contrib/undumprevlog

util: add a file object proxy that can read at most N bytes

Sometimes we have data of a known size within a stream. For
performance reasons, we don't want to pre-read this data (we want
to allow consumers to read on demand). For simplicitly reasons,
we don't want callers to necessarily know their data is coming
from within an outer stream and there is a limit to how much
they should read.

The class introduced by this commit provides a very simple proxy
around an underlying file object that allows the consumer to
.read() up to N bytes from the file object. Attempts to read
past this many bytes results in a simulated EOF.

Differential Revision: https://phab.mercurial-scm.org/D2377

#!/usr/bin/env python
# Undump a dump from dumprevlog
# $ hg init
# $ undumprevlog < repo.dump

from __future__ import absolute_import, print_function

import sys
from mercurial import (
    node,
    revlog,
    transaction,
    util,
    vfs as vfsmod,
)

for fp in (sys.stdin, sys.stdout, sys.stderr):
    util.setbinary(fp)

opener = vfsmod.vfs('.', False)
tr = transaction.transaction(sys.stderr.write, opener, {'store': opener},
                             "undump.journal")
while True:
    l = sys.stdin.readline()
    if not l:
        break
    if l.startswith("file:"):
        f = l[6:-1]
        r = revlog.revlog(opener, f)
        print(f)
    elif l.startswith("node:"):
        n = node.bin(l[6:-1])
    elif l.startswith("linkrev:"):
        lr = int(l[9:-1])
    elif l.startswith("parents:"):
        p = l[9:-1].split()
        p1 = node.bin(p[0])
        p2 = node.bin(p[1])
    elif l.startswith("length:"):
        length = int(l[8:-1])
        sys.stdin.readline() # start marker
        d = sys.stdin.read(length)
        sys.stdin.readline() # end marker
        r.addrevision(d, tr, lr, p1, p2)

tr.close()