mercurial/lock.py
author Gregory Szorc <gregory.szorc@gmail.com>
Mon, 26 Mar 2018 11:00:16 -0700
changeset 37288 9bfcbe4f4745
parent 37120 a8a902d7176e
child 37658 34758397ad1b
permissions -rw-r--r--
wireproto: add streams to frame-based protocol Previously, the frame-based protocol was just a series of frames, with each frame associated with a request ID. In order to scale the protocol, we'll want to enable the use of compression. While it is possible to enable compression at the socket/pipe level, this has its disadvantages. The big one is it undermines the point of frames being standalone, atomic units that can be read and written: if you add compression above the framing protocol, you are back to having a stream-based protocol as opposed to something frame-based. So in order to preserve frames, compression needs to occur at the frame payload level. Compressing each frame's payload individually will limit compression ratios because the window size of the compressor will be limited by the max frame size, which is 32-64kb as currently defined. It will also add CPU overhead, as it is more efficient for compressors to operate on fewer, larger blocks of data than more, smaller blocks. So compressing each frame independently is out. This means we need to compress each frame's payload as if it is part of a larger stream. The simplest approach is to have 1 stream per connection. This could certainly work. However, it has disadvantages (documented below). We could also have 1 stream per RPC/command invocation. (This is the model HTTP/2 goes with.) This also has disadvantages. The main disadvantage to one global stream is that it has the very real potential to create CPU bottlenecks doing compression. Networks are only getting faster and the performance of single CPU cores has been relatively flat. Newer compression formats like zstandard offer better CPU cycle efficiency than predecessors like zlib. But it still all too common to saturate your CPU with compression overhead long before you saturate the network pipe. The main disadvantage with streams per request is that you can't reap the benefits of the compression context for multiple requests. For example, if you send 1000 RPC requests (or HTTP/2 requests for that matter), the response to each would have its own compression context. The overall size of the raw responses would be larger because compression contexts wouldn't be able to reference data from another request or response. The approach for streams as implemented in this commit is to support N streams per connection and for streams to potentially span requests and responses. As explained by the added internals docs, this facilitates servers and clients delegating independent streams and compression to independent threads / CPU cores. This helps alleviate the CPU bottleneck of compression. This design also allows compression contexts to be reused across requests/responses. This can result in improved compression ratios and less overhead for compressors and decompressors having to build new contexts. Another feature that was defined was the ability for individual frames within a stream to declare whether that individual frame's payload uses the content encoding (read: compression) defined by the stream. The idea here is that some servers may serve data from a combination of caches and dynamic resolution. Data coming from caches may be pre-compressed. We want to facilitate servers being able to essentially stream bytes from caches to the wire with minimal overhead. Being able to mix and match with frames are compressed within a stream enables these types of advanced server functionality. This commit defines the new streams mechanism. Basic code for supporting streams in frames has been added. But that code is seriously lacking and doesn't fully conform to the defined protocol. For example, we don't close any streams. And support for content encoding within streams is not yet implemented. The change was rather invasive and I didn't think it would be reasonable to implement the entire feature in a single commit. For the record, I would have loved to reuse an existing multiplexing protocol to build the new wire protocol on top of. However, I couldn't find a protocol that offers the performance and scaling characteristics that I desired. Namely, it should support multiple compression contexts to facilitate scaling out to multiple CPU cores and compression contexts should be able to live longer than single RPC requests. HTTP/2 *almost* fits the bill. But the semantics of HTTP message exchange state that streams can only live for a single request-response. We /could/ tunnel on top of HTTP/2 streams and frames with HEADER and DATA frames. But there's no guarantee that HTTP/2 libraries and proxies would allow us to use HTTP/2 streams and frames without the HTTP message exchange semantics defined in RFC 7540 Section 8. Other RPC protocols like gRPC tunnel are built on top of HTTP/2 and thus preserve its semantics of stream per RPC invocation. Even QUIC does this. We could attempt to invent a higher-level stream that spans HTTP/2 streams. But this would be violating HTTP/2 because there is no guarantee that HTTP/2 streams are routed to the same server. The best we can do - which is what this protocol does - is shoehorn all request and response data into a single HTTP message and create streams within. At that point, we've defined a Content-Type in HTTP parlance. It just so happens our media type can also work as a standalone, stream-based protocol, without leaning on HTTP or similar protocol. Differential Revision: https://phab.mercurial-scm.org/D2907

# lock.py - simple advisory locking scheme for mercurial
#
# Copyright 2005, 2006 Matt Mackall <mpm@selenic.com>
#
# 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 contextlib
import errno
import os
import signal
import socket
import time
import warnings

from .i18n import _

from . import (
    encoding,
    error,
    pycompat,
)

from .utils import (
    procutil,
)

def _getlockprefix():
    """Return a string which is used to differentiate pid namespaces

    It's useful to detect "dead" processes and remove stale locks with
    confidence. Typically it's just hostname. On modern linux, we include an
    extra Linux-specific pid namespace identifier.
    """
    result = encoding.strtolocal(socket.gethostname())
    if pycompat.sysplatform.startswith('linux'):
        try:
            result += '/%x' % os.stat('/proc/self/ns/pid').st_ino
        except OSError as ex:
            if ex.errno not in (errno.ENOENT, errno.EACCES, errno.ENOTDIR):
                raise
    return result

@contextlib.contextmanager
def _delayedinterrupt():
    """Block signal interrupt while doing something critical

    This makes sure that the code block wrapped by this context manager won't
    be interrupted.

