view mercurial/templateutil.py @ 39183:f39efa885a6d

revlog: also detect intermediate snapshots

Also detect intermediate-snapshot done against another previous snapshot.

Doing an intermediate snapshot instead of a full one can reduce the number of
full snapshots we need. They are especially useful for content with a lot of
churn on the same line (eg: the manifest) where having a delta over multiple
revisions can end up being significantly smaller than the sum of these
revision deltas.

A revlog built using intermediate snapshots can be a bit smaller and reuse
snapshot much more efficiently. This last property is useful combined with
constraints on chain length. Using intermediate snapshot can produce
repository with delta chain ten times shorter without impact on the storage
size.  Shorter chain lengths are faster to restore, greatly improving read
performance.

This changesets (and the following ones) focus on getting the core principle
of intermediate snapshots into Mercurial core. Later changeset will introduce
the strategy to create them.

# templateutil.py - utility for template evaluation
#
# Copyright 2005, 2006 Matt Mackall <[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 abc
import types

from .i18n import _
from . import (
    error,
    pycompat,
    util,
)
from .utils import (
    dateutil,
    stringutil,
)

class ResourceUnavailable(error.Abort):
    pass

class TemplateNotFound(error.Abort):
    pass

class wrapped(object):
    """Object requiring extra conversion prior to displaying or processing
    as value

    Use unwrapvalue() or unwrapastype() to obtain the inner object.
    """

    __metaclass__ = abc.ABCMeta

    @abc.abstractmethod
    def contains(self, context, mapping, item):
        """Test if the specified item is in self

        The item argument may be a wrapped object.
        """

    @abc.abstractmethod
    def getmember(self, context, mapping, key):
        """Return a member item for the specified key

        The key argument may be a wrapped object.
        A returned object may be either a wrapped object or a pure value
        depending on the self type.
        """

    @abc.abstractmethod
    def getmin(self, context, mapping):
        """Return the smallest item, which may be either a wrapped or a pure
        value depending on the self type"""

    @abc.abstractmethod
    def getmax(self, context, mapping):
        """Return the largest item, which may be either a wrapped or a pure
        value depending on the self type"""

    @abc.abstractmethod
    def filter(self, context, mapping, select):
        """Return new container of the same type which includes only the
        selected elements

        select() takes each item as a wrapped object and returns True/False.
        """

    @abc.abstractmethod
    def itermaps(self, context):
        """Yield each template mapping"""

    @abc.abstractmethod
    def join(self, context, mapping, sep):
        """Join items with the separator; Returns a bytes or (possibly nested)
        generator of bytes

        A pre-configured template may be rendered per item if this container
        holds unprintable items.
        """

    @abc.abstractmethod
    def show(self, context, mapping):
        """Return a bytes or (possibly nested) generator of bytes representing
        the underlying object

        A pre-configured template may be rendered if the underlying object is
        not printable.
        """

    @abc.abstractmethod
    def tobool(self, context, mapping):
        """Return a boolean representation of the inner value"""

    @abc.abstractmethod
    def tovalue(self, context, mapping):
        """Move the inner value object out or create a value representation

        A returned value must be serializable by templaterfilters.json().
        """

class mappable(object):
    """Object which can be converted to a single template mapping"""

    def itermaps(self, context):
        yield self.tomap(context)

    @abc.abstractmethod
    def tomap(self, context):
        """Create a single template mapping representing this"""

class wrappedbytes(wrapped):
    """Wrapper for byte string"""

    def __init__(self, value):
        self._value = value

    def contains(self, context, mapping, item):
        item = stringify(context, mapping, item)
        return item in self._value

    def getmember(self, context, mapping, key):
        raise error.ParseError(_('%r is not a dictionary')
                               % pycompat.bytestr(self._value))

    def getmin(self, context, mapping):
        return self._getby(context, mapping, min)

    def getmax(self, context, mapping):
        return self._getby(context, mapping, max)

    def _getby(self, context, mapping, func):
        if not self._value:
            raise error.ParseError(_('empty string'))
        return func(pycompat.iterbytestr(self._value))

    def filter(self, context, mapping, select):
        raise error.ParseError(_('%r is not filterable')
                               % pycompat.bytestr(self._value))

