nodemap: document the docket attributes

More documentation rarely hurts.

Differential Revision: https://phab.mercurial-scm.org/D8182

mercurial/revlogutils/nodemap.py

@@ -1,554 +1,565 b''
 # nodemap.py - nodemap related code and utilities
 #
 # Copyright 2019 Pierre-Yves David <[e-mail]>
 # Copyright 2019 George Racinet <[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 __future__ import absolute_import
 
 import errno
 import os
 import re
 import struct
 
 from .. import (
     error,
     node as nodemod,
     util,
 )
 
 
 class NodeMap(dict):
     def __missing__(self, x):
         raise error.RevlogError(b'unknown node: %s' % x)
 
 
 def persisted_data(revlog):
     """read the nodemap for a revlog from disk"""
     if revlog.nodemap_file is None:
         return None
     pdata = revlog.opener.tryread(revlog.nodemap_file)
     if not pdata:
         return None
     offset = 0
     (version,) = S_VERSION.unpack(pdata[offset : offset + S_VERSION.size])
     if version != ONDISK_VERSION:
         return None
     offset += S_VERSION.size
     headers = S_HEADER.unpack(pdata[offset : offset + S_HEADER.size])
     uid_size, tip_rev, data_length, data_unused = headers
     offset += S_HEADER.size
     docket = NodeMapDocket(pdata[offset : offset + uid_size])
     docket.tip_rev = tip_rev
     docket.data_length = data_length
     docket.data_unused = data_unused
 
     filename = _rawdata_filepath(revlog, docket)
     use_mmap = revlog.opener.options.get("exp-persistent-nodemap.mmap")
     try:
         with revlog.opener(filename) as fd:
             if use_mmap:
                 data = util.buffer(util.mmapread(fd, data_length))
             else:
                 data = fd.read(data_length)
     except OSError as e:
         if e.errno != errno.ENOENT:
             raise
     if len(data) < data_length:
         return None
     return docket, data
 
 
 def setup_persistent_nodemap(tr, revlog):
     """Install whatever is needed transaction side to persist a nodemap on disk
 
     (only actually persist the nodemap if this is relevant for this revlog)
     """
     if revlog._inline:
         return  # inlined revlog are too small for this to be relevant
     if revlog.nodemap_file is None:
         return  # we do not use persistent_nodemap on this revlog
     callback_id = b"revlog-persistent-nodemap-%s" % revlog.nodemap_file
     if tr.hasfinalize(callback_id):
         return  # no need to register again
     tr.addfinalize(
         callback_id, lambda tr: _persist_nodemap(tr.addpostclose, revlog)
     )
 
 
 def update_persistent_nodemap(revlog):
     """update the persistent nodemap right now
 
     To be used for updating the nodemap on disk outside of a normal transaction
     setup (eg, `debugupdatecache`).
     """
     cleanups = []
     _persist_nodemap((lambda x, y: cleanups.append(y)), revlog)
     for c in cleanups:
         c(None)
 
 
 def _persist_nodemap(cleaner, revlog):
     """Write nodemap data on disk for a given revlog
     """
     if getattr(revlog, 'filteredrevs', ()):
         raise error.ProgrammingError(
             "cannot persist nodemap of a filtered changelog"
         )
     if revlog.nodemap_file is None:
         msg = "calling persist nodemap on a revlog without the feature enableb"
         raise error.ProgrammingError(msg)
 
     can_incremental = util.safehasattr(revlog.index, "nodemap_data_incremental")
     ondisk_docket = revlog._nodemap_docket
     feed_data = util.safehasattr(revlog.index, "update_nodemap_data")
     use_mmap = revlog.opener.options.get("exp-persistent-nodemap.mmap")
 
     data = None
     # first attemp an incremental update of the data
     if can_incremental and ondisk_docket is not None:
         target_docket = revlog._nodemap_docket.copy()
         (
             src_docket,
             data_changed_count,
             data,
         ) = revlog.index.nodemap_data_incremental()
         if src_docket != target_docket:
             data = None
         else:
             datafile = _rawdata_filepath(revlog, target_docket)
             # EXP-TODO: if this is a cache, this should use a cache vfs, not a
             # store vfs
             new_length = target_docket.data_length + len(data)
             with revlog.opener(datafile, b'r+') as fd:
                 fd.seek(target_docket.data_length)
                 fd.write(data)
                 if feed_data:
                     if use_mmap:
                         fd.seek(0)
                         new_data = fd.read(new_length)
                     else:
                         fd.flush()
                         new_data = util.buffer(util.mmapread(fd, new_length))
             target_docket.data_length = new_length
             target_docket.data_unused += data_changed_count
 
