libboost1.46-dev 1.46.1-7ubuntu3 / usr / share / boost-build / build / targets.jam

# Copyright Vladimir Prus 2002.
# Copyright Rene Rivera 2006.
#
# Distributed under the Boost Software License, Version 1.0.
#    (See accompanying file LICENSE_1_0.txt or copy at
#          http://www.boost.org/LICENSE_1_0.txt)

# Supports 'abstract' targets, which are targets explicitly defined in a
# Jamfile.
#
# Abstract targets are represented by classes derived from 'abstract-target'
# class. The first abstract target is 'project-target', which is created for
# each Jamfile, and can be obtained by the 'target' rule in the Jamfile's module
# (see project.jam).
#
# Project targets keep a list of 'main-target' instances. A main target is what
# the user explicitly defines in a Jamfile. It is possible to have several
# definitions for a main target, for example to have different lists of sources
# for different platforms. So, main targets keep a list of alternatives.
#
# Each alternative is an instance of 'abstract-target'. When a main target
# subvariant is defined by some rule, that rule will decide what class to use,
# create an instance of that class and add it to the list of alternatives for
# the main target.
#
# Rules supplied by the build system will use only targets derived from
# 'basic-target' class, which will provide some default behaviour. There will be
# different classes derived from it such as 'make-target', created by the 'make'
# rule, and 'typed-target', created by rules such as 'exe' and 'lib'.

#
#                         +------------------------+
#                         |abstract-target         |
#                         +========================+
#                         |name                    |
#                         |project                 |
#                         |                        |
#                         |generate(properties) = 0|
#                         +-----------+------------+
#                                     |
#                                     ^
#                                    / \
#                                   +-+-+
#                                     |
#                                     |
#            +------------------------+------+------------------------------+
#            |                               |                              |
#            |                               |                              |
# +----------+-----------+            +------+------+                +------+-------+
# | project-target       |            | main-target |                | basic-target |
# +======================+ 1        * +=============+  alternatives  +==============+
# | generate(properties) |o-----------+ generate    |<>------------->| generate     |
# | main-target          |            +-------------+                | construct = 0|
# +----------------------+                                           +--------------+
#                                                                           |
#                                                                           ^
#                                                                          / \
#                                                                         +-+-+
#                                                                           |
#                                                                           |
#                 ...--+----------------+------------------+----------------+---+
#                      |                |                  |                    |
#                      |                |                  |                    |
#               ... ---+-----+   +------+-------+   +------+------+    +--------+-----+
#                            |   | typed-target |   | make-target |    | stage-target |
#                            .   +==============+   +=============+    +==============+
#                            .   | construct    |   | construct   |    | construct    |
#                                +--------------+   +-------------+    +--------------+

import assert ;
import "class" : new ;
import errors ;
import feature ;
import indirect ;
import path ;
import property ;
import property-set ;
import sequence ;
import set ;
import toolset ;
import build-request ;


# Base class for all abstract targets.
#
class abstract-target
{
    import project ;
    import assert ;
    import "class" ;
    import errors ;

    rule __init__ ( name  # Name of the target in Jamfile.
        : project-target  # The project target to which this one belongs.
    )
    {
        # Note: it might seem that we don't need either name or project at all.
        # However, there are places where we really need it. One example is
        # error messages which should name problematic targets. Another is
        # setting correct paths for sources and generated files.

        self.name = $(name) ;
        self.project = $(project-target) ;
        self.location = [ errors.nearest-user-location ] ;
    }

    # Returns the name of this target.
    rule name ( )
    {
        return $(self.name) ;
    }

    # Returns the project for this target.
    rule project ( )
    {
        return $(self.project) ;
    }

    # Return the location where the target was declared.
    rule location ( )
    {
        return $(self.location) ;
    }

    # Returns a user-readable name for this target.
    rule full-name ( )
    {
        local location = [ $(self.project).get location ] ;
        return $(location)/$(self.name) ;
    }

