text: #!/usr/bin/env python
  # encoding: utf-8
  # Thomas Nagy, 2005-2010 (ita)
  
  """
  Task generators
  
  The class :py:class:`waflib.TaskGen.task_gen` encapsulates the creation of task objects (low-level code)
  The instances can have various parameters, but the creation of task nodes (Task.py)
  is always postponed. To achieve this, various methods are called from the method "apply"
  
  
  """
  
  import copy, re, os
  from waflib import Task, Utils, Logs, Errors, ConfigSet, Node
  
  feats = Utils.defaultdict(set)
  """remember the methods declaring features"""
  
  class task_gen(object):
  	"""
  	Instances of this class create :py:class:`waflib.Task.TaskBase` when
  	calling the method :py:meth:`waflib.TaskGen.task_gen.post` from the main thread.
  	A few notes:
  
  	* The methods to call (*self.meths*) can be specified dynamically (removing, adding, ..)
  	* The 'features' are used to add methods to self.meths and then execute them
  	* The attribute 'path' is a node representing the location of the task generator
  	* The tasks created are added to the attribute *tasks*
  	* The attribute 'idx' is a counter of task generators in the same path
  	"""
  
  	mappings = {}
  	prec = Utils.defaultdict(list)
  
  	def __init__(self, *k, **kw):
  		"""
  		The task generator objects predefine various attributes (source, target) for possible
  		processing by process_rule (make-like rules) or process_source (extensions, misc methods)
  
  		The tasks are stored on the attribute 'tasks'. They are created by calling methods
  		listed in self.meths *or* referenced in the attribute features
  		A topological sort is performed to ease the method re-use.
  
  		The extra key/value elements passed in kw are set as attributes
  		"""
  
  		# so we will have to play with directed acyclic graphs
  		# detect cycles, etc
  		self.source = ''
  		self.target = ''
  
  		self.meths = []
  		"""
  		List of method names to execute (it is usually a good idea to avoid touching this)
  		"""
  
  		self.prec = Utils.defaultdict(list)
  		"""
  		Precedence table for sorting the methods in self.meths
  		"""
  
  		self.mappings = {}
  		"""
  		List of mappings {extension -> function} for processing files by extension
  		"""
  
  		self.features = []
  		"""
  		List of feature names for bringing new methods in
  		"""
  
  		self.tasks = []
  		"""
  		List of tasks created.
  		"""
  
  		if not 'bld' in kw:
  			# task generators without a build context :-/
  			self.env = ConfigSet.ConfigSet()
  			self.idx = 0
  			self.path = None
  		else:
  			self.bld = kw['bld']
  			self.env = self.bld.env.derive()
  			self.path = self.bld.path # emulate chdir when reading scripts
  
  			# provide a unique id
  			try:
  				self.idx = self.bld.idx[id(self.path)] = self.bld.idx.get(id(self.path), 0) + 1
  			except AttributeError:
  				self.bld.idx = {}
  				self.idx = self.bld.idx[id(self.path)] = 1
  
  		for key, val in kw.items():
  			setattr(self, key, val)
  
  	def __str__(self):
  		"""for debugging purposes"""
  		return "<task_gen %r declared in %s>" % (self.name, self.path.abspath())
  
  	def __repr__(self):
  		"""for debugging purposes"""
  		lst = []
  		for x in self.__dict__.keys():
  			if x not in ['env', 'bld', 'compiled_tasks', 'tasks']:
  				lst.append("%s=%s" % (x, repr(getattr(self, x))))
  		return "bld(%s) in %s" % (", ".join(lst), self.path.abspath())
  
  	def get_name(self):
  		"""
  		If not set, the name is computed from the target name::
  
  			def build(bld):
  				x = bld(name='foo')
  				x.get_name() # foo
  				y = bld(target='bar')
  				y.get_name() # bar
  
