changelog-delay: move the appender class next to randomaccessfile

We want to move the delay/divert logic at the revlog level (to have all IO
related logic in the _InnerRevlog) we start with small piece that are easy to
move on their own.

mercurial/changelog.py

@@ -1,644 +1,575 b''
 # changelog.py - changelog class for mercurial
 #
 # Copyright 2005-2007 Olivia Mackall <[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 .i18n import _
 from .node import (
     bin,
     hex,
 )
 from .thirdparty import attr
 
 from . import (
     encoding,
     error,
     metadata,
     pycompat,
     revlog,
 )
 from .utils import (
     dateutil,
     stringutil,
 )
 from .revlogutils import (
     constants as revlog_constants,
     flagutil,
+    randomaccessfile,
 )
 
 _defaultextra = {b'branch': b'default'}
 
 
 def _string_escape(text):
     """
     >>> from .pycompat import bytechr as chr
     >>> d = {b'nl': chr(10), b'bs': chr(92), b'cr': chr(13), b'nul': chr(0)}
     >>> s = b"ab%(nl)scd%(bs)s%(bs)sn%(nul)s12ab%(cr)scd%(bs)s%(nl)s" % d
     >>> s
     'ab\\ncd\\\\\\\\n\\x0012ab\\rcd\\\\\\n'
     >>> res = _string_escape(s)
     >>> s == _string_unescape(res)
     True
     """
     # subset of the string_escape codec
     text = (
         text.replace(b'\\', b'\\\\')
         .replace(b'\n', b'\\n')
         .replace(b'\r', b'\\r')
     )
     return text.replace(b'\0', b'\\0')
 
 
 def _string_unescape(text):
     if b'\\0' in text:
         # fix up \0 without getting into trouble with \\0
         text = text.replace(b'\\\\', b'\\\\\n')
         text = text.replace(b'\\0', b'\0')
         text = text.replace(b'\n', b'')
     return stringutil.unescapestr(text)
 
 
 def decodeextra(text):
     """
     >>> from .pycompat import bytechr as chr
     >>> sorted(decodeextra(encodeextra({b'foo': b'bar', b'baz': chr(0) + b'2'})
     ...                    ).items())
     [('baz', '\\x002'), ('branch', 'default'), ('foo', 'bar')]
     >>> sorted(decodeextra(encodeextra({b'foo': b'bar',
     ...                                 b'baz': chr(92) + chr(0) + b'2'})
     ...                    ).items())
     [('baz', '\\\\\\x002'), ('branch', 'default'), ('foo', 'bar')]
     """
     extra = _defaultextra.copy()
     for l in text.split(b'\0'):
         if l:
             k, v = _string_unescape(l).split(b':', 1)
             extra[k] = v
     return extra
 
 
 def encodeextra(d):
     # keys must be sorted to produce a deterministic changelog entry
     items = [_string_escape(b'%s:%s' % (k, d[k])) for k in sorted(d)]
     return b"\0".join(items)
 
 
 def stripdesc(desc):
     """strip trailing whitespace and leading and trailing empty lines"""
     return b'\n'.join([l.rstrip() for l in desc.splitlines()]).strip(b'\n')
 
 
-class appender:
-    """the changelog index must be updated last on disk, so we use this class
-    to delay writes to it"""
-
-    def __init__(self, vfs, name, mode, buf):
-        self.data = buf
-        fp = vfs(name, mode)
-        self.fp = fp
-        self.offset = fp.tell()
-        self.size = vfs.fstat(fp).st_size
-        self._end = self.size
-
-    def end(self):
-        return self._end
-
-    def tell(self):
-        return self.offset
-
-    def flush(self):
-        pass
-
-    @property
-    def closed(self):
-        return self.fp.closed
-
-    def close(self):
-        self.fp.close()
-
-    def seek(self, offset, whence=0):
-        '''virtual file offset spans real file and data'''
-        if whence == 0:
-            self.offset = offset
-        elif whence == 1:
-            self.offset += offset
-        elif whence == 2:
-            self.offset = self.end() + offset
-        if self.offset < self.size:
-            self.fp.seek(self.offset)
-
-    def read(self, count=-1):
-        '''only trick here is reads that span real file and data'''
-        ret = b""
-        if self.offset < self.size:
-            s = self.fp.read(count)
-            ret = s
-            self.offset += len(s)
-            if count > 0:
-                count -= len(s)
-        if count != 0:
-            doff = self.offset - self.size
-            self.data.insert(0, b"".join(self.data))
-            del self.data[1:]
-            s = self.data[0][doff : doff + count]
-            self.offset += len(s)
-            ret += s
-        return ret
-
-    def write(self, s):
-        self.data.append(bytes(s))
-        self.offset += len(s)
-        self._end += len(s)
-
-    def __enter__(self):
-        self.fp.__enter__()
-        return self
-
-    def __exit__(self, *args):
-        return self.fp.__exit__(*args)
-
-
 class _divertopener:
     def __init__(self, opener, target):
         self._opener = opener
         self._target = target
 