    For Windows developers: It appears not possible to guard time.sleep()
    from CTRL_C_EVENT, so please don't use time.sleep() to test if this is
    working.
    """
    assertedsigs = []
    blocked = False
    orighandlers = {}

    def raiseinterrupt(num):
        if (num == getattr(signal, 'SIGINT', None) or
            num == getattr(signal, 'CTRL_C_EVENT', None)):
            raise KeyboardInterrupt
        else:
            raise error.SignalInterrupt
    def catchterm(num, frame):
        if blocked:
            assertedsigs.append(num)
        else:
            raiseinterrupt(num)

    try:
        # save handlers first so they can be restored even if a setup is
        # interrupted between signal.signal() and orighandlers[] =.
        for name in ['CTRL_C_EVENT', 'SIGINT', 'SIGBREAK', 'SIGHUP', 'SIGTERM']:
            num = getattr(signal, name, None)
            if num and num not in orighandlers:
                orighandlers[num] = signal.getsignal(num)
        try:
            for num in orighandlers:
                signal.signal(num, catchterm)
        except ValueError:
            pass # in a thread? no luck

        blocked = True
        yield
    finally:
        # no simple way to reliably restore all signal handlers because
        # any loops, recursive function calls, except blocks, etc. can be
        # interrupted. so instead, make catchterm() raise interrupt.
        blocked = False
        try:
            for num, handler in orighandlers.items():
                signal.signal(num, handler)
        except ValueError:
            pass # in a thread?

    # re-raise interrupt exception if any, which may be shadowed by a new
    # interrupt occurred while re-raising the first one
    if assertedsigs:
        raiseinterrupt(assertedsigs[0])

def trylock(ui, vfs, lockname, timeout, warntimeout, *args, **kwargs):
    """return an acquired lock or raise an a LockHeld exception

    This function is responsible to issue warnings and or debug messages about
    the held lock while trying to acquires it."""

    def printwarning(printer, locker):
        """issue the usual "waiting on lock" message through any channel"""
        # show more details for new-style locks
        if ':' in locker:
            host, pid = locker.split(":", 1)
            msg = (_("waiting for lock on %s held by process %r on host %r\n")
                   % (pycompat.bytestr(l.desc), pycompat.bytestr(pid),
                      pycompat.bytestr(host)))
        else:
            msg = (_("waiting for lock on %s held by %r\n")
                   % (l.desc, pycompat.bytestr(locker)))
        printer(msg)

    l = lock(vfs, lockname, 0, *args, dolock=False, **kwargs)

    debugidx = 0 if (warntimeout and timeout) else -1
    warningidx = 0
    if not timeout:
        warningidx = -1
    elif warntimeout:
        warningidx = warntimeout

    delay = 0
    while True:
        try:
            l._trylock()
            break
        except error.LockHeld as inst:
            if delay == debugidx:
                printwarning(ui.debug, inst.locker)
            if delay == warningidx:
                printwarning(ui.warn, inst.locker)
            if timeout <= delay:
                raise error.LockHeld(errno.ETIMEDOUT, inst.filename,
                                     l.desc, inst.locker)
            time.sleep(1)
            delay += 1

    l.delay = delay
    if l.delay:
        if 0 <= warningidx <= l.delay:
            ui.warn(_("got lock after %d seconds\n") % l.delay)
        else:
            ui.debug("got lock after %d seconds\n" % l.delay)
    if l.acquirefn:
        l.acquirefn()
    return l

class lock(object):
    '''An advisory lock held by one process to control access to a set
    of files.  Non-cooperating processes or incorrectly written scripts
    can ignore Mercurial's locking scheme and stomp all over the
    repository, so don't do that.

    Typically used via localrepository.lock() to lock the repository
    store (.hg/store/) or localrepository.wlock() to lock everything
    else under .hg/.'''

    # lock is symlink on platforms that support it, file on others.

    # symlink is used because create of directory entry and contents
    # are atomic even over nfs.

    # old-style lock: symlink to pid
    # new-style lock: symlink to hostname:pid

    _host = None

    def __init__(self, vfs, file, timeout=-1, releasefn=None, acquirefn=None,
                 desc=None, inheritchecker=None, parentlock=None,
                 dolock=True):
        self.vfs = vfs
        self.f = file
        self.held = 0
        self.timeout = timeout
        self.releasefn = releasefn
        self.acquirefn = acquirefn
        self.desc = desc
        self._inheritchecker = inheritchecker
        self.parentlock = parentlock
        self._parentheld = False
        self._inherited = False
        self.postrelease  = []
        self.pid = self._getpid()
        if dolock:
            self.delay = self.lock()
            if self.acquirefn:
                self.acquirefn()

