branchcache: skip entries that are topological heads in the on disk file
In the majority of cases, topological heads are also branch heads. We have
efficient way to get the topological heads and efficient way to retrieve
their branch information. So there is little value in putting them in the branch
cache file explicitly. On the contrary, writing them explicitly tend to create
very large cache file that are inefficient to read and update.
So the branch cache v3 format is no longer including them. This changeset focus
on the format aspect and have no focus on the performance aspect. We will cover
that later.
# dirstatedocket.py - docket file for dirstate-v2
#
# 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 struct
from ..revlogutils import docket as docket_mod
from . import v2
V2_FORMAT_MARKER = b"dirstate-v2\n"
# * 12 bytes: format marker
# * 32 bytes: node ID of the working directory's first parent
# * 32 bytes: node ID of the working directory's second parent
# * {TREE_METADATA_SIZE} bytes: tree metadata, parsed separately
# * 4 bytes: big-endian used size of the data file
# * 1 byte: length of the data file's UUID
# * variable: data file's UUID
#
# Node IDs are null-padded if shorter than 32 bytes.
# A data file shorter than the specified used size is corrupted (truncated)
HEADER = struct.Struct(
">{}s32s32s{}sLB".format(len(V2_FORMAT_MARKER), v2.TREE_METADATA_SIZE)
)
class DirstateDocket:
data_filename_pattern = b'dirstate.%s'
def __init__(self, parents, data_size, tree_metadata, uuid):
self.parents = parents
self.data_size = data_size
self.tree_metadata = tree_metadata
self.uuid = uuid
@classmethod
def with_new_uuid(cls, parents, data_size, tree_metadata):
return cls(parents, data_size, tree_metadata, docket_mod.make_uid())
@classmethod
def parse(cls, data, nodeconstants):
if not data:
parents = (nodeconstants.nullid, nodeconstants.nullid)
return cls(parents, 0, b'', None)
marker, p1, p2, meta, data_size, uuid_size = HEADER.unpack_from(data)
if marker != V2_FORMAT_MARKER:
raise ValueError("expected dirstate-v2 marker")
uuid = data[HEADER.size : HEADER.size + uuid_size]
p1 = p1[: nodeconstants.nodelen]
p2 = p2[: nodeconstants.nodelen]
return cls((p1, p2), data_size, meta, uuid)
def serialize(self):
p1, p2 = self.parents
header = HEADER.pack(
V2_FORMAT_MARKER,
p1,
p2,
self.tree_metadata,
self.data_size,
len(self.uuid),
)
return header + self.uuid
def data_filename(self):
return self.data_filename_pattern % self.uuid