    # Generates virtual targets for this abstract target using the specified
    # properties, unless a different value of some feature is required by the
    # target.
    # On success, returns:
    # - a property-set with the usage requirements to be applied to dependants
    # - a list of produced virtual targets, which may be empty.
    # If 'property-set' is empty, performs the default build of this target, in
    # a way specific to the derived class.
    #
    rule generate ( property-set )
    {
        errors.error "method should be defined in derived classes" ;
    }

    rule rename ( new-name )
    {
        self.name = $(new-name) ;
    }
}


if --debug-building in [ modules.peek : ARGV ]
{
    modules.poke : .debug-building : true ;
}


rule indent ( )
{
    return $(.indent:J="") ;
}


rule increase-indent ( )
{
    .indent += "    " ;
}


rule decrease-indent ( )
{
    .indent = $(.indent[2-]) ;
}


#  Project target class (derived from 'abstract-target').
#
#  This class has the following responsibilities:
#  - Maintaining a list of main targets in this project and building them.
#
#  Main targets are constructed in two stages:
#  - When Jamfile is read, a number of calls to 'add-alternative' is made. At
#    that time, alternatives can also be renamed to account for inline targets.
#  - The first time 'main-target' or 'has-main-target' rule is called, all
#    alternatives are enumerated and main targets are created.
#
class project-target : abstract-target
{
    import project ;
    import targets ;
    import path ;
    import print ;
    import property-set ;
    import set ;
    import sequence ;
    import "class" : new ;
    import errors ;

    rule __init__ ( name : project-module parent-project ?
        : requirements * : default-build * )
    {
        abstract-target.__init__ $(name) : $(__name__) ;

        self.project-module = $(project-module) ;
        self.location = [ project.attribute $(project-module) location ] ;
        self.requirements = $(requirements) ;
        self.default-build = $(default-build) ;

        if $(parent-project)
        {
            inherit $(parent-project) ;
        }
    }

    # This is needed only by the 'make' rule. Need to find the way to make
    # 'make' work without this method.
    #
    rule project-module ( )
    {
        return $(self.project-module) ;
    }

    rule get ( attribute )
    {
        return [ project.attribute $(self.project-module) $(attribute) ] ;
    }

    rule build-dir ( )
    {
        if ! $(self.build-dir)
        {
            self.build-dir = [ get build-dir ] ;
            if ! $(self.build-dir)
            {
                self.build-dir = [ path.join [ $(self.project).get location ]
                    bin ] ;
            }
        }
        return $(self.build-dir) ;
    }

    # Generates all possible targets contained in this project.
    #
    rule generate ( property-set * )
    {
        if [ modules.peek : .debug-building ]
        {
            ECHO [ targets.indent ] "building project" [ name ] " ('$(__name__)') with" [ $(property-set).raw ] ;
            targets.increase-indent ;
        }

        local usage-requirements = [ property-set.empty ] ;
        local targets ;

        for local t in [ targets-to-build ]
        {
            local g = [ $(t).generate $(property-set) ] ;
            usage-requirements = [ $(usage-requirements).add $(g[1]) ] ;
            targets += $(g[2-]) ;
        }
        targets.decrease-indent ;
        return $(usage-requirements) [ sequence.unique $(targets) ] ;
    }

    # Computes and returns a list of abstract-target instances which must be
    # built when this project is built.
    #
    rule targets-to-build ( )
    {
        local result ;

        if ! $(self.built-main-targets)
        {
            build-main-targets ;
        }

        # Collect all main targets here, except for "explicit" ones.
        for local t in $(self.main-targets)
        {
            if ! [ $(t).name ] in $(self.explicit-targets)
            {
                result += $(t) ;
            }
        }