     def __call__(self, name, mode=b'r', checkambig=False, **kwargs):
         if name != self._target:
             return self._opener(name, mode, **kwargs)
         return self._opener(name + b".a", mode, **kwargs)
 
     def __getattr__(self, attr):
         return getattr(self._opener, attr)
 
 
 class _delayopener:
     """build an opener that stores chunks in 'buf' instead of 'target'"""
 
     def __init__(self, opener, target, buf):
         self._opener = opener
         self._target = target
         self._buf = buf
 
     def __call__(self, name, mode=b'r', checkambig=False, **kwargs):
         if name != self._target:
             return self._opener(name, mode, **kwargs)
         assert not kwargs
-        return appender(self._opener, name, mode, self._buf)
+        return randomaccessfile.appender(self._opener, name, mode, self._buf)
 
     def __getattr__(self, attr):
         return getattr(self._opener, attr)
 
 
 @attr.s
 class _changelogrevision:
     # Extensions might modify _defaultextra, so let the constructor below pass
     # it in
     extra = attr.ib()
     manifest = attr.ib()
     user = attr.ib(default=b'')
     date = attr.ib(default=(0, 0))
     files = attr.ib(default=attr.Factory(list))
     filesadded = attr.ib(default=None)
     filesremoved = attr.ib(default=None)
     p1copies = attr.ib(default=None)
     p2copies = attr.ib(default=None)
     description = attr.ib(default=b'')
     branchinfo = attr.ib(default=(_defaultextra[b'branch'], False))
 
 
 class changelogrevision:
     """Holds results of a parsed changelog revision.
 
     Changelog revisions consist of multiple pieces of data, including
     the manifest node, user, and date. This object exposes a view into
     the parsed object.
     """
 
     __slots__ = (
         '_offsets',
         '_text',
         '_sidedata',
         '_cpsd',
         '_changes',
     )
 
     def __new__(cls, cl, text, sidedata, cpsd):
         if not text:
             return _changelogrevision(extra=_defaultextra, manifest=cl.nullid)
 
         self = super(changelogrevision, cls).__new__(cls)
         # We could return here and implement the following as an __init__.
         # But doing it here is equivalent and saves an extra function call.
 
         # format used:
         # nodeid\n        : manifest node in ascii
         # user\n          : user, no \n or \r allowed
         # time tz extra\n : date (time is int or float, timezone is int)
         #                 : extra is metadata, encoded and separated by '\0'
         #                 : older versions ignore it
         # files\n\n       : files modified by the cset, no \n or \r allowed
         # (.*)            : comment (free text, ideally utf-8)
         #
         # changelog v0 doesn't use extra
 
         nl1 = text.index(b'\n')
         nl2 = text.index(b'\n', nl1 + 1)
         nl3 = text.index(b'\n', nl2 + 1)
 
         # The list of files may be empty. Which means nl3 is the first of the
         # double newline that precedes the description.
         if text[nl3 + 1 : nl3 + 2] == b'\n':
             doublenl = nl3
         else:
             doublenl = text.index(b'\n\n', nl3 + 1)
 
         self._offsets = (nl1, nl2, nl3, doublenl)
         self._text = text
         self._sidedata = sidedata
         self._cpsd = cpsd
         self._changes = None
 
         return self
 
     @property
     def manifest(self):
         return bin(self._text[0 : self._offsets[0]])
 
     @property
     def user(self):
         off = self._offsets
         return encoding.tolocal(self._text[off[0] + 1 : off[1]])
 
     @property
     def _rawdate(self):
         off = self._offsets
         dateextra = self._text[off[1] + 1 : off[2]]
         return dateextra.split(b' ', 2)[0:2]
 
     @property
     def _rawextra(self):
         off = self._offsets
         dateextra = self._text[off[1] + 1 : off[2]]
         fields = dateextra.split(b' ', 2)
         if len(fields) != 3:
             return None
 
         return fields[2]
 