  		:rtype: string
  		:return: name of this task generator
  		"""
  		try:
  			return self._name
  		except AttributeError:
  			if isinstance(self.target, list):
  				lst = [str(x) for x in self.target]
  				name = self._name = ','.join(lst)
  			else:
  				name = self._name = str(self.target)
  			return name
  	def set_name(self, name):
  		self._name = name
  
  	name = property(get_name, set_name)
  
  	def to_list(self, val):
  		"""
  		Ensure that a parameter is a list
  
  		:type val: string or list of string
  		:param val: input to return as a list
  		:rtype: list
  		"""
  		if isinstance(val, str): return val.split()
  		else: return val
  
  	def post(self):
  		"""
  		Create task objects. The following operations are performed:
  
  		#. The body of this method is called only once and sets the attribute ``posted``
  		#. The attribute ``features`` is used to add more methods in ``self.meths``
  		#. The methods are sorted by the precedence table ``self.prec`` or `:waflib:attr:waflib.TaskGen.task_gen.prec`
  		#. The methods are then executed in order
  		#. The tasks created are added to :py:attr:`waflib.TaskGen.task_gen.tasks`
  		"""
  
  		# we could add a decorator to let the task run once, but then python 2.3 will be difficult to support
  		if getattr(self, 'posted', None):
  			#error("OBJECT ALREADY POSTED" + str( self))
  			return False
  		self.posted = True
  
  		keys = set(self.meths)
  
  		# add the methods listed in the features
  		self.features = Utils.to_list(self.features)
  		for x in self.features + ['*']:
  			st = feats[x]
  			if not st:
  				if not x in Task.classes:
  					Logs.warn('feature %r does not exist - bind at least one method to it' % x)
  			keys.update(list(st)) # ironpython 2.7 wants the cast to list
  
  		# copy the precedence table
  		prec = {}
  		prec_tbl = self.prec or task_gen.prec
  		for x in prec_tbl:
  			if x in keys:
  				prec[x] = prec_tbl[x]
  
  		# elements disconnected
  		tmp = []
  		for a in keys:
  			for x in prec.values():
  				if a in x: break
  			else:
  				tmp.append(a)
  
  		tmp.sort()
  
  		# topological sort
  		out = []
  		while tmp:
  			e = tmp.pop()
  			if e in keys: out.append(e)
  			try:
  				nlst = prec[e]
  			except KeyError:
  				pass
  			else:
  				del prec[e]
  				for x in nlst:
  					for y in prec:
  						if x in prec[y]:
  							break
  					else:
  						tmp.append(x)
  
  		if prec:
  			raise Errors.WafError('Cycle detected in the method execution %r' % prec)
  		out.reverse()
  		self.meths = out
  
  		# then we run the methods in order
  		Logs.debug('task_gen: posting %s %d' % (self, id(self)))
  		for x in out:
  			try:
  				v = getattr(self, x)
  			except AttributeError:
  				raise Errors.WafError('%r is not a valid task generator method' % x)
  			Logs.debug('task_gen: -> %s (%d)' % (x, id(self)))
  			v()
  
  		Logs.debug('task_gen: posted %s' % self.name)
  		return True
  
  	def get_hook(self, node):
  		"""
  		:param node: Input file to process
  		:type node: :py:class:`waflib.Tools.Node.Node`
  		:return: A method able to process the input node by looking at the extension
  		:rtype: function
  		"""
  		name = node.name
  		for k in self.mappings:
  			if name.endswith(k):
  				return self.mappings[k]
  		for k in task_gen.mappings:
  			if name.endswith(k):
  				return task_gen.mappings[k]
  		raise Errors.WafError("File %r has no mapping in %r (did you forget to load a waf tool?)" % (node, task_gen.mappings.keys()))
  
  	def create_task(self, name, src=None, tgt=None):
  		"""
  		Wrapper for creating task instances. The classes are retrieved from the
  		context class if possible, then from the global dict Task.classes.
  