        # Collect all projects referenced via "projects-to-build" attribute.
        local self-location = [ get location ] ;
        for local pn in [ get projects-to-build ]
        {
            result += [ find $(pn)/ ] ;
        }

        return $(result) ;
    }

    # Add 'target' to the list of targets in this project that should be build
    # only by explicit request
    #
    rule mark-target-as-explicit ( target-name * )
    {
        # Record the name of the target, not instance, since this rule is called
        # before main target instances are created.
        self.explicit-targets += $(target-name) ;
    }
    
    rule mark-target-as-always ( target-name * )
    {
        # Record the name of the target, not instance, since this rule is called
        # before main target instances are created.
        self.always-targets += $(target-name) ;
    }

    # Add new target alternative
    #
    rule add-alternative ( target-instance )
    {
        if $(self.built-main-targets)
        {
            errors.error add-alternative called when main targets are already
                created. : in project [ full-name ] ;
        }
        self.alternatives += $(target-instance) ;
    }

    # Returns a 'main-target' class instance corresponding to 'name'.
    #
    rule main-target ( name )
    {
        if ! $(self.built-main-targets)
        {
            build-main-targets ;
        }
        return $(self.main-target.$(name)) ;
    }

    # Returns whether a main target with the specified name exists.
    #
    rule has-main-target ( name )
    {
        if ! $(self.built-main-targets)
        {
            build-main-targets ;
        }

        if $(self.main-target.$(name))
        {
            return true ;
        }
    }

    # Worker function for the find rule not implementing any caching and simply
    # returning nothing in case the target can not be found.
    #
    rule find-really ( id )
    {
        local result ;
        local current-location = [ get location ] ;

        local split = [ MATCH (.*)//(.*) : $(id) ] ;
        local project-part = $(split[1]) ;
        local target-part = $(split[2]) ;

        local extra-error-message ;
        if $(project-part)
        {
            # There is an explicitly specified project part in id. Looks up the
            # project and passes the request to it.
            local pm = [ project.find $(project-part) : $(current-location) ] ;
            if $(pm)
            {
                project-target = [ project.target $(pm) ] ;
                result = [ $(project-target).find $(target-part) : no-error ] ;
            }
            else
            {
                # TODO: This extra error message will not get displayed most
                # likely due to some buggy refactoring. Refactor the code so the
                # message gets diplayed again.
                extra-error-message = error: could not find project
                    '$(project-part)' ;
            }
        }
        else
        {
            # Interpret target-name as name of main target. Need to do this
            # before checking for file. Consider the following scenario with a
            # toolset not modifying its executable's names, e.g. gcc on
            # Unix-like platforms:
            #
            #  exe test : test.cpp ;
            #  install s : test : <location>. ;
            #
            # After the first build we would have a target named 'test' in the
            # Jamfile and a file named 'test' on the disk. We need the target to
            # override the file.
            result = [ main-target $(id) ] ;
            
            # Interpret id as an existing file reference.
            if ! $(result)
            {
                result = [ new file-reference [ path.make $(id) ] :
                    $(self.project) ] ;
                if ! [ $(result).exists ]
                {
                    result = ;
                }
            }

            # Interpret id as project-id.
            if ! $(result)
            {
                local project-module = [ project.find $(id) :
                    $(current-location) ] ;
                if $(project-module)
                {
                    result = [ project.target $(project-module) ] ;
                }
            }
        }

        return $(result) ;
    }

    # Find and return the target with the specified id, treated relative to
    # self. Id may specify either a target or a file name with the target taking
    # priority. May report an error or return nothing if the target is not found
    # depending on the 'no-error' parameter.
    #
    rule find ( id : no-error ? )
    {
        local v = $(.id.$(id)) ;
        if ! $(v)
        {
            v = [ find-really $(id) ] ;
            if ! $(v)
            {
                v = none ;
            }
            .id.$(id) = $(v) ;
        }

        if $(v) != none
        {
            return $(v) ;
        }
        else
        {
            if ! $(no-error)
            {
                local current-location = [ get location ] ;
                ECHO "error: Unable to find file or target named" ;
                ECHO "error:     '$(id)'" ;
                ECHO "error: referred from project at" ;
                ECHO "error:     '$(current-location)'" ;
                ECHO $(extra-error-message) ;
                EXIT ;
            }
        }
    }

