3        4       41      p1         57        180        217   1.20556       217         0    0.00000
-       43       3        5       42      p1         58        226        275   1.21681       275         0    0.00000
-       44       3        6       43      p1         58        272        333   1.22426       333         0    0.00000
-       45       3        7       44      p1         58        318        391   1.22956       391         0    0.00000
-       46       3        8       45      p1         58        364        449   1.23352       449         0    0.00000
-       47       3        9       46      p1         58        410        507   1.23659       507         0    0.00000
-       48       3       10       47      p1         58        456        565   1.23904       565         0    0.00000
-       49       3       11       48      p1         58        502        623   1.24104       623         0    0.00000
-       50       3       12       49      p1         58        548        681   1.24270       681         0    0.00000
-       51       3       13       50      p1         58        594        739   1.24411       739         0    0.00000
-       52       3       14       51      p1         58        640        797   1.24531       797         0    0.00000
-       53       4        1       -1    base          0          0          0   0.00000         0         0    0.00000
-       54       5        1       -1    base        369        640        369   0.57656       369         0    0.00000
+        0       1        1       -1    base         46         45         46   1.02222        46         0    0.00000
+        1       1        2        0      p1         57         90        103   1.14444       103         0    0.00000
+        2       1        3        1      p1         57        135        160   1.18519       160         0    0.00000
+        3       1        4        2      p1         57        180        217   1.20556       217         0    0.00000
+        4       1        5        3      p1         57        225        274   1.21778       274         0    0.00000
+        5       1        6        4      p1         57        270        331   1.22593       331         0    0.00000
+        6       2        1       -1    base         46         45         46   1.02222        46         0    0.00000
+        7       2        2        6      p1         57         90        103   1.14444       103         0    0.00000
+        8       2        3        7      p1         57        135        160   1.18519       160         0    0.00000
+        9       2        4        8      p1         57        180        217   1.20556       217         0    0.00000
+       10       2        5        9      p1         58        226        275   1.21681       275         0    0.00000
+       11       2        6       10      p1         58        272        333   1.22426       333         0    0.00000
+       12       2        7       11      p1         58        318        391   1.22956       391         0    0.00000
+       13       2        8       12      p1         58        364        449   1.23352       449         0    0.00000
+       14       2        9       13      p1         58        410        507   1.23659       507         0    0.00000
+       15       2       10       14      p1         58        456        565   1.23904       565         0    0.00000
+       16       2       11       15      p1         58        502        623   1.24104       623         0    0.00000
+       17       2       12       16      p1         58        548        681   1.24270       681         0    0.00000
+       18       3        1       -1    base         47         46         47   1.02174        47         0    0.00000
+       19       3        2       18      p1         58         92        105   1.14130       105         0    0.00000
+       20       3        3       19      p1         58        138        163   1.18116       163         0    0.00000
+       21       3        4       20      p1         58        184        221   1.20109       221         0    0.00000
+       22       3        5       21      p1         58        230        279   1.21304       279         0    0.00000
+       23       3        6       22      p1         58        276        337   1.22101       337         0    0.00000
+       24       3        7       23      p1         58        322        395   1.22671       395         0    0.00000
+       25       3        8       24      p1         58        368        453   1.23098       453         0    0.00000
+       26       3        9       25      p1         58        414        511   1.23430       511         0    0.00000
+       27       3       10       26      p1         58        460        569   1.23696       569         0    0.00000
+       28       3       11       27      p1         58        506        627   1.23913       627         0    0.00000
+       29       3       12       28      p1         58        552        685   1.24094       685         0    0.00000
+       30       3       13       29      p1         58        598        743   1.24247       743         0    0.00000
+       31       3       14       30      p1         58        644        801   1.24379       801         0    0.00000
+       32       3       15       31      p1         58        690        859   1.24493       859         0    0.00000
+       33       3       16       32      p1         58        736        917   1.24592       917         0    0.00000
+       34       3       17       33      p1         58        782        975   1.24680       975         0    0.00000
+       35       3       18       34      p1         58        828       1033   1.24758      1033         0    0.00000
+       36       3       19       35      p1         58        874       1091   1.24828      1091         0    0.00000
+       37       3       20       36      p1         58        920       1149   1.24891      1149         0    0.00000
+       38       3       21       37      p1         58        966       1207   1.24948      1207         0    0.00000
+       39       3       22       38      p1         58       1012       1265   1.25000      1265         0    0.00000
+       40       3       23       39      p1         58       1058       1323   1.25047      1323         0    0.00000
+       41       3       24       40      p1         58       1104       1381   1.25091      1381         0    0.00000
+       42       3       25       41      p1         58       1150       1439   1.25130      1439         0    0.00000
+       43       3       26       42      p1         58       1196       1497   1.25167      1497         0    0.00000
+       44       3       27       43      p1         58       1242       1555   1.25201      1555         0    0.00000
+       45       3       28       44      p1         58       1288       1613   1.25233      1613         0    0.00000
+       46       3       29       45      p1         58       1334       1671   1.25262      1671         0    0.00000