  		:param name: task class name
  		:type name: string
  		:param src: input nodes
  		:type src: list of :py:class:`waflib.Tools.Node.Node`
  		:param tgt: output nodes
  		:type tgt: list of :py:class:`waflib.Tools.Node.Node`
  		:return: A task object
  		:rtype: :py:class:`waflib.Task.TaskBase`
  		"""
  		task = Task.classes[name](env=self.env.derive(), generator=self)
  		if src:
  			task.set_inputs(src)
  		if tgt:
  			task.set_outputs(tgt)
  		self.tasks.append(task)
  		return task
  
  	def clone(self, env):
  		"""
  		Make a copy of a task generator. Once the copy is made, it is necessary to ensure that the
  		it does not create the same output files as the original, or the same files may
  		be compiled several times.
  
  		:param env: A configuration set
  		:type env: :py:class:`waflib.ConfigSet.ConfigSet`
  		:return: A copy
  		:rtype: :py:class:`waflib.TaskGen.task_gen`
  		"""
  		newobj = self.bld()
  		for x in self.__dict__:
  			if x in ['env', 'bld']:
  				continue
  			elif x in ['path', 'features']:
  				setattr(newobj, x, getattr(self, x))
  			else:
  				setattr(newobj, x, copy.copy(getattr(self, x)))
  
  		newobj.posted = False
  		if isinstance(env, str):
  			newobj.env = self.bld.all_envs[env].derive()
  		else:
  			newobj.env = env.derive()
  
  		return newobj
  
  def declare_chain(name='', rule=None, reentrant=None, color='BLUE',
  	ext_in=[], ext_out=[], before=[], after=[], decider=None, scan=None, install_path=None, shell=False):
  	"""
  	Create a new mapping and a task class for processing files by extension.
  	See Tools/flex.py for an example.
  
  	:param name: name for the task class
  	:type name: string
  	:param rule: function to execute or string to be compiled in a function
  	:type rule: string or function
  	:param reentrant: re-inject the output file in the process (done automatically, set to 0 to disable)
  	:type reentrant: int
  	:param color: color for the task output
  	:type color: string
  	:param ext_in: execute the task only after the files of such extensions are created
  	:type ext_in: list of string
  	:param ext_out: execute the task only before files of such extensions are processed
  	:type ext_out: list of string
  	:param before: execute instances of this task before classes of the given names
  	:type before: list of string
  	:param after: execute instances of this task after classes of the given names
  	:type after: list of string
  	:param decider: if present, use it to create the output nodes for the task
  	:type decider: function
  	:param scan: scanner function for the task
  	:type scan: function
  	:param install_path: installation path for the output nodes
  	:type install_path: string
  	"""
  	ext_in = Utils.to_list(ext_in)
  	ext_out = Utils.to_list(ext_out)
  	if not name:
  		name = rule
  	cls = Task.task_factory(name, rule, color=color, ext_in=ext_in, ext_out=ext_out, before=before, after=after, scan=scan, shell=shell)
  
  	def x_file(self, node):
  		ext = decider and decider(self, node) or cls.ext_out
  		if ext_in:
  			_ext_in = ext_in[0]
  
  		tsk = self.create_task(name, node)
  		cnt = 0
  
  		keys = list(self.mappings.keys()) + list(self.__class__.mappings.keys())
  		for x in ext:
  			k = node.change_ext(x, ext_in=_ext_in)
  			tsk.outputs.append(k)
  
  			if reentrant != None:
  				if cnt < int(reentrant):
  					self.source.append(k)
  			else:
  				for y in keys: # ~ nfile * nextensions :-/
  					if k.name.endswith(y):
  						self.source.append(k)
  						break
  			cnt += 1
  
  		if install_path:
  			self.bld.install_files(install_path, tsk.outputs)
  		return tsk
  
  	for x in cls.ext_in:
  		task_gen.mappings[x] = x_file
  	return x_file
  
  def taskgen_method(func):
  	"""
  	Decorator: register a method as a task generator method.
  	The function must accept a task generator as first parameter::
  
  		from waflib.TaskGen import taskgen_method
  		@taskgen_method
  		def mymethod(self):
  			pass
  