    rule build-main-targets ( )
    {
        self.built-main-targets = true ;
        for local a in $(self.alternatives)
        {
            local name = [ $(a).name ] ;
            local target = $(self.main-target.$(name)) ;
            if ! $(target)
            {
                local t = [ new main-target $(name) : $(self.project) ] ;
                self.main-target.$(name) = $(t) ;
                self.main-targets += $(t) ;
                target = $(self.main-target.$(name)) ;
            }
            
            if $(name) in $(self.always-targets)
            {
                $(a).always ;
            }                

            $(target).add-alternative $(a) ;
        }
    }

    # Accessor, add a constant.
    #
    rule add-constant (
        name       # Variable name of the constant.
        : value +  # Value of the constant.
        : type ?   # Optional type of value.
        )
    {
        switch $(type)
        {
            case path :
              local r ;
              for local v in $(value)
              {
                local l = $(self.location) ;
                if ! $(l)
                {
                    # Project corresponding to config files do not have 
                    # 'location' attribute, but do have source location.
                    # It might be more reasonable to make every project have
                    # a location and use some other approach to prevent buildable
                    # targets in config files, but that's for later.
                    l = [ get source-location ] ;
                }                
                v = [ path.root [ path.make $(v) ] $(l) ] ;
                # Now make the value absolute path.
                v = [ path.root $(v) [ path.pwd ] ] ;
                # Constants should be in platform-native form.
                v = [ path.native $(v) ] ;
                r += $(v) ;
              }
              value = $(r) ;
        }
        if ! $(name) in $(self.constants)
        {
            self.constants += $(name) ;
        }
        self.constant.$(name) = $(value) ;
        # Inject the constant in the scope of the Jamroot module.
        modules.poke $(self.project-module) : $(name) : $(value) ;
    }

    rule inherit ( parent )
    {
        for local c in [ modules.peek $(parent) : self.constants ]
        {
            # No need to pass the type. Path constants were converted to
            # absolute paths already by parent.
            add-constant $(c)
              : [ modules.peek $(parent) : self.constant.$(c) ] ;
        }

        # Import rules from parent.
        local this-module = [ project-module ] ;
        local parent-module = [ $(parent).project-module ] ;
        # Do not import rules coming from 'project-rules' as they must be
        # imported localized.
        local user-rules = [ set.difference
            [ RULENAMES $(parent-module) ] :
            [ RULENAMES project-rules ] ] ;
        IMPORT $(parent-module) : $(user-rules) : $(this-module) : $(user-rules) ;
        EXPORT $(this-module) : $(user-rules) ;
    }
}


# Helper rules to detect cycles in main target references.
#
local rule start-building ( main-target-instance )
{
    if $(main-target-instance) in $(.targets-being-built)
    {
        local names ;
        for local t in $(.targets-being-built) $(main-target-instance)
        {
            names += [ $(t).full-name ] ;
        }

        errors.error "Recursion in main target references"
          : "the following target are being built currently:"
          : $(names) ;
    }
    .targets-being-built += $(main-target-instance) ;
}


local rule end-building ( main-target-instance )
{
    .targets-being-built = $(.targets-being-built[1--2]) ;
}


# A named top-level target in Jamfile.
#
class main-target : abstract-target
{
    import assert ;
    import errors ;
    import feature ;
    import print ;
    import property-set ;
    import sequence ;
    import targets : start-building end-building ;

    rule __init__ ( name : project )
    {
        abstract-target.__init__ $(name) : $(project) ;
    }