    def itermaps(self, context):
        raise error.ParseError(_('%r is not iterable of mappings')
                               % pycompat.bytestr(self._value))

    def join(self, context, mapping, sep):
        return joinitems(pycompat.iterbytestr(self._value), sep)

    def show(self, context, mapping):
        return self._value

    def tobool(self, context, mapping):
        return bool(self._value)

    def tovalue(self, context, mapping):
        return self._value

class wrappedvalue(wrapped):
    """Generic wrapper for pure non-list/dict/bytes value"""

    def __init__(self, value):
        self._value = value

    def contains(self, context, mapping, item):
        raise error.ParseError(_("%r is not iterable") % self._value)

    def getmember(self, context, mapping, key):
        raise error.ParseError(_('%r is not a dictionary') % self._value)

    def getmin(self, context, mapping):
        raise error.ParseError(_("%r is not iterable") % self._value)

    def getmax(self, context, mapping):
        raise error.ParseError(_("%r is not iterable") % self._value)

    def filter(self, context, mapping, select):
        raise error.ParseError(_("%r is not iterable") % self._value)

    def itermaps(self, context):
        raise error.ParseError(_('%r is not iterable of mappings')
                               % self._value)

    def join(self, context, mapping, sep):
        raise error.ParseError(_('%r is not iterable') % self._value)

    def show(self, context, mapping):
        if self._value is None:
            return b''
        return pycompat.bytestr(self._value)

    def tobool(self, context, mapping):
        if self._value is None:
            return False
        if isinstance(self._value, bool):
            return self._value
        # otherwise evaluate as string, which means 0 is True
        return bool(pycompat.bytestr(self._value))

    def tovalue(self, context, mapping):
        return self._value

class date(mappable, wrapped):
    """Wrapper for date tuple"""

    def __init__(self, value, showfmt='%d %d'):
        # value may be (float, int), but public interface shouldn't support
        # floating-point timestamp
        self._unixtime, self._tzoffset = map(int, value)
        self._showfmt = showfmt

    def contains(self, context, mapping, item):
        raise error.ParseError(_('date is not iterable'))

    def getmember(self, context, mapping, key):
        raise error.ParseError(_('date is not a dictionary'))

    def getmin(self, context, mapping):
        raise error.ParseError(_('date is not iterable'))

    def getmax(self, context, mapping):
        raise error.ParseError(_('date is not iterable'))

    def filter(self, context, mapping, select):
        raise error.ParseError(_('date is not iterable'))

    def join(self, context, mapping, sep):
        raise error.ParseError(_("date is not iterable"))

    def show(self, context, mapping):
        return self._showfmt % (self._unixtime, self._tzoffset)

    def tomap(self, context):
        return {'unixtime': self._unixtime, 'tzoffset': self._tzoffset}

    def tobool(self, context, mapping):
        return True

    def tovalue(self, context, mapping):
        return (self._unixtime, self._tzoffset)

class hybrid(wrapped):
    """Wrapper for list or dict to support legacy template

    This class allows us to handle both:
    - "{files}" (legacy command-line-specific list hack) and
    - "{files % '{file}\n'}" (hgweb-style with inlining and function support)
    and to access raw values:
    - "{ifcontains(file, files, ...)}", "{ifcontains(key, extras, ...)}"
    - "{get(extras, key)}"
    - "{files|json}"
    """

    def __init__(self, gen, values, makemap, joinfmt, keytype=None):
        self._gen = gen  # generator or function returning generator
        self._values = values
        self._makemap = makemap
        self._joinfmt = joinfmt
        self._keytype = keytype  # hint for 'x in y' where type(x) is unresolved

    def contains(self, context, mapping, item):
        item = unwrapastype(context, mapping, item, self._keytype)
        return item in self._values

    def getmember(self, context, mapping, key):
        # TODO: maybe split hybrid list/dict types?
        if not util.safehasattr(self._values, 'get'):
            raise error.ParseError(_('not a dictionary'))
        key = unwrapastype(context, mapping, key, self._keytype)
        return self._wrapvalue(key, self._values.get(key))

    def getmin(self, context, mapping):
        return self._getby(context, mapping, min)

    def getmax(self, context, mapping):
        return self._getby(context, mapping, max)

