annotate mercurial/revlogutils/nodemap.py @ 44310:daad3aace942

nodemap: only use persistent nodemap for non-inlined revlog

Revlog are inlined while they are small (to avoid having too many file to deal
with). The persistent nodemap will only provides a significant boost for large
enough revlog index. So it does not make sens to add an extra file to store
nodemap for small revlog.

We could consider inclining the nodemap data inside the revlog itself, but the
benefit is unclear so let it be an adventure for another time.

Differential Revision: https://phab.mercurial-scm.org/D7837

# 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 struct

from .. import (
    error,
    node as nodemod,
    pycompat,
)


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
    return revlog.opener.tryread(revlog.nodemap_file)


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, revlog))


def _persist_nodemap(tr, 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)
    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(revlog.nodemap_file, b'w') as f:
        f.write(data)
    # EXP-TODO: if the transaction abort, we should remove the new data and
    # reinstall the old one. (This will be simpler when the file format get a
    # bit more advanced)


### 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)


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)


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 = {}
    for rev in range(len(index)):
        hex = nodemod.hex(index[rev][7])
        _insert_into_block(index, 0, root, rev, hex)
    return 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
    """
    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
        _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[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)


def _persist_trie(root):
    """turn a nodemap trie into persistent binary data

    See `_build_trie` for nodemap trie structure"""
    block_map = {}
    chunks = []
    for tn in _walk_trie(root):
        block_map[id(tn)] = len(chunks)
        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(block_node.get(i), block_map) for i in range(16))
    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)