  	:param func: task generator method to add
  	:type func: function
  	:rtype: function
  	"""
  	setattr(task_gen, func.__name__, func)
  	return func
  
  def feature(*k):
  	"""
  	Decorator: register a task generator method that will be executed when the
  	object attribute 'feature' contains the corresponding key(s)::
  
  		from waflib.Task import feature
  		@feature('myfeature')
  		def myfunction(self):
  			print('that is my feature!')
  		def build(bld):
  			bld(features='myfeature')
  
  	:param k: feature names
  	:type k: list of string
  	"""
  	def deco(func):
  		setattr(task_gen, func.__name__, func)
  		for name in k:
  			feats[name].update([func.__name__])
  		return func
  	return deco
  
  def before_method(*k):
  	"""
  	Decorator: register a task generator method which will be executed
  	before the functions of given name(s)::
  
  		from waflib.TaskGen import feature, before
  		@feature('myfeature')
  		@before_method('fun2')
  		def fun1(self):
  			print('feature 1!')
  		@feature('myfeature')
  		def fun2(self):
  			print('feature 2!')
  		def build(bld):
  			bld(features='myfeature')
  
  	:param k: method names
  	:type k: list of string
  	"""
  	def deco(func):
  		setattr(task_gen, func.__name__, func)
  		for fun_name in k:
  			if not func.__name__ in task_gen.prec[fun_name]:
  				task_gen.prec[fun_name].append(func.__name__)
  				#task_gen.prec[fun_name].sort()
  		return func
  	return deco
  before = before_method
  
  def after_method(*k):
  	"""
  	Decorator: register a task generator method which will be executed
  	after the functions of given name(s)::
  
  		from waflib.TaskGen import feature, after
  		@feature('myfeature')
  		@after_method('fun2')
  		def fun1(self):
  			print('feature 1!')
  		@feature('myfeature')
  		def fun2(self):
  			print('feature 2!')
  		def build(bld):
  			bld(features='myfeature')
  
  	:param k: method names
  	:type k: list of string
  	"""
  	def deco(func):
  		setattr(task_gen, func.__name__, func)
  		for fun_name in k:
  			if not fun_name in task_gen.prec[func.__name__]:
  				task_gen.prec[func.__name__].append(fun_name)
  				#task_gen.prec[func.__name__].sort()
  		return func
  	return deco
  after = after_method
  
  def extension(*k):
  	"""
  	Decorator: register a task generator method which will be invoked during
  	the processing of source files for the extension given::
  
  		from waflib import Task
  		class mytask(Task):
  			run_str = 'cp ${SRC} ${TGT}'
  		@extension('.moo')
  		def create_maa_file(self, node):
  			self.create_task('mytask', node, node.change_ext('.maa'))
  		def build(bld):
  			bld(source='foo.moo')
  	"""
  	def deco(func):
  		setattr(task_gen, func.__name__, func)
  		for x in k:
  			task_gen.mappings[x] = func
  		return func
  	return deco
  
  # ---------------------------------------------------------------
  # The following methods are task generator methods commonly used
  # they are almost examples, the rest of waf core does not depend on them
  
  @taskgen_method
  def to_nodes(self, lst, path=None):
  	"""
  	Convert the input list into a list of nodes.
  	It is used by :py:func:`waflib.TaskGen.process_source` and :py:func:`waflib.TaskGen.process_rule`.
  	It is designed for source files, for folders, see :py:func:`waflib.Tools.ccroot.to_incnodes`:
  