     if data is None:
         # otherwise fallback to a full new export
         target_docket = NodeMapDocket()
         datafile = _rawdata_filepath(revlog, target_docket)
         if util.safehasattr(revlog.index, "nodemap_data_all"):
             data = revlog.index.nodemap_data_all()
         else:
             data = persistent_data(revlog.index)
         # EXP-TODO: if this is a cache, this should use a cache vfs, not a
         # store vfs
         with revlog.opener(datafile, b'w+') as fd:
             fd.write(data)
             if feed_data:
                 if use_mmap:
                     new_data = data
                 else:
                     fd.flush()
                     new_data = util.buffer(util.mmapread(fd, len(data)))
         target_docket.data_length = len(data)
     target_docket.tip_rev = revlog.tiprev()
     # EXP-TODO: if this is a cache, this should use a cache vfs, not a
     # store vfs
     with revlog.opener(revlog.nodemap_file, b'w', atomictemp=True) as fp:
         fp.write(target_docket.serialize())
     revlog._nodemap_docket = target_docket
     if feed_data:
         revlog.index.update_nodemap_data(target_docket, new_data)
 
     # EXP-TODO: if the transaction abort, we should remove the new data and
     # reinstall the old one.
 
     # search for old index file in all cases, some older process might have
     # left one behind.
     olds = _other_rawdata_filepath(revlog, target_docket)
     if olds:
         realvfs = getattr(revlog, '_realopener', revlog.opener)
 
         def cleanup(tr):
             for oldfile in olds:
                 realvfs.tryunlink(oldfile)
 
         callback_id = b"revlog-cleanup-nodemap-%s" % revlog.nodemap_file
         cleaner(callback_id, cleanup)
 
 
 ### Nodemap docket file
 #
 # The nodemap data are stored on disk using 2 files:
 #
 # * a raw data files containing a persistent nodemap
 #   (see `Nodemap Trie` section)
 #
 # * a small "docket" file containing medatadata
 #
 # While the nodemap data can be multiple tens of megabytes, the "docket" is
 # small, it is easy to update it automatically or to duplicated its content
 # during a transaction.
 #
 # Multiple raw data can exist at the same time (The currently valid one and a
 # new one beind used by an in progress transaction). To accomodate this, the
 # filename hosting the raw data has a variable parts. The exact filename is
 # specified inside the "docket" file.
 #
 # The docket file contains information to find, qualify and validate the raw
 # data. Its content is currently very light, but it will expand as the on disk
 # nodemap gains the necessary features to be used in production.
 
 # version 0 is experimental, no BC garantee, do no use outside of tests.
 ONDISK_VERSION = 0
 S_VERSION = struct.Struct(">B")
 S_HEADER = struct.Struct(">BQQQ")
 
 ID_SIZE = 8
 
 
 def _make_uid():
     """return a new unique identifier.
 
     The identifier is random and composed of ascii characters."""
     return nodemod.hex(os.urandom(ID_SIZE))
 
 
 class NodeMapDocket(object):
     """metadata associated with persistent nodemap data
 
     The persistent data may come from disk or be on their way to disk.
     """
 
     def __init__(self, uid=None):
         if uid is None:
             uid = _make_uid()
+        # a unique identifier for the data file:
+        #   - When new data are appended, it is preserved.
+        #   - When a new data file is created, a new identifier is generated.
         self.uid = uid
+        # the tipmost revision stored in the data file. This revision and all
+        # revision before it are expected to be encoded in the data file.
         self.tip_rev = None
+        # the size (in bytes) of the persisted data to encode the nodemap valid
+        # for `tip_rev`.
+        #   - data file shorter than this are corrupted,
+        #   - any extra data should be ignored.
         self.data_length = None
+        # the amount (in bytes) of "dead" data, still in the data file but no
+        # longer used for the nodemap.
         self.data_unused = 0
 
     def copy(self):
         new = NodeMapDocket(uid=self.uid)
         new.tip_rev = self.tip_rev
         new.data_length = self.data_length
         new.data_unused = self.data_unused
         return new
 
     def __cmp__(self, other):
         if self.uid < other.uid:
             return -1
         if self.uid > other.uid:
             return 1
         elif self.data_length < other.data_length:
             return -1
         elif self.data_length > other.data_length:
             return 1
         return 0
 
     def __eq__(self, other):
         return self.uid == other.uid and self.data_length == other.data_length
 