    def _getby(self, context, mapping, func):
        if not self._values:
            raise error.ParseError(_('empty sequence'))
        val = func(self._values)
        return self._wrapvalue(val, val)

    def _wrapvalue(self, key, val):
        if val is None:
            return
        if util.safehasattr(val, '_makemap'):
            # a nested hybrid list/dict, which has its own way of map operation
            return val
        return hybriditem(None, key, val, self._makemap)

    def filter(self, context, mapping, select):
        if util.safehasattr(self._values, 'get'):
            values = {k: v for k, v in self._values.iteritems()
                      if select(self._wrapvalue(k, v))}
        else:
            values = [v for v in self._values if select(self._wrapvalue(v, v))]
        return hybrid(None, values, self._makemap, self._joinfmt, self._keytype)

    def itermaps(self, context):
        makemap = self._makemap
        for x in self._values:
            yield makemap(x)

    def join(self, context, mapping, sep):
        # TODO: switch gen to (context, mapping) API?
        return joinitems((self._joinfmt(x) for x in self._values), sep)

    def show(self, context, mapping):
        # TODO: switch gen to (context, mapping) API?
        gen = self._gen
        if gen is None:
            return self.join(context, mapping, ' ')
        if callable(gen):
            return gen()
        return gen

    def tobool(self, context, mapping):
        return bool(self._values)

    def tovalue(self, context, mapping):
        # TODO: make it non-recursive for trivial lists/dicts
        xs = self._values
        if util.safehasattr(xs, 'get'):
            return {k: unwrapvalue(context, mapping, v)
                    for k, v in xs.iteritems()}
        return [unwrapvalue(context, mapping, x) for x in xs]

class hybriditem(mappable, wrapped):
    """Wrapper for non-list/dict object to support map operation

    This class allows us to handle both:
    - "{manifest}"
    - "{manifest % '{rev}:{node}'}"
    - "{manifest.rev}"
    """

    def __init__(self, gen, key, value, makemap):
        self._gen = gen  # generator or function returning generator
        self._key = key
        self._value = value  # may be generator of strings
        self._makemap = makemap

    def tomap(self, context):
        return self._makemap(self._key)

    def contains(self, context, mapping, item):
        w = makewrapped(context, mapping, self._value)
        return w.contains(context, mapping, item)

    def getmember(self, context, mapping, key):
        w = makewrapped(context, mapping, self._value)
        return w.getmember(context, mapping, key)

    def getmin(self, context, mapping):
        w = makewrapped(context, mapping, self._value)
        return w.getmin(context, mapping)

    def getmax(self, context, mapping):
        w = makewrapped(context, mapping, self._value)
        return w.getmax(context, mapping)

    def filter(self, context, mapping, select):
        w = makewrapped(context, mapping, self._value)
        return w.filter(context, mapping, select)

    def join(self, context, mapping, sep):
        w = makewrapped(context, mapping, self._value)
        return w.join(context, mapping, sep)

    def show(self, context, mapping):
        # TODO: switch gen to (context, mapping) API?
        gen = self._gen
        if gen is None:
            return pycompat.bytestr(self._value)
        if callable(gen):
            return gen()
        return gen

    def tobool(self, context, mapping):
        w = makewrapped(context, mapping, self._value)
        return w.tobool(context, mapping)

    def tovalue(self, context, mapping):
        return _unthunk(context, mapping, self._value)

class _mappingsequence(wrapped):
    """Wrapper for sequence of template mappings

    This represents an inner template structure (i.e. a list of dicts),
    which can also be rendered by the specified named/literal template.

    Template mappings may be nested.
    """

    def __init__(self, name=None, tmpl=None, sep=''):
        if name is not None and tmpl is not None:
            raise error.ProgrammingError('name and tmpl are mutually exclusive')
        self._name = name
        self._tmpl = tmpl
        self._defaultsep = sep

    def contains(self, context, mapping, item):
        raise error.ParseError(_('not comparable'))

    def getmember(self, context, mapping, key):
        raise error.ParseError(_('not a dictionary'))

    def getmin(self, context, mapping):
        raise error.ParseError(_('not comparable'))

    def getmax(self, context, mapping):
        raise error.ParseError(_('not comparable'))

    def filter(self, context, mapping, select):
        # implement if necessary; we'll need a wrapped type for a mapping dict
        raise error.ParseError(_('not filterable without template'))