    # Add a new alternative for this target
    rule add-alternative ( target )
    {
        local d = [ $(target).default-build ] ;
        if $(self.alternatives) && ( $(self.default-build) != $(d) )
        {
            errors.error "default build must be identical in all alternatives"
              : "main target is" [ full-name ]
              : "with" [ $(d).raw ]
              : "differing from previous default build" [ $(self.default-build).raw ] ;
        }
        else
        {
            self.default-build = $(d) ;
        }
        self.alternatives += $(target) ;
    }

    # Returns the best viable alternative for this property-set. See the
    # documentation for selection rules.
    #
    local rule select-alternatives ( property-set debug ? )
    {
        # When selecting alternatives we have to consider defaults, for example:
        #    lib l : l.cpp : <variant>debug ;
        #    lib l : l_opt.cpp : <variant>release ;
        # won't work unless we add default value <variant>debug.
        property-set = [ $(p).add-defaults ] ;

        # The algorithm: we keep the current best viable alternative. When we've
        # got a new best viable alternative, we compare it with the current one.

        local best ;
        local best-properties ;

        if $(self.alternatives[2-])
        {
            local bad ;
            local worklist = $(self.alternatives) ;
            while $(worklist) && ! $(bad)
            {
                local v = $(worklist[1]) ;
                local properties = [ $(v).match $(property-set) $(debug) ] ;

                if $(properties) != no-match
                {
                    if ! $(best)
                    {
                        best = $(v) ;
                        best-properties = $(properties) ;
                    }
                    else
                    {
                        if $(properties) = $(best-properties)
                        {
                            bad = true ;
                        }
                        else if $(properties) in $(best-properties)
                        {
                            # Do nothing, this alternative is worse
                        }
                        else if $(best-properties) in $(properties)
                        {
                            best = $(v) ;
                            best-properties = $(properties) ;
                        }
                        else
                        {
                            bad = true ;
                        }
                    }
                }
                worklist = $(worklist[2-]) ;
            }
            if ! $(bad)
            {
                return $(best) ;
            }
        }
        else
        {
            return $(self.alternatives) ;
        }
    }

    rule apply-default-build ( property-set )
    {
        return [ targets.apply-default-build $(property-set) 
          : $(self.default-build) ] ;
    }

    # Select an alternative for this main target, by finding all alternatives
    # which requirements are satisfied by 'properties' and picking the one with
    # the longest requirements set. Returns the result of calling 'generate' on
    # that alternative.
    #
    rule generate ( property-set )
    {
        start-building $(__name__) ;

        # We want composite properties in build request act as if all the
        # properties it expands too are explicitly specified.
        property-set = [ $(property-set).expand ] ;

        local all-property-sets = [ apply-default-build $(property-set) ] ;
        local usage-requirements = [ property-set.empty ] ;
        local result ;
        for local p in $(all-property-sets)
        {
            local r = [ generate-really $(p) ] ;
            if $(r)
            {
                usage-requirements = [ $(usage-requirements).add $(r[1]) ] ;
                result += $(r[2-]) ;
            }
        }
        end-building $(__name__) ;
        return $(usage-requirements) [ sequence.unique $(result) ] ;
    }

    # Generates the main target with the given property set and returns a list
    # which first element is property-set object containing usage-requirements
    # of generated target and with generated virtual target in other elements.
    # It is possible that no targets are generated.
    #
    local rule generate-really ( property-set )
    {
        local best-alternatives = [ select-alternatives $(property-set) ] ;
        if ! $(best-alternatives)
        {
            ECHO "error: No best alternative for" [ full-name ] ;
            select-alternatives $(property-set) debug ;
            return [ property-set.empty ] ;
        }
        else
        {
            # Now return virtual targets for the only alternative.
            return [ $(best-alternatives).generate $(property-set) ] ;
        }
    }

    rule rename ( new-name )
    {
        abstract-target.rename $(new-name) ;
        for local a in $(self.alternatives)
        {
            $(a).rename $(new-name) ;
        }
    }
}


