sqlitestore: support for storing revisions without their parents

This commit kinda/sorta implements the equivalent of ellipsis nodes for
the SQLite storage backend.

Without implementing full blown ellipsis nodes (and the necessary support for
them in the wire protocol), we instead teach the store to rewrite the p1 and
p2 nodes to nullid when the incoming parent isn't in the local store. This allows
servers to remain dumb and send the real parent and have the clients deal
with the missing parent problem.

This obviously isn't ideal because a benefit of ellipsis nodes is we can
insert a fake parent to ellide missing changesets. But neither solution is
ideal because it drops the original parent from storage. We could probably
teach the SQLite store to retain the original parent and handle missing
parents at read time. However, parent revisions are stored as integers and
it isn't trivial to store an "empty" revision in the store yet, which would
be necessary to represent the "missing" parent.

The store is somewhat intelligent in trying to remove the missing parents
metadata when the revision is re-added. But, revision numbers will be all
messed up in that case, so I'm not sure it is worth it. At some point we'll
likely want to remove the concept of revision numbers from the database and
have the store invent them at index generation time. Or even better, we can
do away with revision numbers from the file storage interface completely.
We'll get there eventually...

Differential Revision: https://phab.mercurial-scm.org/D5168

hgext/sqlitestore.py

@@ -1,1134 +1,1169
 # sqlitestore.py - Storage backend that uses SQLite
 #
 # 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.
 
 """store repository data in SQLite (EXPERIMENTAL)
 
 The sqlitestore extension enables the storage of repository data in SQLite.
 
 This extension is HIGHLY EXPERIMENTAL. There are NO BACKWARDS COMPATIBILITY
 GUARANTEES. This means that repositories created with this extension may
 only be usable with the exact version of this extension/Mercurial that was
 used. The extension attempts to enforce this in order to prevent repository
 corruption.
 
 In addition, several features are not yet supported or have known bugs:
 
 * Only some data is stored in SQLite. Changeset, manifest, and other repository
   data is not yet stored in SQLite.
 * Transactions are not robust. If the process is aborted at the right time
   during transaction close/rollback, the repository could be in an inconsistent
   state. This problem will diminish once all repository data is tracked by
   SQLite.
 * Bundle repositories do not work (the ability to use e.g.
   `hg -R <bundle-file> log` to automatically overlay a bundle on top of the
   existing repository).
 * Various other features don't work.
 
 This extension should work for basic clone/pull, update, and commit workflows.
 Some history rewriting operations may fail due to lack of support for bundle
 repositories.
 
 To use, activate the extension and set the ``storage.new-repo-backend`` config
 option to ``sqlite`` to enable new repositories to use SQLite for storage.
 """
 
 # To run the test suite with repos using SQLite by default, execute the
 # following:
 #
 # HGREPOFEATURES="sqlitestore" run-tests.py \
 #     --extra-config-opt extensions.sqlitestore= \
 #     --extra-config-opt storage.new-repo-backend=sqlite
 
 from __future__ import absolute_import
 
 import hashlib
 import sqlite3
 import struct
 import threading
 import zlib
 
 from mercurial.i18n import _
 from mercurial.node import (
     nullid,
     nullrev,
     short,
 )
 from mercurial.thirdparty import (
     attr,
 )
 from mercurial import (
     ancestor,
     dagop,
     error,
     extensions,
     localrepo,
     mdiff,
     pycompat,
     registrar,
     repository,
     util,
     verify,
 )
 from mercurial.utils import (
     interfaceutil,
     storageutil,
 )
 
 try:
     from mercurial import zstd
     zstd.__version__
 except ImportError:
     zstd = None
 
 configtable = {}
 configitem = registrar.configitem(configtable)
 
 # experimental config: storage.sqlite.compression
 configitem('storage', 'sqlite.compression',
            default='zstd' if zstd else 'zlib')
 
 # Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
 # extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
 # be specifying the version(s) of Mercurial they are tested with, or
 # leave the attribute unspecified.
 testedwith = 'ships-with-hg-core'
 
 REQUIREMENT = b'exp-sqlite-001'
 REQUIREMENT_ZSTD = b'exp-sqlite-comp-001=zstd'
 REQUIREMENT_ZLIB = b'exp-sqlite-comp-001=zlib'
 REQUIREMENT_NONE = b'exp-sqlite-comp-001=none'
 REQUIREMENT_SHALLOW_FILES = b'exp-sqlite-shallow-files'
 
 CURRENT_SCHEMA_VERSION = 1
 
 COMPRESSION_NONE = 1
 COMPRESSION_ZSTD = 2
 COMPRESSION_ZLIB = 3
 
 FLAG_CENSORED = 1
+FLAG_MISSING_P1 = 2
+FLAG_MISSING_P2 = 4
 
 CREATE_SCHEMA = [
     # Deltas are stored as content-indexed blobs.
     # compression column holds COMPRESSION_* constant for how the
     # delta is encoded.
 