  	:param lst: input list
  	:type lst: list of string and nodes
  	:param path: path from which to search the nodes (by default, :py:attr:`waflib.TaskGen.task_gen.path`)
  	:type path: :py:class:`waflib.Tools.Node.Node`
  	:rtype: list of :py:class:`waflib.Tools.Node.Node`
  	"""
  	tmp = []
  	path = path or self.path
  	find = path.find_resource
  
  	if isinstance(lst, self.path.__class__):
  		lst = [lst]
  
  	# either a list or a string, convert to a list of nodes
  	for x in Utils.to_list(lst):
  		if isinstance(x, str):
  			node = find(x)
  		else:
  			node = x
  		if not node:
  			raise Errors.WafError("source not found: %r in %r" % (x, self))
  		tmp.append(node)
  	return tmp
  
  @feature('*')
  def process_source(self):
  	"""
  	Process each element in the attribute ``source`` by extension.
  
  	#. The *source* list is converted through :py:meth:`waflib.TaskGen.to_nodes` to a list of :py:class:`waflib.Node.Node` first.
  	#. File extensions are mapped to methods having the signature: ``def meth(self, node)`` by :py:meth:`waflib.TaskGen.extension`
  	#. The method is retrieved through :py:meth:`waflib.TaskGen.task_gen.get_hook`
  	#. When called, the methods may modify self.source to append more source to process
  	#. The mappings can map an extension or a filename (see the code below)
  	"""
  	self.source = self.to_nodes(getattr(self, 'source', []))
  	for node in self.source:
  		self.get_hook(node)(self, node)
  
  @feature('*')
  @before_method('process_source')
  def process_rule(self):
  	"""
  	Process the attribute ``rule``. When present, :py:meth:`waflib.TaskGen.process_source` is disabled::
  
  		def build(bld):
  			bld(rule='cp ${SRC} ${TGT}', source='wscript', target='bar.txt')
  	"""
  	if not getattr(self, 'rule', None):
  		return
  
  	# create the task class
  	name = str(getattr(self, 'name', None) or self.target or getattr(self.rule, '__name__', self.rule))
  
  	# or we can put the class in a cache for performance reasons
  	try:
  		cache = self.bld.cache_rule_attr
  	except AttributeError:
  		cache = self.bld.cache_rule_attr = {}
  
  	cls = None
  	if getattr(self, 'cache_rule', 'True'):
  		try:
  			cls = cache[(name, self.rule)]
  		except KeyError:
  			pass
  	if not cls:
  		cls = Task.task_factory(name, self.rule,
  			getattr(self, 'vars', []),
  			shell=getattr(self, 'shell', True), color=getattr(self, 'color', 'BLUE'),
  			scan = getattr(self, 'scan', None))
  		if getattr(self, 'scan', None):
  			cls.scan = self.scan
  		elif getattr(self, 'deps', None):
  			def scan(self):
  				nodes = []
  				for x in self.generator.to_list(getattr(self.generator, 'deps', None)):
  					node = self.generator.path.find_resource(x)
  					if not node:
  						self.generator.bld.fatal('Could not find %r (was it declared?)' % x)
  					nodes.append(node)
  				return [nodes, []]
  			cls.scan = scan
  
  		if getattr(self, 'update_outputs', None):
  			Task.update_outputs(cls)
  
  		if getattr(self, 'always', None):
  			Task.always_run(cls)
  
  		for x in ['after', 'before', 'ext_in', 'ext_out']:
  			setattr(cls, x, getattr(self, x, []))
  
  		if getattr(self, 'cache_rule', 'True'):
  			cache[(name, self.rule)] = cls
  
  	# now create one instance
  	tsk = self.create_task(name)
  
  	if getattr(self, 'target', None):
  		if isinstance(self.target, str):
  			self.target = self.target.split()
  		if not isinstance(self.target, list):
  			self.target = [self.target]
  		for x in self.target:
  			if isinstance(x, str):
  				tsk.outputs.append(self.path.find_or_declare(x))
  			else:
  				x.parent.mkdir() # if a node was given, create the required folders
  				tsk.outputs.append(x)
  		if getattr(self, 'install_path', None):
  			# from waf 1.5
  			# although convenient, it does not 1. allow to name the target file and 2. symlinks
  			# TODO remove in waf 1.7
  			self.bld.install_files(self.install_path, tsk.outputs)
  