# Abstract target refering to a source file. This is an artificial entity
# allowing sources to a target to be represented using a list of abstract target
# instances.
#
class file-reference : abstract-target
{
    import virtual-target ;
    import property-set ;
    import path ;

    rule __init__ ( file : project )
    {
        abstract-target.__init__ $(file) : $(project) ;
    }

    rule generate ( properties )
    {
        return [ property-set.empty ] [ virtual-target.from-file $(self.name) :
            [ location ] : $(self.project) ] ;
    }

    # Returns true if the referred file really exists.
    rule exists ( )
    {
        location ;
        return $(self.file-path) ;
    }

    # Returns the location of target. Needed by 'testing.jam'.
    rule location ( )
    {
        if ! $(self.file-location)
        {
            local source-location = [ $(self.project).get source-location ] ;
            for local src-dir in $(source-location)
            {
                if ! $(self.file-location)
                {
                    local location = [ path.root $(self.name) $(src-dir) ] ;
                    if [ CHECK_IF_FILE [ path.native $(location) ] ]
                    {
                         self.file-location = $(src-dir) ;
                         self.file-path = $(location) ;
                    }
                }
            }
        }
        return $(self.file-location) ;
    }
}


# Given a target-reference, made in context of 'project', returns the
# abstract-target instance that is referred to, as well as properties explicitly
# specified for this reference.
#
rule resolve-reference ( target-reference : project )
{
    # Separate target name from properties override.
    local split = [ MATCH "^([^<]*)(/(<.*))?$" : $(target-reference) ] ;
    local id = $(split[1]) ;
    local sproperties = ;
    if $(split[3])
    {
        sproperties = [ property.make [ feature.split $(split[3]) ] ] ;
        sproperties = [ feature.expand-composites $(sproperties) ] ;
    }

    # Find the target.
    local target = [ $(project).find $(id) ] ;

    return $(target) [ property-set.create $(sproperties) ] ;
}


# Attempts to generate the target given by target reference, which can refer
# both to a main target or to a file. Returns a list consisting of
# - usage requirements
# - generated virtual targets, if any
#
rule generate-from-reference (
    target-reference  # Target reference.
    : project         # Project where the reference is made.
    : property-set    # Properties of the main target that makes the reference.
)
{
    local r = [ resolve-reference $(target-reference) : $(project) ] ;
    local target = $(r[1]) ;
    local sproperties = $(r[2]) ;

    # Take properties which should be propagated and refine them with
    # source-specific requirements.
    local propagated = [ $(property-set).propagated ] ;
    local rproperties = [ $(propagated).refine $(sproperties) ] ;
    if $(rproperties[1]) = "@error"
    {
        errors.error
            "When building" [ full-name ] " with properties " $(properties) :
            "Invalid properties specified for " $(source) ":"
            $(rproperties[2-]) ;
    }
    return [ $(target).generate $(rproperties) ] ;
}

rule apply-default-build ( property-set : default-build )
{
    # 1. First, see what properties from default-build are already present
    # in property-set.

    local raw = [ $(property-set).raw ] ;
    local specified-features = $(raw:G) ;

    local defaults-to-apply ;
    for local d in [ $(default-build).raw ]
    {
        if ! $(d:G) in $(specified-features)
        {
            defaults-to-apply += $(d) ;
        }
    }

    # 2. If there are any defaults to be applied, form a new build request.
    # Pass it through to 'expand-no-defaults' since default-build might
    # contain "release debug" resulting in two property-sets.
    local result ;
    if $(defaults-to-apply)
    {
        properties = [
          build-request.expand-no-defaults

            # We have to compress subproperties here to prevent property
            # lists like:
            #
            #    <toolset>msvc <toolset-msvc:version>7.1 <threading>multi
            #
            # from being expanded into:
            #
            #    <toolset-msvc:version>7.1/<threading>multi
            #    <toolset>msvc/<toolset-msvc:version>7.1/<threading>multi
            #
            # due to a cross-product property combination. That may be an
            # indication that build-request.expand-no-defaults is the wrong
            # rule to use here.
            [ feature.compress-subproperties $(raw) ]
              $(defaults-to-apply)
        ] ;

        if $(properties)
        {
            for local p in $(properties)
            {
                result += [ property-set.create
                    [ feature.expand [ feature.split $(p) ] ] ] ;
            }
        }
        else
        {
            result = [ property-set.empty ] ;
        }
    }
    else
    {
        result = $(property-set) ;
    }
    return $(result) ;
}