     r'CREATE TABLE delta ('
     r'    id INTEGER PRIMARY KEY, '
     r'    compression INTEGER NOT NULL, '
     r'    hash BLOB UNIQUE ON CONFLICT ABORT, '
     r'    delta BLOB NOT NULL '
     r')',
 
     # Tracked paths are denormalized to integers to avoid redundant
     # storage of the path name.
     r'CREATE TABLE filepath ('
     r'    id INTEGER PRIMARY KEY, '
     r'    path BLOB NOT NULL '
     r')',
 
     r'CREATE UNIQUE INDEX filepath_path '
     r'    ON filepath (path)',
 
     # We have a single table for all file revision data.
     # Each file revision is uniquely described by a (path, rev) and
     # (path, node).
     #
     # Revision data is stored as a pointer to the delta producing this
     # revision and the file revision whose delta should be applied before
     # that one. One can reconstruct the delta chain by recursively following
     # the delta base revision pointers until one encounters NULL.
     #
     # flags column holds bitwise integer flags controlling storage options.
     # These flags are defined by the FLAG_* constants.
     r'CREATE TABLE fileindex ('
     r'    id INTEGER PRIMARY KEY, '
     r'    pathid INTEGER REFERENCES filepath(id), '
     r'    revnum INTEGER NOT NULL, '
     r'    p1rev INTEGER NOT NULL, '
     r'    p2rev INTEGER NOT NULL, '
     r'    linkrev INTEGER NOT NULL, '
     r'    flags INTEGER NOT NULL, '
     r'    deltaid INTEGER REFERENCES delta(id), '
     r'    deltabaseid INTEGER REFERENCES fileindex(id), '
     r'    node BLOB NOT NULL '
     r')',
 
     r'CREATE UNIQUE INDEX fileindex_pathrevnum '
     r'    ON fileindex (pathid, revnum)',
 
     r'CREATE UNIQUE INDEX fileindex_pathnode '
     r'    ON fileindex (pathid, node)',
 
     # Provide a view over all file data for convenience.
     r'CREATE VIEW filedata AS '
     r'SELECT '
     r'    fileindex.id AS id, '
     r'    filepath.id AS pathid, '
     r'    filepath.path AS path, '
     r'    fileindex.revnum AS revnum, '
     r'    fileindex.node AS node, '
     r'    fileindex.p1rev AS p1rev, '
     r'    fileindex.p2rev AS p2rev, '
     r'    fileindex.linkrev AS linkrev, '
     r'    fileindex.flags AS flags, '
     r'    fileindex.deltaid AS deltaid, '
     r'    fileindex.deltabaseid AS deltabaseid '
     r'FROM filepath, fileindex '
     r'WHERE fileindex.pathid=filepath.id',
 
     r'PRAGMA user_version=%d' % CURRENT_SCHEMA_VERSION,
 ]
 
 def resolvedeltachain(db, pathid, node, revisioncache,
                       stoprids, zstddctx=None):
     """Resolve a delta chain for a file node."""
 
     # TODO the "not in ({stops})" here is possibly slowing down the query
     # because it needs to perform the lookup on every recursive invocation.
     # This could possibly be faster if we created a temporary query with
     # baseid "poisoned" to null and limited the recursive filter to
     # "is not null".
     res = db.execute(
         r'WITH RECURSIVE '
         r'    deltachain(deltaid, baseid) AS ('
         r'        SELECT deltaid, deltabaseid FROM fileindex '
         r'            WHERE pathid=? AND node=? '
         r'        UNION ALL '
         r'        SELECT fileindex.deltaid, deltabaseid '
         r'            FROM fileindex, deltachain '
         r'            WHERE '
         r'                fileindex.id=deltachain.baseid '
         r'                AND deltachain.baseid IS NOT NULL '
         r'                AND fileindex.id NOT IN ({stops}) '
         r'    ) '
         r'SELECT deltachain.baseid, compression, delta '
         r'FROM deltachain, delta '
         r'WHERE delta.id=deltachain.deltaid'.format(
             stops=r','.join([r'?'] * len(stoprids))),
         tuple([pathid, node] + list(stoprids.keys())))
 
     deltas = []
     lastdeltabaseid = None
 
     for deltabaseid, compression, delta in res:
         lastdeltabaseid = deltabaseid
 
         if compression == COMPRESSION_ZSTD:
             delta = zstddctx.decompress(delta)
         elif compression == COMPRESSION_NONE:
             delta = delta
         elif compression == COMPRESSION_ZLIB:
             delta = zlib.decompress(delta)
         else:
             raise SQLiteStoreError('unhandled compression type: %d' %
                                    compression)
 
         deltas.append(delta)
 
     if lastdeltabaseid in stoprids:
         basetext = revisioncache[stoprids[lastdeltabaseid]]
     else:
         basetext = deltas.pop()
 
     deltas.reverse()
     fulltext = mdiff.patches(basetext, deltas)
 
