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changelog-delay: move the appender class next to randomaccessfile...
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.

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randomaccessfile.py
234 lines | 7.1 KiB | text/x-python | PythonLexer
# 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