    def join(self, context, mapping, sep):
        mapsiter = _iteroverlaymaps(context, mapping, self.itermaps(context))
        if self._name:
            itemiter = (context.process(self._name, m) for m in mapsiter)
        elif self._tmpl:
            itemiter = (context.expand(self._tmpl, m) for m in mapsiter)
        else:
            raise error.ParseError(_('not displayable without template'))
        return joinitems(itemiter, sep)

    def show(self, context, mapping):
        return self.join(context, mapping, self._defaultsep)

    def tovalue(self, context, mapping):
        knownres = context.knownresourcekeys()
        items = []
        for nm in self.itermaps(context):
            # drop internal resources (recursively) which shouldn't be displayed
            lm = context.overlaymap(mapping, nm)
            items.append({k: unwrapvalue(context, lm, v)
                          for k, v in nm.iteritems() if k not in knownres})
        return items

class mappinggenerator(_mappingsequence):
    """Wrapper for generator of template mappings

    The function ``make(context, *args)`` should return a generator of
    mapping dicts.
    """

    def __init__(self, make, args=(), name=None, tmpl=None, sep=''):
        super(mappinggenerator, self).__init__(name, tmpl, sep)
        self._make = make
        self._args = args

    def itermaps(self, context):
        return self._make(context, *self._args)

    def tobool(self, context, mapping):
        return _nonempty(self.itermaps(context))

class mappinglist(_mappingsequence):
    """Wrapper for list of template mappings"""

    def __init__(self, mappings, name=None, tmpl=None, sep=''):
        super(mappinglist, self).__init__(name, tmpl, sep)
        self._mappings = mappings

    def itermaps(self, context):
        return iter(self._mappings)

    def tobool(self, context, mapping):
        return bool(self._mappings)

class mappedgenerator(wrapped):
    """Wrapper for generator of strings which acts as a list

    The function ``make(context, *args)`` should return a generator of
    byte strings, or a generator of (possibly nested) generators of byte
    strings (i.e. a generator for a list of byte strings.)
    """

    def __init__(self, make, args=()):
        self._make = make
        self._args = args

    def contains(self, context, mapping, item):
        item = stringify(context, mapping, item)
        return item in self.tovalue(context, mapping)

    def _gen(self, context):
        return self._make(context, *self._args)

    def getmember(self, context, mapping, key):
        raise error.ParseError(_('not a dictionary'))

    def getmin(self, context, mapping):
        return self._getby(context, mapping, min)

    def getmax(self, context, mapping):
        return self._getby(context, mapping, max)

    def _getby(self, context, mapping, func):
        xs = self.tovalue(context, mapping)
        if not xs:
            raise error.ParseError(_('empty sequence'))
        return func(xs)

    @staticmethod
    def _filteredgen(context, mapping, make, args, select):
        for x in make(context, *args):
            s = stringify(context, mapping, x)
            if select(wrappedbytes(s)):
                yield s

    def filter(self, context, mapping, select):
        args = (mapping, self._make, self._args, select)
        return mappedgenerator(self._filteredgen, args)

    def itermaps(self, context):
        raise error.ParseError(_('list of strings is not mappable'))

    def join(self, context, mapping, sep):
        return joinitems(self._gen(context), sep)

    def show(self, context, mapping):
        return self.join(context, mapping, '')

    def tobool(self, context, mapping):
        return _nonempty(self._gen(context))

    def tovalue(self, context, mapping):
        return [stringify(context, mapping, x) for x in self._gen(context)]

def hybriddict(data, key='key', value='value', fmt=None, gen=None):
    """Wrap data to support both dict-like and string-like operations"""
    prefmt = pycompat.identity
    if fmt is None:
        fmt = '%s=%s'
        prefmt = pycompat.bytestr
    return hybrid(gen, data, lambda k: {key: k, value: data[k]},
                  lambda k: fmt % (prefmt(k), prefmt(data[k])))

def hybridlist(data, name, fmt=None, gen=None):
    """Wrap data to support both list-like and string-like operations"""
    prefmt = pycompat.identity
    if fmt is None:
        fmt = '%s'
        prefmt = pycompat.bytestr
    return hybrid(gen, data, lambda x: {name: x}, lambda x: fmt % prefmt(x))

def compatdict(context, mapping, name, data, key='key', value='value',
               fmt=None, plural=None, separator=' '):
    """Wrap data like hybriddict(), but also supports old-style list template