  	if getattr(self, 'source', None):
  		tsk.inputs = self.to_nodes(self.source)
  		# bypass the execution of process_source by setting the source to an empty list
  		self.source = []
  
  	if getattr(self, 'cwd', None):
  		tsk.cwd = self.cwd
  
  @feature('seq')
  def sequence_order(self):
  	"""
  	Add a strict sequential constraint between the tasks generated by task generators.
  	It works because task generators are posted in order.
  	It will not post objects which belong to other folders.
  
  	Example::
  
  		bld(features='javac seq')
  		bld(features='jar seq')
  
  	To start a new sequence, set the attribute seq_start, for example::
  
  		obj = bld(features='seq')
  		obj.seq_start = True
  
  	Note that the method is executed in last position. This is more an
  	example than a widely-used solution.
  	"""
  	if self.meths and self.meths[-1] != 'sequence_order':
  		self.meths.append('sequence_order')
  		return
  
  	if getattr(self, 'seq_start', None):
  		return
  
  	# all the tasks previously declared must be run before these
  	if getattr(self.bld, 'prev', None):
  		self.bld.prev.post()
  		for x in self.bld.prev.tasks:
  			for y in self.tasks:
  				y.set_run_after(x)
  
  	self.bld.prev = self
  
  
  re_m4 = re.compile('@(\w+)@', re.M)
  
  class subst_pc(Task.Task):
  	"""
  	Create *.pc* files from *.pc.in*. The task is executed whenever an input variable used
  	in the substitution changes.
  	"""
  
  	def run(self):
  		"Substitutes variables in a .in file"
  
  		if getattr(self.generator, 'is_copy', None):
  			self.outputs[0].write(self.inputs[0].read('rb'), 'wb')
  			if getattr(self.generator, 'chmod', None):
  				os.chmod(self.outputs[0].abspath(), self.generator.chmod)
  			return None
  
  		if getattr(self.generator, 'fun', None):
  			self.generator.fun(self)
  
  		code = self.inputs[0].read(encoding=getattr(self.generator, 'encoding', 'ISO8859-1'))
  		if getattr(self.generator, 'subst_fun', None):
  			code = self.generator.subst_fun(self, code)
  			if code:
  				self.outputs[0].write(code, encoding=getattr(self.generator, 'encoding', 'ISO8859-1'))
  			return
  
  		# replace all % by %% to prevent errors by % signs
  		code = code.replace('%', '%%')
  
  		# extract the vars foo into lst and replace @foo@ by %(foo)s
  		lst = []
  		def repl(match):
  			g = match.group
  			if g(1):
  				lst.append(g(1))
  				return "%%(%s)s" % g(1)
  			return ''
  		code = re_m4.sub(repl, code)
  
  		try:
  			d = self.generator.dct
  		except AttributeError:
  			d = {}
  			for x in lst:
  				tmp = getattr(self.generator, x, '') or self.env.get_flat(x) or self.env.get_flat(x.upper())
  				d[x] = str(tmp)
  
  		code = code % d
  		self.outputs[0].write(code, encoding=getattr(self.generator, 'encoding', 'ISO8859-1'))
  		self.generator.bld.raw_deps[self.uid()] = self.dep_vars = lst
  
  		# make sure the signature is updated
  		try: delattr(self, 'cache_sig')
  		except AttributeError: pass
  
  		if getattr(self.generator, 'chmod', None):
  			os.chmod(self.outputs[0].abspath(), self.generator.chmod)
  
  	def sig_vars(self):
  		"""
  		Compute a hash (signature) of the variables used in the substitution
  		"""
  		bld = self.generator.bld
  		env = self.env
  		upd = self.m.update
  
  		if getattr(self.generator, 'fun', None):
  			upd(Utils.h_fun(self.generator.fun).encode())
  		if getattr(self.generator, 'subst_fun', None):
  			upd(Utils.h_fun(self.generator.subst_fun).encode())
  