     @property
     def date(self):
         raw = self._rawdate
         time = float(raw[0])
         # Various tools did silly things with the timezone.
         try:
             timezone = int(raw[1])
         except ValueError:
             timezone = 0
 
         return time, timezone
 
     @property
     def extra(self):
         raw = self._rawextra
         if raw is None:
             return _defaultextra
 
         return decodeextra(raw)
 
     @property
     def changes(self):
         if self._changes is not None:
             return self._changes
         if self._cpsd:
             changes = metadata.decode_files_sidedata(self._sidedata)
         else:
             changes = metadata.ChangingFiles(
                 touched=self.files or (),
                 added=self.filesadded or (),
                 removed=self.filesremoved or (),
                 p1_copies=self.p1copies or {},
                 p2_copies=self.p2copies or {},
             )
         self._changes = changes
         return changes
 
     @property
     def files(self):
         if self._cpsd:
             return sorted(self.changes.touched)
         off = self._offsets
         if off[2] == off[3]:
             return []
 
         return self._text[off[2] + 1 : off[3]].split(b'\n')
 
     @property
     def filesadded(self):
         if self._cpsd:
             return self.changes.added
         else:
             rawindices = self.extra.get(b'filesadded')
         if rawindices is None:
             return None
         return metadata.decodefileindices(self.files, rawindices)
 
     @property
     def filesremoved(self):
         if self._cpsd:
             return self.changes.removed
         else:
             rawindices = self.extra.get(b'filesremoved')
         if rawindices is None:
             return None
         return metadata.decodefileindices(self.files, rawindices)
 
     @property
     def p1copies(self):
         if self._cpsd:
             return self.changes.copied_from_p1
         else:
             rawcopies = self.extra.get(b'p1copies')
         if rawcopies is None:
             return None
         return metadata.decodecopies(self.files, rawcopies)
 
     @property
     def p2copies(self):
         if self._cpsd:
             return self.changes.copied_from_p2
         else:
             rawcopies = self.extra.get(b'p2copies')
         if rawcopies is None:
             return None
         return metadata.decodecopies(self.files, rawcopies)
 
     @property
     def description(self):
         return encoding.tolocal(self._text[self._offsets[3] + 2 :])
 
     @property
     def branchinfo(self):
         extra = self.extra
         return encoding.tolocal(extra.get(b"branch")), b'close' in extra
 
 
 class changelog(revlog.revlog):
     def __init__(self, opener, trypending=False, concurrencychecker=None):
         """Load a changelog revlog using an opener.
 
         If ``trypending`` is true, we attempt to load the index from a
         ``00changelog.i.a`` file instead of the default ``00changelog.i``.
         The ``00changelog.i.a`` file contains index (and possibly inline
         revision) data for a transaction that hasn't been finalized yet.
         It exists in a separate file to facilitate readers (such as
         hooks processes) accessing data before a transaction is finalized.
 
         ``concurrencychecker`` will be passed to the revlog init function, see
         the documentation there.
         """
         revlog.revlog.__init__(
             self,
             opener,
             target=(revlog_constants.KIND_CHANGELOG, None),
             radix=b'00changelog',
             checkambig=True,
             mmaplargeindex=True,
             persistentnodemap=opener.options.get(b'persistent-nodemap', False),
             concurrencychecker=concurrencychecker,
             trypending=trypending,
         )
 
         if self._initempty and (self._format_version == revlog.REVLOGV1):
             # changelogs don't benefit from generaldelta.
 
             self._format_flags &= ~revlog.FLAG_GENERALDELTA
             self.delta_config.general_delta = False
 
         # Delta chains for changelogs tend to be very small because entries
         # tend to be small and don't delta well with each. So disable delta
         # chains.
         self._storedeltachains = False
 
         self._realopener = opener
         self._delayed = False
         self._delaybuf = None
         self._divert = False
         self._filteredrevs = frozenset()
         self._filteredrevs_hashcache = {}
         self._copiesstorage = opener.options.get(b'copies-storage')
 