+       47       3       30       46      p1         58       1380       1729   1.25290      1729         0    0.00000
+       48       3       31       47      p1         58       1426       1787   1.25316      1787         0    0.00000
+       49       4        1       -1    base        197        316        197   0.62342       197         0    0.00000
+       50       4        2       49      p1         58        362        255   0.70442       255         0    0.00000
+       51       2       13       17      p1         58        594        739   1.24411      2781      2042    2.76319
+       52       5        1       -1    base        369        640        369   0.57656       369         0    0.00000
+       53       6        1       -1    base          0          0          0   0.00000         0         0    0.00000
+       54       7        1       -1    base        369        640        369   0.57656       369         0    0.00000
   $ hg clone --pull source-repo --config experimental.maxdeltachainspan=0 noconst-chain --config format.generaldelta=yes
   requesting all changes
   adding changesets
   adding manifests
   adding file changes
   added 55 changesets with 53 changes to 53 files (+2 heads)
   new changesets 61246295ee1e:c930ac4a5b32
   updating to branch default
   14 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg -R noconst-chain debugdeltachain -m
       rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
-        0       1        1       -1    base         47         46         47   1.02174        47         0    0.00000
-        1       1        2        0      p1         58         92        105   1.14130       105         0    0.00000
-        2       1        3        1      p1         58        138        163   1.18116       163         0    0.00000
-        3       1        4        2      p1         58        184        221   1.20109       221         0    0.00000
-        4       1        5        3      p1         58        230        279   1.21304       279         0    0.00000
-        5       1        6        4      p1         58        276        337   1.22101       337         0    0.00000
-        6       1        7        5      p1         58        322        395   1.22671       395         0    0.00000
-        7       1        8        6      p1         58        368        453   1.23098       453         0    0.00000
-        8       1        9        7      p1         58        414        511   1.23430       511         0    0.00000
-        9       1       10        8      p1         58        460        569   1.23696       569         0    0.00000
-       10       1       11        9      p1         58        506        627   1.23913       627         0    0.00000
-       11       1       12       10      p1         58        552        685   1.24094       685         0    0.00000
-       12       1       13       11      p1         58        598        743   1.24247       743         0    0.00000
-       13       1       14       12      p1         58        644        801   1.24379       801         0    0.00000
-       14       1       15       13      p1         58        690        859   1.24493       859         0    0.00000
-       15       1       16       14      p1         58        736        917   1.24592       917         0    0.00000
-       16       1       17       15      p1         58        782        975   1.24680       975         0    0.00000
-       17       1       18       16      p1         58        828       1033   1.24758      1033         0    0.00000
-       18       1       19       17      p1         58        874       1091   1.24828      1091         0    0.00000
-       19       1       20       18      p1         58        920       1149   1.24891      1149         0    0.00000
-       20       1       21       19      p1         58        966       1207   1.24948      1207         0    0.00000
-       21       1       22       20      p1         58       1012       1265   1.25000      1265         0    0.00000
-       22       1       23       21      p1         58       1058       1323   1.25047      1323         0    0.00000
-       23       1       24       22      p1         58       1104       1381   1.25091      1381         0    0.00000
-       24       1       25       23      p1         58       1150       1439   1.25130      1439         0    0.00000
-       25       1       26       24      p1         58       1196       1497   1.25167      1497         0    0.00000
-       26       1       27       25      p1         58       1242       1555   1.25201      1555         0    0.00000
-       27       1       28       26      p1         58       1288       1613   1.25233      1613         0    0.00000
-       28       1       29       27      p1         58       1334       1671   1.25262      1671         0    0.00000
-       29       1       30       28      p1         58       1380       1729   1.25290      1729         0    0.00000
-       30       1       31       29      p1         58       1426       1787   1.25316      1787         0    0.00000
-       31       2        1       -1    base         46         45         46   1.02222        46         0    0.00000
-       32       2        2       31      p1         57         90        103   1.14444       103         0    0.00000
-       33       2        3       32      p1         57        135        160   1.18519       160         0    0.00000
-       34       2        4       33      p1         57        180        217   1.20556       217         0    0.00000
-       35       2        5       34      p1         57        225        274   1.21778       274         0    0.00000
-       36       2        6       35      p1         57        270        331   1.22593       331         0    0.00000
-       37       2        7       36      p1         58        316        389   1.23101       389         0    0.00000
-       38       2        8       37      p1         58        362        447   1.23481       447         0    0.00000
-       39       3        1       -1    base         46         45         46   1.02222        46         0    0.00000
-       40       3        2       39      p1         57         90        103   1.14444       103         0    0.00000
-       41       3        3       40      p1         57        135        160   1.18519       160         0    0.00000
-       42       3        4       41      p1         57        180        217   1.20556       217         0    0.00000
-       43       3        5       42      p1         58        226        275   1.21681       275         0    0.00000
-       44       3        6       43      p1         58        272        333   1.22426       333         0    0.00000
-       45       3        7       44      p1         58        318        391   1.22956       391         0    0.00000
-       46       3        8       45      p1         58        364        449   1.23352       449         0    0.00000
-       47       3        9       46      p1         58        410        507   1.23659       507         0    0.00000