  		# raw_deps: persistent custom values returned by the scanner
  		vars = self.generator.bld.raw_deps.get(self.uid(), [])
  
  		# hash both env vars and task generator attributes
  		act_sig = bld.hash_env_vars(env, vars)
  		upd(act_sig)
  
  		lst = [getattr(self.generator, x, '') for x in vars]
  		upd(Utils.h_list(lst))
  
  		return self.m.digest()
  
  @extension('.pc.in')
  def add_pcfile(self, node):
  	"""
  	Process *.pc.in* files to *.pc*. Install the results to ``${PREFIX}/lib/pkgconfig/``
  
  		def build(bld):
  			bld(source='foo.pc.in', install_path='${LIBDIR}/pkgconfig/')
  	"""
  	tsk = self.create_task('subst_pc', node, node.change_ext('.pc', '.pc.in'))
  	self.bld.install_files(getattr(self, 'install_path', '${LIBDIR}/pkgconfig/'), tsk.outputs)
  
  class subst(subst_pc):
  	pass
  
  @feature('subst')
  @before_method('process_source', 'process_rule')
  def process_subst(self):
  	"""
  	Define a transformation that substitutes the contents of *source* files to *target* files::
  
  		def build(bld):
  			bld(
  				features='subst',
  				source='foo.c.in',
  				target='foo.c',
  				install_path='${LIBDIR}/pkgconfig',
  				VAR = 'val'
  			)
  
  	The input files are supposed to contain macros of the form *@VAR@*, where *VAR* is an argument
  	of the task generator object.
  
  	This method overrides the processing by :py:meth:`waflib.TaskGen.process_source`.
  	"""
  
  	src = Utils.to_list(getattr(self, 'source', []))
  	if isinstance(src, Node.Node):
  		src = [src]
  	tgt = Utils.to_list(getattr(self, 'target', []))
  	if isinstance(tgt, Node.Node):
  		tgt = [tgt]
  	if len(src) != len(tgt):
  		raise Errors.WafError('invalid number of source/target for %r' % self)
  
  	for x, y in zip(src, tgt):
  		if not x or not y:
  			raise Errors.WafError('null source or target for %r' % self)
  		a, b = None, None
  
  		if isinstance(x, str) and isinstance(y, str) and x == y:
  			a = self.path.find_node(x)
  			b = self.path.get_bld().make_node(y)
  			if not os.path.isfile(b.abspath()):
  				b.sig = None
  				b.parent.mkdir()
  		else:
  			if isinstance(x, str):
  				a = self.path.find_resource(x)
  			elif isinstance(x, Node.Node):
  				a = x
  			if isinstance(y, str):
  				b = self.path.find_or_declare(y)
  			elif isinstance(y, Node.Node):
  				b = y
  
  		if not a:
  			raise Errors.WafError('cound not find %r for %r' % (x, self))
  
  		has_constraints = False
  		tsk = self.create_task('subst', a, b)
  		for k in ('after', 'before', 'ext_in', 'ext_out'):
  			val = getattr(self, k, None)
  			if val:
  				has_constraints = True
  				setattr(tsk, k, val)
  
  		# paranoid safety measure for the general case foo.in->foo.h with ambiguous dependencies
  		if not has_constraints and b.name.endswith('.h'):
  			tsk.before = [k for k in ('c', 'cxx') if k in Task.classes]
  
  		inst_to = getattr(self, 'install_path', None)
  		if inst_to:
  			self.bld.install_files(inst_to, b, chmod=getattr(self, 'chmod', Utils.O644))
  
  	self.source = []
  
  