[PATCH 4 of 4] revlog: skeleton support for version 2 revlogs

Gregory Szorc gregory.szorc at gmail.com
Fri May 19 23:59:00 EDT 2017


# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1495250951 25200
#      Fri May 19 20:29:11 2017 -0700
# Node ID 410eab1d00eef1f90ec7ab5383f29c29c9f31c3e
# Parent  653a47e013ae71470ae943e5c9a84e92eae0676e
revlog: skeleton support for version 2 revlogs

There are a number of improvements we want to make to revlogs
that will require a new version - version 2. It is unclear what the
full set of improvements will be or when we'll be done with them.
What I do know is that the process will likely take longer than a
single release, will require input from various stakeholders to
evaluate changes, and will have many contentious debates and
bikeshedding.

It is unrealistic to develop revlog version 2 up front: there
are just too many uncertainties that we won't know until things
are implemented and experiments are run. Some changes will also
be invasive and prone to bit rot, so sitting on dozens of patches
is not practical.

This commit introduces skeleton support for version 2 revlogs in
a way that is flexible and not bound by backwards compatibility
concerns.

An experimental repo requirement for denoting revlog v2 has been
added. The requirement string has a sub-version component to it.
This will allow us to declare multiple requirements in the course
of developing revlog v2. Whenever we change the in-development
revlog v2 format, we can tweak the string, creating a new
requirement and locking out old clients. This will allow us to
make as many backwards incompatible changes and experiments to
revlog v2 as we want. In other words, we can land code and make
meaningful progress towards revlog v2 while still maintaining
extreme format flexibility up until the point we freeze the
format and remove the experimental labels.

To enable the new repo requirement, you must supply an experimental
and undocumented config option. But not just any boolean flag
will do: you need to explicitly use a value that no sane person
should ever type. This is an additional guard against enabling
revlog v2 on an installation it shouldn't be enabled on. The
specific scenario I'm trying to prevent is say a user with a
4.4 client with a frozen format enabling the option but then
downgrading to 4.3 and accidentally creating repos with an
outdated and unsupported repo format. Requiring a "challenge"
string should prevent this.

Because the format is not yet finalized and I don't want to take
any chances, revlog v2's version is currently 0xDEAD. I figure
squatting on a value we're likely never to use as an actual revlog
version to mean "internal testing only" is acceptable. And
"dead" is easily recognized as something meaningful.

There is a bunch of cleanup that is needed before work on revlog
v2 begins in earnest. I plan on doing that work once this patch
is accepted and we're comfortable with the idea of starting down
this path.

diff --git a/mercurial/help/internals/revlogs.txt b/mercurial/help/internals/revlogs.txt
--- a/mercurial/help/internals/revlogs.txt
+++ b/mercurial/help/internals/revlogs.txt
@@ -45,6 +45,12 @@ 0
 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 @@ length from bytes 8-11 define how to acc
 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
 ============
 
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -245,6 +245,10 @@ class locallegacypeer(localpeer):
     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 = {
@@ -252,6 +256,7 @@ class localrepository(object):
         'generaldelta',
         'treemanifest',
         'manifestv2',
+        REVLOGV2_REQUIREMENT,
     }
     _basesupported = supportedformats | {
         'store',
@@ -441,6 +446,10 @@ class localrepository(object):
             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 @@ def newreporequirements(repo):
     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
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -46,12 +46,16 @@ from . import (
 # 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
@@ -286,7 +290,10 @@ class revlog(object):
         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:
@@ -336,6 +343,11 @@ class revlog(object):
                 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))
         elif fmt > REVLOGV1:
             raise RevlogError(_('unknown version (%d) in revlog %s') %
                               (fmt, self.indexfile))
diff --git a/tests/test-revlog-v2.t b/tests/test-revlog-v2.t
new file mode 100644
--- /dev/null
+++ b/tests/test-revlog-v2.t
@@ -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                                     |....|


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