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, exc_tb):
        self.release()

    def __del__(self):
        if self.held:
            warnings.warn("use lock.release instead of del lock",
                    category=DeprecationWarning,
                    stacklevel=2)

            # ensure the lock will be removed
            # even if recursive locking did occur
            self.held = 1

        self.release()

    def _getpid(self):
        # wrapper around procutil.getpid() to make testing easier
        return procutil.getpid()

    def lock(self):
        timeout = self.timeout
        while True:
            try:
                self._trylock()
                return self.timeout - timeout
            except error.LockHeld as inst:
                if timeout != 0:
                    time.sleep(1)
                    if timeout > 0:
                        timeout -= 1
                    continue
                raise error.LockHeld(errno.ETIMEDOUT, inst.filename, self.desc,
                                     inst.locker)

    def _trylock(self):
        if self.held:
            self.held += 1
            return
        if lock._host is None:
            lock._host = _getlockprefix()
        lockname = '%s:%d' % (lock._host, self.pid)
        retry = 5
        while not self.held and retry:
            retry -= 1
            try:
                with _delayedinterrupt():
                    self.vfs.makelock(lockname, self.f)
                    self.held = 1
            except (OSError, IOError) as why:
                if why.errno == errno.EEXIST:
                    locker = self._readlock()
                    if locker is None:
                        continue

                    # special case where a parent process holds the lock -- this
                    # is different from the pid being different because we do
                    # want the unlock and postrelease functions to be called,
                    # but the lockfile to not be removed.
                    if locker == self.parentlock:
                        self._parentheld = True
                        self.held = 1
                        return
                    locker = self._testlock(locker)
                    if locker is not None:
                        raise error.LockHeld(errno.EAGAIN,
                                             self.vfs.join(self.f), self.desc,
                                             locker)
                else:
                    raise error.LockUnavailable(why.errno, why.strerror,
                                                why.filename, self.desc)

        if not self.held:
            # use empty locker to mean "busy for frequent lock/unlock
            # by many processes"
            raise error.LockHeld(errno.EAGAIN,
                                 self.vfs.join(self.f), self.desc, "")

    def _readlock(self):
        """read lock and return its value

        Returns None if no lock exists, pid for old-style locks, and host:pid
        for new-style locks.
        """
        try:
            return self.vfs.readlock(self.f)
        except (OSError, IOError) as why:
            if why.errno == errno.ENOENT:
                return None
            raise

    def _testlock(self, locker):
        if locker is None:
            return None
        try:
            host, pid = locker.split(":", 1)
        except ValueError:
            return locker
        if host != lock._host:
            return locker
        try:
            pid = int(pid)
        except ValueError:
            return locker
        if procutil.testpid(pid):
            return locker
        # if locker dead, break lock.  must do this with another lock
        # held, or can race and break valid lock.
        try:
            l = lock(self.vfs, self.f + '.break', timeout=0)
            self.vfs.unlink(self.f)
            l.release()
        except error.LockError:
            return locker

    def testlock(self):
        """return id of locker if lock is valid, else None.

        If old-style lock, we cannot tell what machine locker is on.
        with new-style lock, if locker is on this machine, we can
        see if locker is alive.  If locker is on this machine but
        not alive, we can safely break lock.

        The lock file is only deleted when None is returned.

        """
        locker = self._readlock()
        return self._testlock(locker)

    @contextlib.contextmanager
    def inherit(self):
        """context for the lock to be inherited by a Mercurial subprocess.

        Yields a string that will be recognized by the lock in the subprocess.
        Communicating this string to the subprocess needs to be done separately
        -- typically by an environment variable.
        """
        if not self.held:
            raise error.LockInheritanceContractViolation(
                'inherit can only be called while lock is held')
        if self._inherited:
            raise error.LockInheritanceContractViolation(
                'inherit cannot be called while lock is already inherited')
        if self._inheritchecker is not None:
            self._inheritchecker()
        if self.releasefn:
            self.releasefn()
        if self._parentheld:
            lockname = self.parentlock
        else:
            lockname = '%s:%s' % (lock._host, self.pid)
        self._inherited = True
        try:
            yield lockname
        finally:
            if self.acquirefn:
                self.acquirefn()
            self._inherited = False

    def release(self):
        """release the lock and execute callback function if any

        If the lock has been acquired multiple times, the actual release is
        delayed to the last release call."""
        if self.held > 1:
            self.held -= 1
        elif self.held == 1:
            self.held = 0
            if self._getpid() != self.pid:
                # we forked, and are not the parent
                return
            try:
                if self.releasefn:
                    self.releasefn()
            finally:
                if not self._parentheld:
                    try:
                        self.vfs.unlink(self.f)
                    except OSError:
                        pass
            # The postrelease functions typically assume the lock is not held
            # at all.
            if not self._parentheld:
                for callback in self.postrelease:
                    callback()
                # Prevent double usage and help clear cycles.
                self.postrelease = None

def release(*locks):
    for lock in locks:
        if lock is not None:
            lock.release()