     @property
     def filteredrevs(self):
         return self._filteredrevs
 
     @filteredrevs.setter
     def filteredrevs(self, val):
         # Ensure all updates go through this function
         assert isinstance(val, frozenset)
         self._filteredrevs = val
         self._filteredrevs_hashcache = {}
 
     def _write_docket(self, tr):
         if not self._delayed:
             super(changelog, self)._write_docket(tr)
 
     def delayupdate(self, tr):
         """delay visibility of index updates to other readers"""
         assert not self._inner.is_open
         if self._docket is None and not self._delayed:
             if len(self) == 0:
                 self._divert = True
                 if self._realopener.exists(self._indexfile + b'.a'):
                     self._realopener.unlink(self._indexfile + b'.a')
                 self.opener = _divertopener(self._realopener, self._indexfile)
             else:
                 self._delaybuf = []
                 self.opener = _delayopener(
                     self._realopener, self._indexfile, self._delaybuf
                 )
             self._inner.opener = self.opener
             self._inner._segmentfile.opener = self.opener
             self._inner._segmentfile_sidedata.opener = self.opener
         self._delayed = True
         tr.addpending(b'cl-%i' % id(self), self._writepending)
         tr.addfinalize(b'cl-%i' % id(self), self._finalize)
 
     def _finalize(self, tr):
         """finalize index updates"""
         assert not self._inner.is_open
         self._delayed = False
         self.opener = self._realopener
         self._inner.opener = self.opener
         self._inner._segmentfile.opener = self.opener
         self._inner._segmentfile_sidedata.opener = self.opener
         # move redirected index data back into place
         if self._docket is not None:
             self._write_docket(tr)
         elif self._divert:
             assert not self._delaybuf
             tmpname = self._indexfile + b".a"
             nfile = self.opener.open(tmpname)
             nfile.close()
             self.opener.rename(tmpname, self._indexfile, checkambig=True)
         elif self._delaybuf:
             fp = self.opener(self._indexfile, b'a', checkambig=True)
             fp.write(b"".join(self._delaybuf))
             fp.close()
             self._delaybuf = None
         self._divert = False
         # split when we're done
         self._enforceinlinesize(tr, side_write=False)
 
     def _writepending(self, tr):
         """create a file containing the unfinalized state for
         pretxnchangegroup"""
         assert not self._inner.is_open
         if self._docket:
             return self._docket.write(tr, pending=True)
         if self._delaybuf:
             # make a temporary copy of the index
             fp1 = self._realopener(self._indexfile)
             pendingfilename = self._indexfile + b".a"
             # register as a temp file to ensure cleanup on failure
             tr.registertmp(pendingfilename)
             # write existing data
             fp2 = self._realopener(pendingfilename, b"w")
             fp2.write(fp1.read())
             # add pending data
             fp2.write(b"".join(self._delaybuf))
             fp2.close()
             # switch modes so finalize can simply rename
             self._delaybuf = None
             self._divert = True
             self.opener = _divertopener(self._realopener, self._indexfile)
             self._inner.opener = self.opener
             self._inner._segmentfile.opener = self.opener
             self._inner._segmentfile_sidedata.opener = self.opener
 
         if self._divert:
             return True
 
         return False
 
     def _enforceinlinesize(self, tr, side_write=True):
         if not self._delayed:
             revlog.revlog._enforceinlinesize(self, tr, side_write=side_write)
 
     def read(self, nodeorrev):
         """Obtain data from a parsed changelog revision.
 
         Returns a 6-tuple of:
 
            - manifest node in binary
            - author/user as a localstr
            - date as a 2-tuple of (time, timezone)
            - list of files
            - commit message as a localstr
            - dict of extra metadata
 
         Unless you need to access all fields, consider calling
         ``changelogrevision`` instead, as it is faster for partial object
         access.
         """
         d = self._revisiondata(nodeorrev)
         sidedata = self.sidedata(nodeorrev)
         copy_sd = self._copiesstorage == b'changeset-sidedata'
         c = changelogrevision(self, d, sidedata, copy_sd)
         return (c.manifest, c.user, c.date, c.files, c.description, c.extra)
 
     def changelogrevision(self, nodeorrev):
         """Obtain a ``changelogrevision`` for a node or revision."""
         text = self._revisiondata(nodeorrev)
         sidedata = self.sidedata(nodeorrev)
         return changelogrevision(
             self, text, sidedata, self._copiesstorage == b'changeset-sidedata'
         )
 
     def readfiles(self, nodeorrev):
         """
         short version of read that only returns the files modified by the cset
         """
         text = self.revision(nodeorrev)
         if not text:
             return []
         last = text.index(b"\n\n")
         l = text[:last].split(b'\n')
         return l[3:]
 