     # SQLite returns buffer instances for blob columns on Python 2. This
     # type can propagate through the delta application layer. Because
     # downstream callers assume revisions are bytes, cast as needed.
     if not isinstance(fulltext, bytes):
         fulltext = bytes(delta)
 
     return fulltext
 
 def insertdelta(db, compression, hash, delta):
     try:
         return db.execute(
             r'INSERT INTO delta (compression, hash, delta) '
             r'VALUES (?, ?, ?)',
             (compression, hash, delta)).lastrowid
     except sqlite3.IntegrityError:
         return db.execute(
             r'SELECT id FROM delta WHERE hash=?',
             (hash,)).fetchone()[0]
 
 class SQLiteStoreError(error.StorageError):
     pass
 
 @attr.s
 class revisionentry(object):
     rid = attr.ib()
     rev = attr.ib()
     node = attr.ib()
     p1rev = attr.ib()
     p2rev = attr.ib()
     p1node = attr.ib()
     p2node = attr.ib()
     linkrev = attr.ib()
     flags = attr.ib()
 
 @interfaceutil.implementer(repository.irevisiondelta)
 @attr.s(slots=True)
 class sqliterevisiondelta(object):
     node = attr.ib()
     p1node = attr.ib()
     p2node = attr.ib()
     basenode = attr.ib()
     flags = attr.ib()
     baserevisionsize = attr.ib()
     revision = attr.ib()
     delta = attr.ib()
     linknode = attr.ib(default=None)
 
 @interfaceutil.implementer(repository.iverifyproblem)
 @attr.s(frozen=True)
 class sqliteproblem(object):
     warning = attr.ib(default=None)
     error = attr.ib(default=None)
     node = attr.ib(default=None)
 
 @interfaceutil.implementer(repository.ifilestorage)
 class sqlitefilestore(object):
     """Implements storage for an individual tracked path."""
 
     def __init__(self, db, path, compression):
         self._db = db
         self._path = path
 
         self._pathid = None
 
         # revnum -> node
         self._revtonode = {}
         # node -> revnum
         self._nodetorev = {}
         # node -> data structure
         self._revisions = {}
 
         self._revisioncache = util.lrucachedict(10)
 
         self._compengine = compression
 
         if compression == 'zstd':
             self._cctx = zstd.ZstdCompressor(level=3)
             self._dctx = zstd.ZstdDecompressor()
         else:
             self._cctx = None
             self._dctx = None
 
         self._refreshindex()
 
     def _refreshindex(self):
         self._revtonode = {}
         self._nodetorev = {}
         self._revisions = {}
 
         res = list(self._db.execute(
             r'SELECT id FROM filepath WHERE path=?', (self._path,)))
 
         if not res:
             self._pathid = None
             return
 
         self._pathid = res[0][0]
 
         res = self._db.execute(
             r'SELECT id, revnum, node, p1rev, p2rev, linkrev, flags '
             r'FROM fileindex '
             r'WHERE pathid=? '
             r'ORDER BY revnum ASC',
             (self._pathid,))
 
         for i, row in enumerate(res):
             rid, rev, node, p1rev, p2rev, linkrev, flags = row
 
             if i != rev:
                 raise SQLiteStoreError(_('sqlite database has inconsistent '
                                          'revision numbers'))
 
             if p1rev == nullrev:
                 p1node = nullid
             else:
                 p1node = self._revtonode[p1rev]
 
             if p2rev == nullrev:
                 p2node = nullid
             else:
                 p2node = self._revtonode[p2rev]
 
             entry = revisionentry(
                 rid=rid,
                 rev=rev,
                 node=node,
                 p1rev=p1rev,
                 p2rev=p2rev,
                 p1node=p1node,
                 p2node=p2node,
                 linkrev=linkrev,
                 flags=flags)
 
             self._revtonode[rev] = node
             self._nodetorev[node] = rev
             self._revisions[node] = entry
 
     # Start of ifileindex interface.
 
     def __len__(self):
         return len(self._revisions)
 
     def __iter__(self):
         return iter(pycompat.xrange(len(self._revisions)))
 
     def hasnode(self, node):
         if node == nullid:
             return False
 
         return node in self._nodetorev
 
     def revs(self, start=0, stop=None):
         return storageutil.iterrevs(len(self._revisions), start=start,
                                     stop=stop)
 
     def parents(self, node):
         if node == nullid:
             return nullid, nullid
 
         if node not in self._revisions:
             raise error.LookupError(node, self._path, _('no node'))
 
         entry = self._revisions[node]
         return entry.p1node, entry.p2node
 
     def parentrevs(self, rev):
         if rev == nullrev:
             return nullrev, nullrev
 