    This exists for backward compatibility with the old-style template. Use
    hybriddict() for new template keywords.
    """
    c = [{key: k, value: v} for k, v in data.iteritems()]
    f = _showcompatlist(context, mapping, name, c, plural, separator)
    return hybriddict(data, key=key, value=value, fmt=fmt, gen=f)

def compatlist(context, mapping, name, data, element=None, fmt=None,
               plural=None, separator=' '):
    """Wrap data like hybridlist(), but also supports old-style list template

    This exists for backward compatibility with the old-style template. Use
    hybridlist() for new template keywords.
    """
    f = _showcompatlist(context, mapping, name, data, plural, separator)
    return hybridlist(data, name=element or name, fmt=fmt, gen=f)

def _showcompatlist(context, mapping, name, values, plural=None, separator=' '):
    """Return a generator that renders old-style list template

    name is name of key in template map.
    values is list of strings or dicts.
    plural is plural of name, if not simply name + 's'.
    separator is used to join values as a string

    expansion works like this, given name 'foo'.

    if values is empty, expand 'no_foos'.

    if 'foo' not in template map, return values as a string,
    joined by 'separator'.

    expand 'start_foos'.

    for each value, expand 'foo'. if 'last_foo' in template
    map, expand it instead of 'foo' for last key.

    expand 'end_foos'.
    """
    if not plural:
        plural = name + 's'
    if not values:
        noname = 'no_' + plural
        if context.preload(noname):
            yield context.process(noname, mapping)
        return
    if not context.preload(name):
        if isinstance(values[0], bytes):
            yield separator.join(values)
        else:
            for v in values:
                r = dict(v)
                r.update(mapping)
                yield r
        return
    startname = 'start_' + plural
    if context.preload(startname):
        yield context.process(startname, mapping)
    def one(v, tag=name):
        vmapping = {}
        try:
            vmapping.update(v)
        # Python 2 raises ValueError if the type of v is wrong. Python
        # 3 raises TypeError.
        except (AttributeError, TypeError, ValueError):
            try:
                # Python 2 raises ValueError trying to destructure an e.g.
                # bytes. Python 3 raises TypeError.
                for a, b in v:
                    vmapping[a] = b
            except (TypeError, ValueError):
                vmapping[name] = v
        vmapping = context.overlaymap(mapping, vmapping)
        return context.process(tag, vmapping)
    lastname = 'last_' + name
    if context.preload(lastname):
        last = values.pop()
    else:
        last = None
    for v in values:
        yield one(v)
    if last is not None:
        yield one(last, tag=lastname)
    endname = 'end_' + plural
    if context.preload(endname):
        yield context.process(endname, mapping)

def flatten(context, mapping, thing):
    """Yield a single stream from a possibly nested set of iterators"""
    if isinstance(thing, wrapped):
        thing = thing.show(context, mapping)
    if isinstance(thing, bytes):
        yield thing
    elif isinstance(thing, str):
        # We can only hit this on Python 3, and it's here to guard
        # against infinite recursion.
        raise error.ProgrammingError('Mercurial IO including templates is done'
                                     ' with bytes, not strings, got %r' % thing)
    elif thing is None:
        pass
    elif not util.safehasattr(thing, '__iter__'):
        yield pycompat.bytestr(thing)
    else:
        for i in thing:
            if isinstance(i, wrapped):
                i = i.show(context, mapping)
            if isinstance(i, bytes):
                yield i
            elif i is None:
                pass
            elif not util.safehasattr(i, '__iter__'):
                yield pycompat.bytestr(i)
            else:
                for j in flatten(context, mapping, i):
                    yield j

def stringify(context, mapping, thing):
    """Turn values into bytes by converting into text and concatenating them"""
    if isinstance(thing, bytes):
        return thing  # retain localstr to be round-tripped
    return b''.join(flatten(context, mapping, thing))

def findsymbolicname(arg):
    """Find symbolic name for the given compiled expression; returns None
    if nothing found reliably"""
    while True:
        func, data = arg
        if func is runsymbol:
            return data
        elif func is runfilter:
            arg = data[0]
        else:
            return None