     def serialize(self):
         """return serialized bytes for a docket using the passed uid"""
         data = []
         data.append(S_VERSION.pack(ONDISK_VERSION))
         headers = (
             len(self.uid),
             self.tip_rev,
             self.data_length,
             self.data_unused,
         )
         data.append(S_HEADER.pack(*headers))
         data.append(self.uid)
         return b''.join(data)
 
 
 def _rawdata_filepath(revlog, docket):
     """The (vfs relative) nodemap's rawdata file for a given uid"""
     prefix = revlog.nodemap_file[:-2]
     return b"%s-%s.nd" % (prefix, docket.uid)
 
 
 def _other_rawdata_filepath(revlog, docket):
     prefix = revlog.nodemap_file[:-2]
     pattern = re.compile(br"(^|/)%s-[0-9a-f]+\.nd$" % prefix)
     new_file_path = _rawdata_filepath(revlog, docket)
     new_file_name = revlog.opener.basename(new_file_path)
     dirpath = revlog.opener.dirname(new_file_path)
     others = []
     for f in revlog.opener.listdir(dirpath):
         if pattern.match(f) and f != new_file_name:
             others.append(f)
     return others
 
 
 ### Nodemap Trie
 #
 # This is a simple reference implementation to compute and persist a nodemap
 # trie. This reference implementation is write only. The python version of this
 # is not expected to be actually used, since it wont provide performance
 # improvement over existing non-persistent C implementation.
 #
 # The nodemap is persisted as Trie using 4bits-address/16-entries block. each
 # revision can be adressed using its node shortest prefix.
 #
 # The trie is stored as a sequence of block. Each block contains 16 entries
 # (signed 64bit integer, big endian). Each entry can be one of the following:
 #
 #  * value >=  0 -> index of sub-block
 #  * value == -1 -> no value
 #  * value <  -1 -> a revision value: rev = -(value+10)
 #
 # The implementation focus on simplicity, not on performance. A Rust
 # implementation should provide a efficient version of the same binary
 # persistence. This reference python implementation is never meant to be
 # extensively use in production.
 
 
 def persistent_data(index):
     """return the persistent binary form for a nodemap for a given index
     """
     trie = _build_trie(index)
     return _persist_trie(trie)
 
 
 def update_persistent_data(index, root, max_idx, last_rev):
     """return the incremental update for persistent nodemap from a given index
     """
     changed_block, trie = _update_trie(index, root, last_rev)
     return (
         changed_block * S_BLOCK.size,
         _persist_trie(trie, existing_idx=max_idx),
     )
 
 
 S_BLOCK = struct.Struct(">" + ("l" * 16))
 
 NO_ENTRY = -1
 # rev 0 need to be -2 because 0 is used by block, -1 is a special value.
 REV_OFFSET = 2
 
 
 def _transform_rev(rev):
     """Return the number used to represent the rev in the tree.
 
     (or retrieve a rev number from such representation)
 
     Note that this is an involution, a function equal to its inverse (i.e.
     which gives the identity when applied to itself).
     """
     return -(rev + REV_OFFSET)
 
 
 def _to_int(hex_digit):
     """turn an hexadecimal digit into a proper integer"""
     return int(hex_digit, 16)
 
 
 class Block(dict):
     """represent a block of the Trie
 
     contains up to 16 entry indexed from 0 to 15"""
 
     def __init__(self):
         super(Block, self).__init__()
         # If this block exist on disk, here is its ID
         self.ondisk_id = None
 
     def __iter__(self):
         return iter(self.get(i) for i in range(16))
 
 
 def _build_trie(index):
     """build a nodemap trie
 
     The nodemap stores revision number for each unique prefix.
 
     Each block is a dictionary with keys in `[0, 15]`. Values are either
     another block or a revision number.
     """
     root = Block()
     for rev in range(len(index)):
         hex = nodemod.hex(index[rev][7])
         _insert_into_block(index, 0, root, rev, hex)
     return root
 
 
 def _update_trie(index, root, last_rev):
     """consume"""
     changed = 0
     for rev in range(last_rev + 1, len(index)):
         hex = nodemod.hex(index[rev][7])
         changed += _insert_into_block(index, 0, root, rev, hex)
     return changed, root
 
 
 def _insert_into_block(index, level, block, current_rev, current_hex):
     """insert a new revision in a block
 