         if rev not in self._revtonode:
             raise IndexError(rev)
 
         entry = self._revisions[self._revtonode[rev]]
         return entry.p1rev, entry.p2rev
 
     def rev(self, node):
         if node == nullid:
             return nullrev
 
         if node not in self._nodetorev:
             raise error.LookupError(node, self._path, _('no node'))
 
         return self._nodetorev[node]
 
     def node(self, rev):
         if rev == nullrev:
             return nullid
 
         if rev not in self._revtonode:
             raise IndexError(rev)
 
         return self._revtonode[rev]
 
     def lookup(self, node):
         return storageutil.fileidlookup(self, node, self._path)
 
     def linkrev(self, rev):
         if rev == nullrev:
             return nullrev
 
         if rev not in self._revtonode:
             raise IndexError(rev)
 
         entry = self._revisions[self._revtonode[rev]]
         return entry.linkrev
 
     def iscensored(self, rev):
         if rev == nullrev:
             return False
 
         if rev not in self._revtonode:
             raise IndexError(rev)
 
         return self._revisions[self._revtonode[rev]].flags & FLAG_CENSORED
 
     def commonancestorsheads(self, node1, node2):
         rev1 = self.rev(node1)
         rev2 = self.rev(node2)
 
         ancestors = ancestor.commonancestorsheads(self.parentrevs, rev1, rev2)
         return pycompat.maplist(self.node, ancestors)
 
     def descendants(self, revs):
         # TODO we could implement this using a recursive SQL query, which
         # might be faster.
         return dagop.descendantrevs(revs, self.revs, self.parentrevs)
 
     def heads(self, start=None, stop=None):
         if start is None and stop is None:
             if not len(self):
                 return [nullid]
 
         startrev = self.rev(start) if start is not None else nullrev
         stoprevs = {self.rev(n) for n in stop or []}
 
         revs = dagop.headrevssubset(self.revs, self.parentrevs,
                                     startrev=startrev, stoprevs=stoprevs)
 
         return [self.node(rev) for rev in revs]
 
     def children(self, node):
         rev = self.rev(node)
 
         res = self._db.execute(
             r'SELECT'
             r'  node '
             r'  FROM filedata '
             r'  WHERE path=? AND (p1rev=? OR p2rev=?) '
             r'  ORDER BY revnum ASC',
             (self._path, rev, rev))
 
         return [row[0] for row in res]
 
     # End of ifileindex interface.
 
     # Start of ifiledata interface.
 
     def size(self, rev):
         if rev == nullrev:
             return 0
 
         if rev not in self._revtonode:
             raise IndexError(rev)
 
         node = self._revtonode[rev]
 
         if self.renamed(node):
             return len(self.read(node))
 
         return len(self.revision(node))
 
     def revision(self, node, raw=False, _verifyhash=True):
         if node in (nullid, nullrev):
             return b''
 
         if isinstance(node, int):
             node = self.node(node)
 
         if node not in self._nodetorev:
             raise error.LookupError(node, self._path, _('no node'))
 
         if node in self._revisioncache:
             return self._revisioncache[node]
 
         # Because we have a fulltext revision cache, we are able to
         # short-circuit delta chain traversal and decompression as soon as
         # we encounter a revision in the cache.
 
         stoprids = {self._revisions[n].rid: n
                     for n in self._revisioncache}
 
         if not stoprids:
             stoprids[-1] = None
 
         fulltext = resolvedeltachain(self._db, self._pathid, node,
                                      self._revisioncache, stoprids,
                                      zstddctx=self._dctx)
 
+        # Don't verify hashes if parent nodes were rewritten, as the hash
+        # wouldn't verify.
+        if self._revisions[node].flags & (FLAG_MISSING_P1 | FLAG_MISSING_P2):
+            _verifyhash = False
+
         if _verifyhash:
             self._checkhash(fulltext, node)
             self._revisioncache[node] = fulltext
 
         return fulltext
 
     def read(self, node):
         return storageutil.filtermetadata(self.revision(node))
 
     def renamed(self, node):
         return storageutil.filerevisioncopied(self, node)
 
     def cmp(self, node, fulltext):
         return not storageutil.filedataequivalent(self, node, fulltext)
 
     def emitrevisions(self, nodes, nodesorder=None, revisiondata=False,
                       assumehaveparentrevisions=False, deltaprevious=False):
         if nodesorder not in ('nodes', 'storage', None):
             raise error.ProgrammingError('unhandled value for nodesorder: %s' %
                                          nodesorder)
 
         nodes = [n for n in nodes if n != nullid]
 
         if not nodes:
             return
 
         # TODO perform in a single query.
         res = self._db.execute(
             r'SELECT revnum, deltaid FROM fileindex '
             r'WHERE pathid=? '
             r'    AND node in (%s)' % (r','.join([r'?'] * len(nodes))),
             tuple([self._pathid] + nodes))
 
         deltabases = {}
 
         for rev, deltaid in res:
             res = self._db.execute(
                 r'SELECT revnum from fileindex WHERE pathid=? AND deltaid=?',
                 (self._pathid, deltaid))
             deltabases[rev] = res.fetchone()[0]
 