def _nonempty(xiter):
    try:
        next(xiter)
        return True
    except StopIteration:
        return False

def _unthunk(context, mapping, thing):
    """Evaluate a lazy byte string into value"""
    if not isinstance(thing, types.GeneratorType):
        return thing
    return stringify(context, mapping, thing)

def evalrawexp(context, mapping, arg):
    """Evaluate given argument as a bare template object which may require
    further processing (such as folding generator of strings)"""
    func, data = arg
    return func(context, mapping, data)

def evalwrapped(context, mapping, arg):
    """Evaluate given argument to wrapped object"""
    thing = evalrawexp(context, mapping, arg)
    return makewrapped(context, mapping, thing)

def makewrapped(context, mapping, thing):
    """Lift object to a wrapped type"""
    if isinstance(thing, wrapped):
        return thing
    thing = _unthunk(context, mapping, thing)
    if isinstance(thing, bytes):
        return wrappedbytes(thing)
    return wrappedvalue(thing)

def evalfuncarg(context, mapping, arg):
    """Evaluate given argument as value type"""
    return unwrapvalue(context, mapping, evalrawexp(context, mapping, arg))

def unwrapvalue(context, mapping, thing):
    """Move the inner value object out of the wrapper"""
    if isinstance(thing, wrapped):
        return thing.tovalue(context, mapping)
    # evalrawexp() may return string, generator of strings or arbitrary object
    # such as date tuple, but filter does not want generator.
    return _unthunk(context, mapping, thing)

def evalboolean(context, mapping, arg):
    """Evaluate given argument as boolean, but also takes boolean literals"""
    func, data = arg
    if func is runsymbol:
        thing = func(context, mapping, data, default=None)
        if thing is None:
            # not a template keyword, takes as a boolean literal
            thing = stringutil.parsebool(data)
    else:
        thing = func(context, mapping, data)
    return makewrapped(context, mapping, thing).tobool(context, mapping)

def evaldate(context, mapping, arg, err=None):
    """Evaluate given argument as a date tuple or a date string; returns
    a (unixtime, offset) tuple"""
    thing = evalrawexp(context, mapping, arg)
    return unwrapdate(context, mapping, thing, err)

def unwrapdate(context, mapping, thing, err=None):
    if isinstance(thing, date):
        return thing.tovalue(context, mapping)
    # TODO: update hgweb to not return bare tuple; then just stringify 'thing'
    thing = unwrapvalue(context, mapping, thing)
    try:
        return dateutil.parsedate(thing)
    except AttributeError:
        raise error.ParseError(err or _('not a date tuple nor a string'))
    except error.ParseError:
        if not err:
            raise
        raise error.ParseError(err)

def evalinteger(context, mapping, arg, err=None):
    thing = evalrawexp(context, mapping, arg)
    return unwrapinteger(context, mapping, thing, err)

def unwrapinteger(context, mapping, thing, err=None):
    thing = unwrapvalue(context, mapping, thing)
    try:
        return int(thing)
    except (TypeError, ValueError):
        raise error.ParseError(err or _('not an integer'))

def evalstring(context, mapping, arg):
    return stringify(context, mapping, evalrawexp(context, mapping, arg))

def evalstringliteral(context, mapping, arg):
    """Evaluate given argument as string template, but returns symbol name
    if it is unknown"""
    func, data = arg
    if func is runsymbol:
        thing = func(context, mapping, data, default=data)
    else:
        thing = func(context, mapping, data)
    return stringify(context, mapping, thing)

_unwrapfuncbytype = {
    None: unwrapvalue,
    bytes: stringify,
    date: unwrapdate,
    int: unwrapinteger,
}

def unwrapastype(context, mapping, thing, typ):
    """Move the inner value object out of the wrapper and coerce its type"""
    try:
        f = _unwrapfuncbytype[typ]
    except KeyError:
        raise error.ProgrammingError('invalid type specified: %r' % typ)
    return f(context, mapping, thing)

def runinteger(context, mapping, data):
    return int(data)

def runstring(context, mapping, data):
    return data

def _recursivesymbolblocker(key):
    def showrecursion(context, mapping):
        raise error.Abort(_("recursive reference '%s' in template") % key)
    showrecursion._requires = ()  # mark as new-style templatekw
    return showrecursion