     index: the index we are adding revision for
     level: the depth of the current block in the trie
     block: the block currently being considered
     current_rev: the revision number we are adding
     current_hex: the hexadecimal representation of the of that revision
     """
     changed = 1
     if block.ondisk_id is not None:
         block.ondisk_id = None
     hex_digit = _to_int(current_hex[level : level + 1])
     entry = block.get(hex_digit)
     if entry is None:
         # no entry, simply store the revision number
         block[hex_digit] = current_rev
     elif isinstance(entry, dict):
         # need to recurse to an underlying block
         changed += _insert_into_block(
             index, level + 1, entry, current_rev, current_hex
         )
     else:
         # collision with a previously unique prefix, inserting new
         # vertices to fit both entry.
         other_hex = nodemod.hex(index[entry][7])
         other_rev = entry
         new = Block()
         block[hex_digit] = new
         _insert_into_block(index, level + 1, new, other_rev, other_hex)
         _insert_into_block(index, level + 1, new, current_rev, current_hex)
     return changed
 
 
 def _persist_trie(root, existing_idx=None):
     """turn a nodemap trie into persistent binary data
 
     See `_build_trie` for nodemap trie structure"""
     block_map = {}
     if existing_idx is not None:
         base_idx = existing_idx + 1
     else:
         base_idx = 0
     chunks = []
     for tn in _walk_trie(root):
         if tn.ondisk_id is not None:
             block_map[id(tn)] = tn.ondisk_id
         else:
             block_map[id(tn)] = len(chunks) + base_idx
             chunks.append(_persist_block(tn, block_map))
     return b''.join(chunks)
 
 
 def _walk_trie(block):
     """yield all the block in a trie
 
     Children blocks are always yield before their parent block.
     """
     for (_, item) in sorted(block.items()):
         if isinstance(item, dict):
             for sub_block in _walk_trie(item):
                 yield sub_block
     yield block
 
 
 def _persist_block(block_node, block_map):
     """produce persistent binary data for a single block
 
     Children block are assumed to be already persisted and present in
     block_map.
     """
     data = tuple(_to_value(v, block_map) for v in block_node)
     return S_BLOCK.pack(*data)
 
 
 def _to_value(item, block_map):
     """persist any value as an integer"""
     if item is None:
         return NO_ENTRY
     elif isinstance(item, dict):
         return block_map[id(item)]
     else:
         return _transform_rev(item)
 
 
 def parse_data(data):
     """parse parse nodemap data into a nodemap Trie"""
     if (len(data) % S_BLOCK.size) != 0:
         msg = "nodemap data size is not a multiple of block size (%d): %d"
         raise error.Abort(msg % (S_BLOCK.size, len(data)))
     if not data:
         return Block(), None
     block_map = {}
     new_blocks = []
     for i in range(0, len(data), S_BLOCK.size):
         block = Block()
         block.ondisk_id = len(block_map)
         block_map[block.ondisk_id] = block
         block_data = data[i : i + S_BLOCK.size]
         values = S_BLOCK.unpack(block_data)
         new_blocks.append((block, values))
     for b, values in new_blocks:
         for idx, v in enumerate(values):
             if v == NO_ENTRY:
                 continue
             elif v >= 0:
                 b[idx] = block_map[v]
             else:
                 b[idx] = _transform_rev(v)
     return block, i // S_BLOCK.size
 
 
 # debug utility
 
 
 def check_data(ui, index, data):
     """verify that the provided nodemap data are valid for the given idex"""
     ret = 0
     ui.status((b"revision in index:   %d\n") % len(index))
     root, __ = parse_data(data)
     all_revs = set(_all_revisions(root))
     ui.status((b"revision in nodemap: %d\n") % len(all_revs))
     for r in range(len(index)):
         if r not in all_revs:
             msg = b"  revision missing from nodemap: %d\n" % r
             ui.write_err(msg)
             ret = 1
         else:
             all_revs.remove(r)
         nm_rev = _find_node(root, nodemod.hex(index[r][7]))
         if nm_rev is None:
             msg = b"  revision node does not match any entries: %d\n" % r
             ui.write_err(msg)
             ret = 1
         elif nm_rev != r:
             msg = (
                 b"  revision node does not match the expected revision: "
                 b"%d != %d\n" % (r, nm_rev)
             )
             ui.write_err(msg)
             ret = 1
 
     if all_revs:
         for r in sorted(all_revs):
             msg = b"  extra revision in  nodemap: %d\n" % r
             ui.write_err(msg)
         ret = 1
     return ret
 
 
 def _all_revisions(root):
     """return all revisions stored in a Trie"""
     for block in _walk_trie(root):
         for v in block:
             if v is None or isinstance(v, Block):
                 continue
             yield v
 
 
 def _find_node(block, node):
     """find the revision associated with a given node"""
     entry = block.get(_to_int(node[0:1]))
     if isinstance(entry, dict):
         return _find_node(entry, node[1:])
     return entry