         # TODO define revdifffn so we can use delta from storage.
         for delta in storageutil.emitrevisions(
             self, nodes, nodesorder, sqliterevisiondelta,
             deltaparentfn=deltabases.__getitem__,
             revisiondata=revisiondata,
             assumehaveparentrevisions=assumehaveparentrevisions,
             deltaprevious=deltaprevious):
 
             yield delta
 
     # End of ifiledata interface.
 
     # Start of ifilemutation interface.
 
     def add(self, filedata, meta, transaction, linkrev, p1, p2):
         if meta or filedata.startswith(b'\x01\n'):
             filedata = storageutil.packmeta(meta, filedata)
 
         return self.addrevision(filedata, transaction, linkrev, p1, p2)
 
     def addrevision(self, revisiondata, transaction, linkrev, p1, p2, node=None,
                     flags=0, cachedelta=None):
         if flags:
             raise SQLiteStoreError(_('flags not supported on revisions'))
 
         validatehash = node is not None
         node = node or storageutil.hashrevisionsha1(revisiondata, p1, p2)
 
         if validatehash:
             self._checkhash(revisiondata, node, p1, p2)
 
         if node in self._nodetorev:
             return node
 
         node = self._addrawrevision(node, revisiondata, transaction, linkrev,
                                     p1, p2)
 
         self._revisioncache[node] = revisiondata
         return node
 
     def addgroup(self, deltas, linkmapper, transaction, addrevisioncb=None,
                  maybemissingparents=False):
-        if maybemissingparents:
-            raise error.Abort(_('SQLite storage does not support missing '
-                                'parents write mode'))
-
         nodes = []
 
         for node, p1, p2, linknode, deltabase, delta, wireflags in deltas:
             storeflags = 0
 
             if wireflags & repository.REVISION_FLAG_CENSORED:
                 storeflags |= FLAG_CENSORED
 
             if wireflags & ~repository.REVISION_FLAG_CENSORED:
                 raise SQLiteStoreError('unhandled revision flag')
 
+            if maybemissingparents:
+                if p1 != nullid and not self.hasnode(p1):
+                    p1 = nullid
+                    storeflags |= FLAG_MISSING_P1
+
+                if p2 != nullid and not self.hasnode(p2):
+                    p2 = nullid
+                    storeflags |= FLAG_MISSING_P2
+
             baserev = self.rev(deltabase)
 
             # If base is censored, delta must be full replacement in a single
             # patch operation.
             if baserev != nullrev and self.iscensored(baserev):
                 hlen = struct.calcsize('>lll')
                 oldlen = len(self.revision(deltabase, raw=True,
                                            _verifyhash=False))
                 newlen = len(delta) - hlen
 
                 if delta[:hlen] != mdiff.replacediffheader(oldlen, newlen):
                     raise error.CensoredBaseError(self._path,
                                                   deltabase)
 
             if (not (storeflags & FLAG_CENSORED)
                 and storageutil.deltaiscensored(
                     delta, baserev, lambda x: len(self.revision(x, raw=True)))):
                 storeflags |= FLAG_CENSORED
 
             linkrev = linkmapper(linknode)
 
             nodes.append(node)
 
             if node in self._revisions:
+                # Possibly reset parents to make them proper.
+                entry = self._revisions[node]
+
+                if entry.flags & FLAG_MISSING_P1 and p1 != nullid:
+                    entry.p1node = p1
+                    entry.p1rev = self._nodetorev[p1]
+                    entry.flags &= ~FLAG_MISSING_P1
+
+                    self._db.execute(
+                        r'UPDATE fileindex SET p1rev=?, flags=? '
+                        r'WHERE id=?',
+                        (self._nodetorev[p1], entry.flags, entry.rid))
+
+                if entry.flags & FLAG_MISSING_P2 and p2 != nullid:
+                    entry.p2node = p2
+                    entry.p2rev = self._nodetorev[p2]
+                    entry.flags &= ~FLAG_MISSING_P2
+
+                    self._db.execute(
+                        r'UPDATE fileindex SET p2rev=?, flags=? '
+                        r'WHERE id=?',
+                        (self._nodetorev[p1], entry.flags, entry.rid))
+
                 continue
 
             if deltabase == nullid:
                 text = mdiff.patch(b'', delta)
                 storedelta = None
             else:
                 text = None
                 storedelta = (deltabase, delta)
 
             self._addrawrevision(node, text, transaction, linkrev, p1, p2,
                                  storedelta=storedelta, flags=storeflags)
 
             if addrevisioncb:
                 addrevisioncb(self, node)
 
         return nodes
 
     def censorrevision(self, tr, censornode, tombstone=b''):
         tombstone = storageutil.packmeta({b'censored': tombstone}, b'')
 