     def add(
         self,
         manifest,
         files,
         desc,
         transaction,
         p1,
         p2,
         user,
         date=None,
         extra=None,
     ):
         # Convert to UTF-8 encoded bytestrings as the very first
         # thing: calling any method on a localstr object will turn it
         # into a str object and the cached UTF-8 string is thus lost.
         user, desc = encoding.fromlocal(user), encoding.fromlocal(desc)
 
         user = user.strip()
         # An empty username or a username with a "\n" will make the
         # revision text contain two "\n\n" sequences -> corrupt
         # repository since read cannot unpack the revision.
         if not user:
             raise error.StorageError(_(b"empty username"))
         if b"\n" in user:
             raise error.StorageError(
                 _(b"username %r contains a newline") % pycompat.bytestr(user)
             )
 
         desc = stripdesc(desc)
 
         if date:
             parseddate = b"%d %d" % dateutil.parsedate(date)
         else:
             parseddate = b"%d %d" % dateutil.makedate()
         if extra:
             branch = extra.get(b"branch")
             if branch in (b"default", b""):
                 del extra[b"branch"]
             elif branch in (b".", b"null", b"tip"):
                 raise error.StorageError(
                     _(b'the name \'%s\' is reserved') % branch
                 )
         sortedfiles = sorted(files.touched)
         flags = 0
         sidedata = None
         if self._copiesstorage == b'changeset-sidedata':
             if files.has_copies_info:
                 flags |= flagutil.REVIDX_HASCOPIESINFO
             sidedata = metadata.encode_files_sidedata(files)
 
         if extra:
             extra = encodeextra(extra)
             parseddate = b"%s %s" % (parseddate, extra)
         l = [hex(manifest), user, parseddate] + sortedfiles + [b"", desc]
         text = b"\n".join(l)
         rev = self.addrevision(
             text, transaction, len(self), p1, p2, sidedata=sidedata, flags=flags
         )
         return self.node(rev)
 
     def branchinfo(self, rev):
         """return the branch name and open/close state of a revision
 
         This function exists because creating a changectx object
         just to access this is costly."""
         return self.changelogrevision(rev).branchinfo
 
     def _nodeduplicatecallback(self, transaction, rev):
         # keep track of revisions that got "re-added", eg: unbunde of know rev.
         #
         # We track them in a list to preserve their order from the source bundle
         duplicates = transaction.changes.setdefault(b'revduplicates', [])
         duplicates.append(rev)

mercurial/revlogutils/randomaccessfile.py

@@ -1,164 +1,234 b''
 # Copyright Mercurial Contributors
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 import contextlib
 
 from ..i18n import _
 from .. import (
     error,
     util,
 )
 
 
 _MAX_CACHED_CHUNK_SIZE = 1048576  # 1 MiB
 
 PARTIAL_READ_MSG = _(
     b'partial read of revlog %s; expected %d bytes from offset %d, got %d'
 )
 
 
 def _is_power_of_two(n):
     return (n & (n - 1) == 0) and n != 0
 
 
+class appender:
+    """the changelog index must be updated last on disk, so we use this class
+    to delay writes to it"""
+
+    def __init__(self, vfs, name, mode, buf):
+        self.data = buf
+        fp = vfs(name, mode)
+        self.fp = fp
+        self.offset = fp.tell()
+        self.size = vfs.fstat(fp).st_size
+        self._end = self.size
+
+    def end(self):
+        return self._end
+
+    def tell(self):
+        return self.offset
+
+    def flush(self):
+        pass
+
+    @property
+    def closed(self):
+        return self.fp.closed
+
+    def close(self):
+        self.fp.close()
+
+    def seek(self, offset, whence=0):
+        '''virtual file offset spans real file and data'''
+        if whence == 0:
+            self.offset = offset
+        elif whence == 1:
+            self.offset += offset
+        elif whence == 2:
+            self.offset = self.end() + offset
+        if self.offset < self.size:
+            self.fp.seek(self.offset)
+
+    def read(self, count=-1):
+        '''only trick here is reads that span real file and data'''
+        ret = b""
+        if self.offset < self.size:
+            s = self.fp.read(count)
+            ret = s
+            self.offset += len(s)
+            if count > 0:
+                count -= len(s)
+        if count != 0:
+            doff = self.offset - self.size
+            self.data.insert(0, b"".join(self.data))
+            del self.data[1:]
+            s = self.data[0][doff : doff + count]
+            self.offset += len(s)
+            ret += s
+        return ret
+
+    def write(self, s):
+        self.data.append(bytes(s))
+        self.offset += len(s)
+        self._end += len(s)
+
+    def __enter__(self):
+        self.fp.__enter__()
+        return self
+
+    def __exit__(self, *args):
+        return self.fp.__exit__(*args)
+
+
 class randomaccessfile:
     """Accessing arbitrary chuncks of data within a file, with some caching"""
 
