effectflag: store an empty effect flag for the moment

The idea behind effect flag is to store additional information in obs-markers
about what changed between a changeset and its successor(s). It's a low-level
information that comes without guarantees.

This information can be computed a posteriori, but only if we have all
changesets locally. This is not the case with distributed workflows where you
work with several people or on several computers (eg: laptop + build server).

Storing the effect-flag as a bitfield has several advantages:

- It's compact, we are using one byte per obs-marker at most for the effect-
  flag.
- It's compoundable, the obsfate log approach needs to display evolve history
  that could spans several obs-markers. Computing the effect-flag between a
  changeset and its grand-grand-grand-successor is simple thanks to the
  bitfield.

The effect-flag design has also some limitations:

- Evolving a changeset and reverting these changes just after would lead to
  two obs-markers with the same effect-flag without information that the first
  and third changesets are the same.

The effect-flag current design is a trade-off between compactness and
usefulness.

Storing this information helps commands to display a more complete and
understandable evolve history. For example, obslog (an Evolve command) use it
to improve its output:

x  62206adfd571 (34302) obscache: skip updating outdated obscache...
|    rewritten(parent) by Matthieu Laneuville <matthieu.laneuville@octobus...
|    rewritten(content) by Boris Feld <[e-mail]>

The effect flag is stored in obs-markers metadata while we iterate on the
information we want to store. We plan to extend the existing obsmarkers
bit-field when the effect flag design will be stabilized.

It's different from the CommitCustody concept, effect-flag are not signed and
can be forged. It's also different from the operation metadata as the command
name (for example: amend) could alter a changeset in different ways (changing
the content with hg amend, changing the description with hg amend -e, changing
the user with hg amend -U). Also it's compatible with every custom command
that writes obs-markers without needing to be updated.

The effect-flag is placed behind an experimental flag set to off by default.

Hook the saving of effect flag in create markers, but store only an empty one
for the moment, I will refine the values in effect flag in following patches.

For more information, see:

  https://www.mercurial-scm.org/wiki/ChangesetEvolutionDevel#Record_types_of_operation

Differential Revision: https://phab.mercurial-scm.org/D533

mercurial/obsolete.py

@@ -1043,6 +1043,11 b' def createmarkers(repo, relations, flag='
     if useoperation and operation:
         metadata['operation'] = operation
 
+    # Effect flag metadata handling
+    saveeffectflag = repo.ui.configbool('experimental',
+                                        'effect-flags',
+                                        False)
+
     tr = repo.transaction('add-obsolescence-marker')
     try:
         markerargs = []
@@ -1066,6 +1071,13 b' def createmarkers(repo, relations, flag='
                 raise error.Abort(_("changeset %s cannot obsolete itself")
                                   % prec)
 
+            # Effect flag can be different by relation
+            if saveeffectflag:
+                # The effect flag is saved in a versioned field name for future
+                # evolution
+                effectflag = obsutil.geteffectflag(rel)
+                localmetadata[obsutil.EFFECTFLAGFIELD] = "%d" % effectflag
+
             # Creating the marker causes the hidden cache to become invalid,
             # which causes recomputation when we ask for prec.parents() above.
             # Resulting in n^2 behavior.  So let's prepare all of the args

mercurial/obsutil.py

@@ -305,6 +305,19 b' def foreground(repo, nodes):'
             foreground = set(repo.set('%ln::', known))
     return set(c.node() for c in foreground)
 
+# logic around storing and using effect flags
+EFFECTFLAGFIELD = "ef1"
+
+def geteffectflag(relation):
+    """ From an obs-marker relation, compute what changed between the
+    predecessor and the successor.
+    """
+    effects = 0
+
+    source = relation[0]
+
+    return effects
+
 def getobsoleted(repo, tr):
     """return the set of pre-existing revisions obsoleted by a transaction"""
     torev = repo.unfiltered().changelog.nodemap.get