         # This restriction is cargo culted from revlogs and makes no sense for
         # SQLite, since columns can be resized at will.
         if len(tombstone) > len(self.revision(censornode, raw=True)):
             raise error.Abort(_('censor tombstone must be no longer than '
                                 'censored data'))
 
         # We need to replace the censored revision's data with the tombstone.
         # But replacing that data will have implications for delta chains that
         # reference it.
         #
         # While "better," more complex strategies are possible, we do something
         # simple: we find delta chain children of the censored revision and we
         # replace those incremental deltas with fulltexts of their corresponding
         # revision. Then we delete the now-unreferenced delta and original
         # revision and insert a replacement.
 
         # Find the delta to be censored.
         censoreddeltaid = self._db.execute(
             r'SELECT deltaid FROM fileindex WHERE id=?',
             (self._revisions[censornode].rid,)).fetchone()[0]
 
         # Find all its delta chain children.
         # TODO once we support storing deltas for !files, we'll need to look
         # for those delta chains too.
         rows = list(self._db.execute(
             r'SELECT id, pathid, node FROM fileindex '
             r'WHERE deltabaseid=? OR deltaid=?',
             (censoreddeltaid, censoreddeltaid)))
 
         for row in rows:
             rid, pathid, node = row
 
             fulltext = resolvedeltachain(self._db, pathid, node, {}, {-1: None},
                                          zstddctx=self._dctx)
 
             deltahash = hashlib.sha1(fulltext).digest()
 
             if self._compengine == 'zstd':
                 deltablob = self._cctx.compress(fulltext)
                 compression = COMPRESSION_ZSTD
             elif self._compengine == 'zlib':
                 deltablob = zlib.compress(fulltext)
                 compression = COMPRESSION_ZLIB
             elif self._compengine == 'none':
                 deltablob = fulltext
                 compression = COMPRESSION_NONE
             else:
                 raise error.ProgrammingError('unhandled compression engine: %s'
                                              % self._compengine)
 
             if len(deltablob) >= len(fulltext):
                 deltablob = fulltext
                 compression = COMPRESSION_NONE
 
             deltaid = insertdelta(self._db, compression, deltahash, deltablob)
 
             self._db.execute(
                 r'UPDATE fileindex SET deltaid=?, deltabaseid=NULL '
                 r'WHERE id=?', (deltaid, rid))
 
         # Now create the tombstone delta and replace the delta on the censored
         # node.
         deltahash = hashlib.sha1(tombstone).digest()
         tombstonedeltaid = insertdelta(self._db, COMPRESSION_NONE,
                                        deltahash, tombstone)
 
         flags = self._revisions[censornode].flags
         flags |= FLAG_CENSORED
 
         self._db.execute(
             r'UPDATE fileindex SET flags=?, deltaid=?, deltabaseid=NULL '
             r'WHERE pathid=? AND node=?',
             (flags, tombstonedeltaid, self._pathid, censornode))
 
         self._db.execute(
             r'DELETE FROM delta WHERE id=?', (censoreddeltaid,))
 
         self._refreshindex()
         self._revisioncache.clear()
 
     def getstrippoint(self, minlink):
         return storageutil.resolvestripinfo(minlink, len(self) - 1,
                                             [self.rev(n) for n in self.heads()],
                                             self.linkrev,
                                             self.parentrevs)
 
     def strip(self, minlink, transaction):
         if not len(self):
             return
 
         rev, _ignored = self.getstrippoint(minlink)
 
         if rev == len(self):
             return
 
         for rev in self.revs(rev):
             self._db.execute(
                 r'DELETE FROM fileindex WHERE pathid=? AND node=?',
                 (self._pathid, self.node(rev)))
 
         # TODO how should we garbage collect data in delta table?
 
         self._refreshindex()
 
     # End of ifilemutation interface.
 
     # Start of ifilestorage interface.
 
     def files(self):
         return []
 
     def storageinfo(self, exclusivefiles=False, sharedfiles=False,
                     revisionscount=False, trackedsize=False,
                     storedsize=False):
         d = {}
 
         if exclusivefiles:
             d['exclusivefiles'] = []
 
         if sharedfiles:
             # TODO list sqlite file(s) here.
             d['sharedfiles'] = []
 
         if revisionscount:
             d['revisionscount'] = len(self)
 
         if trackedsize:
             d['trackedsize'] = sum(len(self.revision(node))
                                        for node in self._nodetorev)
 
         if storedsize:
             # TODO implement this?
             d['storedsize'] = None
 
         return d
 
     def verifyintegrity(self, state):
         state['skipread'] = set()
 
         for rev in self:
             node = self.node(rev)
 
             try:
                 self.revision(node)
             except Exception as e:
                 yield sqliteproblem(
                     error=_('unpacking %s: %s') % (short(node), e),
                     node=node)
 
                 state['skipread'].add(node)
 
