--- a/mercurial/help/internals/revlogs.txt	Tue Jun 06 08:58:27 2017 -0700
+++ b/mercurial/help/internals/revlogs.txt	Fri May 19 20:29:11 2017 -0700
@@ -45,6 +45,12 @@
 1
    RevlogNG (*next generation*). It replaced version 0 when it was
    implemented in 2006.
+2
+   In-development version incorporating accumulated knowledge and
+   missing features from 10+ years of revlog version 1.
+57005 (0xdead)
+   Reserved for internal testing of new versions. No defined format
+   beyond 32-bit header.
 
 The feature flags short consists of bit flags. Where 0 is the least
 significant bit, the following bit offsets define flags:
@@ -142,6 +148,14 @@
 The first 4 bytes of the revlog are shared between the revlog header
 and the 6 byte absolute offset field from the first revlog entry.
 
+Version 2 Format
+================
+
+(In development. Format not finalized or stable.)
+
+Version 2 is currently identical to version 1. This will obviously
+change.
+
 Delta Chains
 ============
 

--- a/mercurial/localrepo.py	Tue Jun 06 08:58:27 2017 -0700
+++ b/mercurial/localrepo.py	Fri May 19 20:29:11 2017 -0700
@@ -244,6 +244,10 @@
     def changegroupsubset(self, bases, heads, source):
         return changegroup.changegroupsubset(self._repo, bases, heads, source)
 
+# Increment the sub-version when the revlog v2 format changes to lock out old
+# clients.
+REVLOGV2_REQUIREMENT = 'exp-revlogv2.0'
+
 class localrepository(object):
 
     supportedformats = {
@@ -251,6 +255,7 @@
         'generaldelta',
         'treemanifest',
         'manifestv2',
+        REVLOGV2_REQUIREMENT,
     }
     _basesupported = supportedformats | {
         'store',
@@ -440,6 +445,10 @@
             if r.startswith('exp-compression-'):
                 self.svfs.options['compengine'] = r[len('exp-compression-'):]
 
+        # TODO move "revlogv2" to openerreqs once finalized.
+        if REVLOGV2_REQUIREMENT in self.requirements:
+            self.svfs.options['revlogv2'] = True
+
     def _writerequirements(self):
         scmutil.writerequires(self.vfs, self.requirements)
 
@@ -2070,4 +2079,11 @@
     if ui.configbool('experimental', 'manifestv2', False):
         requirements.add('manifestv2')
 
+    revlogv2 = ui.config('experimental', 'revlogv2')
+    if revlogv2 == 'enable-unstable-format-and-corrupt-my-data':
+        requirements.remove('revlogv1')
+        # generaldelta is implied by revlogv2.
+        requirements.discard('generaldelta')
+        requirements.add(REVLOGV2_REQUIREMENT)
+
     return requirements

--- a/mercurial/revlog.py	Tue Jun 06 08:58:27 2017 -0700
+++ b/mercurial/revlog.py	Fri May 19 20:29:11 2017 -0700
@@ -51,12 +51,16 @@
 # revlog header flags
 REVLOGV0 = 0
 REVLOGV1 = 1
+# Dummy value until file format is finalized.
+# Reminder: change the bounds check in revlog.__init__ when this is changed.
+REVLOGV2 = 0xDEAD
 FLAG_INLINE_DATA = (1 << 16)
 FLAG_GENERALDELTA = (1 << 17)
 REVLOG_DEFAULT_FLAGS = FLAG_INLINE_DATA
 REVLOG_DEFAULT_FORMAT = REVLOGV1
 REVLOG_DEFAULT_VERSION = REVLOG_DEFAULT_FORMAT | REVLOG_DEFAULT_FLAGS
 REVLOGV1_FLAGS = FLAG_INLINE_DATA | FLAG_GENERALDELTA
+REVLOGV2_FLAGS = REVLOGV1_FLAGS
 
 # revlog index flags
 REVIDX_ISCENSORED = (1 << 15) # revision has censor metadata, must be verified
@@ -291,7 +295,10 @@
         v = REVLOG_DEFAULT_VERSION
         opts = getattr(opener, 'options', None)
         if opts is not None:
-            if 'revlogv1' in opts:
+            if 'revlogv2' in opts:
+                # version 2 revlogs always use generaldelta.
+                v = REVLOGV2 | FLAG_GENERALDELTA | FLAG_INLINE_DATA
+            elif 'revlogv1' in opts:
                 if 'generaldelta' in opts:
                     v |= FLAG_GENERALDELTA
             else:
@@ -341,6 +348,11 @@
                 raise RevlogError(_('unknown flags (%#04x) in version %d '
                                     'revlog %s') %
                                   (flags >> 16, fmt, self.indexfile))
+        elif fmt == REVLOGV2:
+            if flags & ~REVLOGV2_FLAGS:
+                raise RevlogError(_('unknown flags (%#04x) in version %d '
+                                    'revlog %s') %
+                                  (flags >> 16, fmt, self.indexfile))
         else:
             raise RevlogError(_('unknown version (%d) in revlog %s') %
                               (fmt, self.indexfile))

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-revlog-v2.t	Fri May 19 20:29:11 2017 -0700
@@ -0,0 +1,62 @@
+A repo with unknown revlogv2 requirement string cannot be opened
+
+  $ hg init invalidreq
+  $ cd invalidreq
+  $ echo exp-revlogv2.unknown >> .hg/requires
+  $ hg log
+  abort: repository requires features unknown to this Mercurial: exp-revlogv2.unknown!
+  (see https://mercurial-scm.org/wiki/MissingRequirement for more information)
+  [255]
+  $ cd ..
+
+Can create and open repo with revlog v2 requirement
+
+  $ cat >> $HGRCPATH << EOF
+  > [experimental]
+  > revlogv2 = enable-unstable-format-and-corrupt-my-data
+  > EOF
+
+  $ hg init empty-repo
+  $ cd empty-repo
+  $ cat .hg/requires
+  dotencode
+  exp-revlogv2.0
+  fncache
+  store
+
+  $ hg log
+
+Unknown flags to revlog are rejected
+
+  >>> with open('.hg/store/00changelog.i', 'wb') as fh:
+  ...     fh.write('\x00\x04\xde\xad')
+
+  $ hg log
+  abort: unknown flags (0x04) in version 57005 revlog 00changelog.i!
+  [255]
+
+  $ cd ..
+
+Writing a simple revlog v2 works
+
+  $ hg init simple
+  $ cd simple
+  $ touch foo
+  $ hg -q commit -A -m initial
+
+  $ hg log
+  changeset:   0:96ee1d7354c4
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     initial
+  
+Header written as expected (changelog always disables generaldelta)
+
+  $ f --hexdump --bytes 4 .hg/store/00changelog.i
+  .hg/store/00changelog.i:
+  0000: 00 01 de ad                                     |....|
+
+  $ f --hexdump --bytes 4 .hg/store/data/foo.i
+  .hg/store/data/foo.i:
+  0000: 00 03 de ad                                     |....|