-       48       3       10       47      p1         58        456        565   1.23904       565         0    0.00000
-       49       3       11       48      p1         58        502        623   1.24104       623         0    0.00000
-       50       3       12       49      p1         58        548        681   1.24270       681         0    0.00000
-       51       3       13       50      p1         58        594        739   1.24411       739         0    0.00000
-       52       3       14       51      p1         58        640        797   1.24531       797         0    0.00000
+        0       1        1       -1    base         46         45         46   1.02222        46         0    0.00000
+        1       1        2        0      p1         57         90        103   1.14444       103         0    0.00000
+        2       1        3        1      p1         57        135        160   1.18519       160         0    0.00000
+        3       1        4        2      p1         57        180        217   1.20556       217         0    0.00000
+        4       1        5        3      p1         57        225        274   1.21778       274         0    0.00000
+        5       1        6        4      p1         57        270        331   1.22593       331         0    0.00000
+        6       2        1       -1    base         46         45         46   1.02222        46         0    0.00000
+        7       2        2        6      p1         57         90        103   1.14444       103         0    0.00000
+        8       2        3        7      p1         57        135        160   1.18519       160         0    0.00000
+        9       2        4        8      p1         57        180        217   1.20556       217         0    0.00000
+       10       2        5        9      p1         58        226        275   1.21681       275         0    0.00000
+       11       2        6       10      p1         58        272        333   1.22426       333         0    0.00000
+       12       2        7       11      p1         58        318        391   1.22956       391         0    0.00000
+       13       2        8       12      p1         58        364        449   1.23352       449         0    0.00000
+       14       2        9       13      p1         58        410        507   1.23659       507         0    0.00000
+       15       2       10       14      p1         58        456        565   1.23904       565         0    0.00000
+       16       2       11       15      p1         58        502        623   1.24104       623         0    0.00000
+       17       2       12       16      p1         58        548        681   1.24270       681         0    0.00000
+       18       3        1       -1    base         47         46         47   1.02174        47         0    0.00000
+       19       3        2       18      p1         58         92        105   1.14130       105         0    0.00000
+       20       3        3       19      p1         58        138        163   1.18116       163         0    0.00000
+       21       3        4       20      p1         58        184        221   1.20109       221         0    0.00000
+       22       3        5       21      p1         58        230        279   1.21304       279         0    0.00000
+       23       3        6       22      p1         58        276        337   1.22101       337         0    0.00000
+       24       3        7       23      p1         58        322        395   1.22671       395         0    0.00000
+       25       3        8       24      p1         58        368        453   1.23098       453         0    0.00000
+       26       3        9       25      p1         58        414        511   1.23430       511         0    0.00000
+       27       3       10       26      p1         58        460        569   1.23696       569         0    0.00000
+       28       3       11       27      p1         58        506        627   1.23913       627         0    0.00000
+       29       3       12       28      p1         58        552        685   1.24094       685         0    0.00000
+       30       3       13       29      p1         58        598        743   1.24247       743         0    0.00000
+       31       3       14       30      p1         58        644        801   1.24379       801         0    0.00000
+       32       3       15       31      p1         58        690        859   1.24493       859         0    0.00000
+       33       3       16       32      p1         58        736        917   1.24592       917         0    0.00000
+       34       3       17       33      p1         58        782        975   1.24680       975         0    0.00000
+       35       3       18       34      p1         58        828       1033   1.24758      1033         0    0.00000
+       36       3       19       35      p1         58        874       1091   1.24828      1091         0    0.00000
+       37       3       20       36      p1         58        920       1149   1.24891      1149         0    0.00000
+       38       3       21       37      p1         58        966       1207   1.24948      1207         0    0.00000
+       39       3       22       38      p1         58       1012       1265   1.25000      1265         0    0.00000
+       40       3       23       39      p1         58       1058       1323   1.25047      1323         0    0.00000
+       41       3       24       40      p1         58       1104       1381   1.25091      1381         0    0.00000
+       42       3       25       41      p1         58       1150       1439   1.25130      1439         0    0.00000
+       43       3       26       42      p1         58       1196       1497   1.25167      1497         0    0.00000
+       44       3       27       43      p1         58       1242       1555   1.25201      1555         0    0.00000
+       45       3       28       44      p1         58       1288       1613   1.25233      1613         0    0.00000
+       46       3       29       45      p1         58       1334       1671   1.25262      1671         0    0.00000
+       47       3       30       46      p1         58       1380       1729   1.25290      1729         0    0.00000
+       48       3       31       47      p1         58       1426       1787   1.25316      1787         0    0.00000
+       49       1        7        5      p1         58        316        389   1.23101      2857      2468    6.34447
+       50       1        8       49      p1         58        362        447   1.23481      2915      2468    5.52125
+       51       2       13       17      p1         58        594        739   1.24411      2642      1903    2.57510
+       52       2       14       51      p1         58        640        797   1.24531      2700      1903    2.38770
        53       4        1       -1    base          0          0          0   0.00000         0         0    0.00000
        54       5        1       -1    base        369        640        369   0.57656       369         0    0.00000