     def __init__(
         self,
         opener,
         filename,
         default_cached_chunk_size,
         initial_cache=None,
     ):
         # Required by bitwise manipulation below
         assert _is_power_of_two(default_cached_chunk_size)
 
         self.opener = opener
         self.filename = filename
         self.default_cached_chunk_size = default_cached_chunk_size
         self.writing_handle = None  # This is set from revlog.py
         self.reading_handle = None
         self._cached_chunk = b''
         self._cached_chunk_position = 0  # Offset from the start of the file
         if initial_cache:
             self._cached_chunk_position, self._cached_chunk = initial_cache
 
     def clear_cache(self):
         self._cached_chunk = b''
         self._cached_chunk_position = 0
 
     @property
     def is_open(self):
         """True if any file handle is being held
 
         Used for assert and debug in the python code"""
         return (
             self.reading_handle is not None or self.writing_handle is not None
         )
 
     def _open(self, mode=b'r'):
         """Return a file object"""
         return self.opener(self.filename, mode=mode)
 
     @contextlib.contextmanager
     def _read_handle(self):
         """File object suitable for reading data"""
         # Use a file handle being actively used for writes, if available.
         # There is some danger to doing this because reads will seek the
         # file. However, revlog._writeentry performs a SEEK_END before all
         # writes, so we should be safe.
         if self.writing_handle:
             yield self.writing_handle
 
         elif self.reading_handle:
             yield self.reading_handle
 
         # Otherwise open a new file handle.
         else:
             with self._open() as fp:
                 yield fp
 
     @contextlib.contextmanager
     def reading(self):
         """Context manager that keeps the file open for reading"""
         if (
             self.reading_handle is None
             and self.writing_handle is None
             and self.filename is not None
         ):
             with self._open() as fp:
                 self.reading_handle = fp
                 try:
                     yield
                 finally:
                     self.reading_handle = None
         else:
             yield
 
     def read_chunk(self, offset, length):
         """Read a chunk of bytes from the file.
 
         Accepts an absolute offset, length to read, and an optional existing
         file handle to read from.
 
         If an existing file handle is passed, it will be seeked and the
         original seek position will NOT be restored.
 
         Returns a str or buffer of raw byte data.
 
         Raises if the requested number of bytes could not be read.
         """
         end = offset + length
         cache_start = self._cached_chunk_position
         cache_end = cache_start + len(self._cached_chunk)
         # Is the requested chunk within the cache?
         if cache_start <= offset and end <= cache_end:
             if cache_start == offset and end == cache_end:
                 return self._cached_chunk  # avoid a copy
             relative_start = offset - cache_start
             return util.buffer(self._cached_chunk, relative_start, length)
 
         return self._read_and_update_cache(offset, length)
 
     def _read_and_update_cache(self, offset, length):
         # Cache data both forward and backward around the requested
         # data, in a fixed size window. This helps speed up operations
         # involving reading the revlog backwards.
         real_offset = offset & ~(self.default_cached_chunk_size - 1)
         real_length = (
             (offset + length + self.default_cached_chunk_size)
             & ~(self.default_cached_chunk_size - 1)
         ) - real_offset
         with self._read_handle() as file_obj:
             file_obj.seek(real_offset)
             data = file_obj.read(real_length)
 
         self._add_cached_chunk(real_offset, data)
 
         relative_offset = offset - real_offset
         got = len(data) - relative_offset
         if got < length:
             message = PARTIAL_READ_MSG % (self.filename, length, offset, got)
             raise error.RevlogError(message)
 
         if offset != real_offset or real_length != length:
             return util.buffer(data, relative_offset, length)
         return data
 
     def _add_cached_chunk(self, offset, data):
         """Add to or replace the cached data chunk.
 
         Accepts an absolute offset and the data that is at that location.
         """
         if (
             self._cached_chunk_position + len(self._cached_chunk) == offset
             and len(self._cached_chunk) + len(data) < _MAX_CACHED_CHUNK_SIZE
         ):
             # add to existing cache
             self._cached_chunk += data
         else:
             self._cached_chunk = data
             self._cached_chunk_position = offset