     # End of ifilestorage interface.
 
     def _checkhash(self, fulltext, node, p1=None, p2=None):
         if p1 is None and p2 is None:
             p1, p2 = self.parents(node)
 
         if node == storageutil.hashrevisionsha1(fulltext, p1, p2):
             return
 
         try:
             del self._revisioncache[node]
         except KeyError:
             pass
 
         if storageutil.iscensoredtext(fulltext):
             raise error.CensoredNodeError(self._path, node, fulltext)
 
         raise SQLiteStoreError(_('integrity check failed on %s') %
                                self._path)
 
     def _addrawrevision(self, node, revisiondata, transaction, linkrev,
                         p1, p2, storedelta=None, flags=0):
         if self._pathid is None:
             res = self._db.execute(
                 r'INSERT INTO filepath (path) VALUES (?)', (self._path,))
             self._pathid = res.lastrowid
 
         # For simplicity, always store a delta against p1.
         # TODO we need a lot more logic here to make behavior reasonable.
 
         if storedelta:
             deltabase, delta = storedelta
 
             if isinstance(deltabase, int):
                 deltabase = self.node(deltabase)
 
         else:
             assert revisiondata is not None
             deltabase = p1
 
             if deltabase == nullid:
                 delta = revisiondata
             else:
                 delta = mdiff.textdiff(self.revision(self.rev(deltabase)),
                                        revisiondata)
 
         # File index stores a pointer to its delta and the parent delta.
         # The parent delta is stored via a pointer to the fileindex PK.
         if deltabase == nullid:
             baseid = None
         else:
             baseid = self._revisions[deltabase].rid
 
         # Deltas are stored with a hash of their content. This allows
         # us to de-duplicate. The table is configured to ignore conflicts
         # and it is faster to just insert and silently noop than to look
         # first.
         deltahash = hashlib.sha1(delta).digest()
 
         if self._compengine == 'zstd':
             deltablob = self._cctx.compress(delta)
             compression = COMPRESSION_ZSTD
         elif self._compengine == 'zlib':
             deltablob = zlib.compress(delta)
             compression = COMPRESSION_ZLIB
         elif self._compengine == 'none':
             deltablob = delta
             compression = COMPRESSION_NONE
         else:
             raise error.ProgrammingError('unhandled compression engine: %s' %
                                          self._compengine)
 
         # Don't store compressed data if it isn't practical.
         if len(deltablob) >= len(delta):
             deltablob = delta
             compression = COMPRESSION_NONE
 
         deltaid = insertdelta(self._db, compression, deltahash, deltablob)
 
         rev = len(self)
 
         if p1 == nullid:
             p1rev = nullrev
         else:
             p1rev = self._nodetorev[p1]
 
         if p2 == nullid:
             p2rev = nullrev
         else:
             p2rev = self._nodetorev[p2]
 
         rid = self._db.execute(
             r'INSERT INTO fileindex ('
             r'    pathid, revnum, node, p1rev, p2rev, linkrev, flags, '
             r'    deltaid, deltabaseid) '
             r'    VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)',
             (self._pathid, rev, node, p1rev, p2rev, linkrev, flags,
              deltaid, baseid)
         ).lastrowid
 
         entry = revisionentry(
             rid=rid,
             rev=rev,
             node=node,
             p1rev=p1rev,
             p2rev=p2rev,
             p1node=p1,
             p2node=p2,
             linkrev=linkrev,
             flags=flags)
 
         self._nodetorev[node] = rev
         self._revtonode[rev] = node
         self._revisions[node] = entry
 
         return node
 
 class sqliterepository(localrepo.localrepository):
     def cancopy(self):
         return False
 
     def transaction(self, *args, **kwargs):
         current = self.currenttransaction()
 
         tr = super(sqliterepository, self).transaction(*args, **kwargs)
 
         if current:
             return tr
 
         self._dbconn.execute(r'BEGIN TRANSACTION')
 
         def committransaction(_):
             self._dbconn.commit()
 
         tr.addfinalize('sqlitestore', committransaction)
 
         return tr
 
     @property
     def _dbconn(self):
         # SQLite connections can only be used on the thread that created
         # them. In most cases, this "just works." However, hgweb uses
         # multiple threads.
         tid = threading.current_thread().ident
 
         if self._db:
             if self._db[0] == tid:
                 return self._db[1]
 
         db = makedb(self.svfs.join('db.sqlite'))
         self._db = (tid, db)
 
         return db
 
 def makedb(path):
     """Construct a database handle for a database at path."""
 
     db = sqlite3.connect(path)
     db.text_factory = bytes
 
     res = db.execute(r'PRAGMA user_version').fetchone()[0]
 