def runsymbol(context, mapping, key, default=''):
    v = context.symbol(mapping, key)
    if v is None:
        # put poison to cut recursion. we can't move this to parsing phase
        # because "x = {x}" is allowed if "x" is a keyword. (issue4758)
        safemapping = mapping.copy()
        safemapping[key] = _recursivesymbolblocker(key)
        try:
            v = context.process(key, safemapping)
        except TemplateNotFound:
            v = default
    if callable(v) and getattr(v, '_requires', None) is None:
        # old templatekw: expand all keywords and resources
        # (TODO: drop support for old-style functions. 'f._requires = ()'
        #  can be removed.)
        props = {k: context._resources.lookup(context, mapping, k)
                 for k in context._resources.knownkeys()}
        # pass context to _showcompatlist() through templatekw._showlist()
        props['templ'] = context
        props.update(mapping)
        ui = props.get('ui')
        if ui:
            ui.deprecwarn("old-style template keyword '%s'" % key, '4.8')
        return v(**pycompat.strkwargs(props))
    if callable(v):
        # new templatekw
        try:
            return v(context, mapping)
        except ResourceUnavailable:
            # unsupported keyword is mapped to empty just like unknown keyword
            return None
    return v

def runtemplate(context, mapping, template):
    for arg in template:
        yield evalrawexp(context, mapping, arg)

def runfilter(context, mapping, data):
    arg, filt = data
    thing = evalrawexp(context, mapping, arg)
    intype = getattr(filt, '_intype', None)
    try:
        thing = unwrapastype(context, mapping, thing, intype)
        return filt(thing)
    except error.ParseError as e:
        raise error.ParseError(bytes(e), hint=_formatfiltererror(arg, filt))

def _formatfiltererror(arg, filt):
    fn = pycompat.sysbytes(filt.__name__)
    sym = findsymbolicname(arg)
    if not sym:
        return _("incompatible use of template filter '%s'") % fn
    return (_("template filter '%s' is not compatible with keyword '%s'")
            % (fn, sym))

def _iteroverlaymaps(context, origmapping, newmappings):
    """Generate combined mappings from the original mapping and an iterable
    of partial mappings to override the original"""
    for i, nm in enumerate(newmappings):
        lm = context.overlaymap(origmapping, nm)
        lm['index'] = i
        yield lm

def _applymap(context, mapping, d, darg, targ):
    try:
        diter = d.itermaps(context)
    except error.ParseError as err:
        sym = findsymbolicname(darg)
        if not sym:
            raise
        hint = _("keyword '%s' does not support map operation") % sym
        raise error.ParseError(bytes(err), hint=hint)
    for lm in _iteroverlaymaps(context, mapping, diter):
        yield evalrawexp(context, lm, targ)

def runmap(context, mapping, data):
    darg, targ = data
    d = evalwrapped(context, mapping, darg)
    return mappedgenerator(_applymap, args=(mapping, d, darg, targ))

def runmember(context, mapping, data):
    darg, memb = data
    d = evalwrapped(context, mapping, darg)
    if isinstance(d, mappable):
        lm = context.overlaymap(mapping, d.tomap(context))
        return runsymbol(context, lm, memb)
    try:
        return d.getmember(context, mapping, memb)
    except error.ParseError as err:
        sym = findsymbolicname(darg)
        if not sym:
            raise
        hint = _("keyword '%s' does not support member operation") % sym
        raise error.ParseError(bytes(err), hint=hint)

def runnegate(context, mapping, data):
    data = evalinteger(context, mapping, data,
                       _('negation needs an integer argument'))
    return -data

def runarithmetic(context, mapping, data):
    func, left, right = data
    left = evalinteger(context, mapping, left,
                       _('arithmetic only defined on integers'))
    right = evalinteger(context, mapping, right,
                        _('arithmetic only defined on integers'))
    try:
        return func(left, right)
    except ZeroDivisionError:
        raise error.Abort(_('division by zero is not defined'))

def joinitems(itemiter, sep):
    """Join items with the separator; Returns generator of bytes"""
    first = True
    for x in itemiter:
        if first:
            first = False
        elif sep:
            yield sep
        yield x