     # New database.
     if res == 0:
         for statement in CREATE_SCHEMA:
             db.execute(statement)
 
         db.commit()
 
     elif res == CURRENT_SCHEMA_VERSION:
         pass
 
     else:
         raise error.Abort(_('sqlite database has unrecognized version'))
 
     db.execute(r'PRAGMA journal_mode=WAL')
 
     return db
 
 def featuresetup(ui, supported):
     supported.add(REQUIREMENT)
 
     if zstd:
         supported.add(REQUIREMENT_ZSTD)
 
     supported.add(REQUIREMENT_ZLIB)
     supported.add(REQUIREMENT_NONE)
     supported.add(REQUIREMENT_SHALLOW_FILES)
     supported.add(repository.NARROW_REQUIREMENT)
 
 def newreporequirements(orig, ui, createopts):
     if createopts['backend'] != 'sqlite':
         return orig(ui, createopts)
 
     # This restriction can be lifted once we have more confidence.
     if 'sharedrepo' in createopts:
         raise error.Abort(_('shared repositories not supported with SQLite '
                             'store'))
 
     # This filtering is out of an abundance of caution: we want to ensure
     # we honor creation options and we do that by annotating exactly the
     # creation options we recognize.
     known = {
         'narrowfiles',
         'backend',
         'shallowfilestore',
     }
 
     unsupported = set(createopts) - known
     if unsupported:
         raise error.Abort(_('SQLite store does not support repo creation '
                             'option: %s') % ', '.join(sorted(unsupported)))
 
     # Since we're a hybrid store that still relies on revlogs, we fall back
     # to using the revlogv1 backend's storage requirements then adding our
     # own requirement.
     createopts['backend'] = 'revlogv1'
     requirements = orig(ui, createopts)
     requirements.add(REQUIREMENT)
 
     compression = ui.config('storage', 'sqlite.compression')
 
     if compression == 'zstd' and not zstd:
         raise error.Abort(_('storage.sqlite.compression set to "zstd" but '
                             'zstandard compression not available to this '
                             'Mercurial install'))
 
     if compression == 'zstd':
         requirements.add(REQUIREMENT_ZSTD)
     elif compression == 'zlib':
         requirements.add(REQUIREMENT_ZLIB)
     elif compression == 'none':
         requirements.add(REQUIREMENT_NONE)
     else:
         raise error.Abort(_('unknown compression engine defined in '
                             'storage.sqlite.compression: %s') % compression)
 
     if createopts.get('shallowfilestore'):
         requirements.add(REQUIREMENT_SHALLOW_FILES)
 
     return requirements
 
 @interfaceutil.implementer(repository.ilocalrepositoryfilestorage)
 class sqlitefilestorage(object):
     """Repository file storage backed by SQLite."""
     def file(self, path):
         if path[0] == b'/':
             path = path[1:]
 
         if REQUIREMENT_ZSTD in self.requirements:
             compression = 'zstd'
         elif REQUIREMENT_ZLIB in self.requirements:
             compression = 'zlib'
         elif REQUIREMENT_NONE in self.requirements:
             compression = 'none'
         else:
             raise error.Abort(_('unable to determine what compression engine '
                                 'to use for SQLite storage'))
 
         return sqlitefilestore(self._dbconn, path, compression)
 
 def makefilestorage(orig, requirements, features, **kwargs):
     """Produce a type conforming to ``ilocalrepositoryfilestorage``."""
     if REQUIREMENT in requirements:
         if REQUIREMENT_SHALLOW_FILES in requirements:
             features.add(repository.REPO_FEATURE_SHALLOW_FILE_STORAGE)
 
         return sqlitefilestorage
     else:
         return orig(requirements=requirements, features=features, **kwargs)
 
 def makemain(orig, ui, requirements, **kwargs):
     if REQUIREMENT in requirements:
         if REQUIREMENT_ZSTD in requirements and not zstd:
             raise error.Abort(_('repository uses zstandard compression, which '
                                 'is not available to this Mercurial install'))
 
         return sqliterepository
 
     return orig(requirements=requirements, **kwargs)
 
 def verifierinit(orig, self, *args, **kwargs):
     orig(self, *args, **kwargs)
 
     # We don't care that files in the store don't align with what is
     # advertised. So suppress these warnings.
     self.warnorphanstorefiles = False
 
 def extsetup(ui):
     localrepo.featuresetupfuncs.add(featuresetup)
     extensions.wrapfunction(localrepo, 'newreporequirements',
                             newreporequirements)
     extensions.wrapfunction(localrepo, 'makefilestorage',
                             makefilestorage)
     extensions.wrapfunction(localrepo, 'makemain',
                             makemain)
     extensions.wrapfunction(verify.verifier, '__init__',
                             verifierinit)
 
 def reposetup(ui, repo):
     if isinstance(repo, sqliterepository):
         repo._db = None
 
     # TODO check for bundlerepository?