# Given a build request and requirements, return properties common to dependency
# build request and target requirements.
#
# TODO: Document exactly what 'common properties' are, whether they should
# include default property values, whether they should contain any conditional
# properties or should those be already processed, etc. See whether there are
# any differences between use cases with empty and non-empty build-request as
# well as with requirements containing and those not containing any non-free
# features.
#
rule common-properties ( build-request requirements )
{
    # For optimization, we add free requirements directly, without using a
    # complex algorithm. This gives the complex algorithm a better chance of
    # caching results.
    local free = [ $(requirements).free ] ;
    local non-free = [ property-set.create [ $(requirements).base ]
        [ $(requirements).incidental ] ] ;

    local key = .rp.$(build-request)-$(non-free) ;
    if ! $($(key))
    {
        $(key) = [ common-properties2 $(build-request) $(non-free) ] ;
    }
    result = [ $($(key)).add-raw $(free) ] ;
}


# Given a 'context' -- a set of already present properties, and 'requirements',
# decide which extra properties should be applied to 'context'. For conditional
# requirements, this means evaluating the condition. For indirect conditional
# requirements, this means calling a rule. Ordinary requirements are always
# applied.
#
# Handles the situation where evaluating one conditional requirement affects
# conditions of another conditional requirements, such as:
#     <toolset>gcc:<variant>release <variant>release:<define>RELEASE
#
# If 'what' is 'refined' returns context refined with new requirements. If
# 'what' is 'added' returns just the requirements to be applied.
#
rule evaluate-requirements ( requirements : context : what )
{
    # Apply non-conditional requirements. It is possible that further
    # conditional requirement change a value set by non-conditional
    # requirements. For example:
    #
    #    exe a : a.cpp : <threading>single <toolset>foo:<threading>multi ;
    #
    # I am not sure if this should be an error, or not, especially given that
    #
    #    <threading>single
    #
    # might come from project's requirements.

    local unconditional = [ feature.expand [ $(requirements).non-conditional ] ] ;
 
    local raw = [ $(context).raw ] ;
    raw = [ property.refine $(raw) : $(unconditional) ] ;

    # We have collected properties that surely must be present in common
    # properties. We now try to figure out what other properties should be added
    # in order to satisfy rules (4)-(6) from the docs.

    local conditionals = [ $(requirements).conditional ] ;
    # The 'count' variable has one element for each conditional feature and for
    # each occurrence of '<indirect-conditional>' feature. It is used as a loop
    # counter: for each iteration of the loop before we remove one element and
    # the property set should stabilize before we are done. It is assumed that
    # #conditionals iterations should be enough for properties to propagate
    # along conditions in any direction.
    local count = $(conditionals)
                  [ $(requirements).get <conditional> ]
                  and-once-more ;

    local added-requirements ;

    local current = $(raw) ;

    # It is assumed that ordinary conditional requirements can not add
    # <conditional> properties (a.k.a. indirect conditional properties), and
    # that rules referred to by <conditional> properties can not add new
    # <conditional> properties. So the list of indirect conditionals does not
    # change.
    local indirect = [ $(requirements).get <conditional> ] ;
    indirect = [ MATCH ^@(.*) : $(indirect) ] ;

    local ok ;
    while $(count)
    {
        # Evaluate conditionals in context of current properties.
        local e = [ property.evaluate-conditionals-in-context $(conditionals)
            : $(current) ] ;

        # Evaluate indirect conditionals.
        for local i in $(indirect)
        {
            e += [ indirect.call $(i) $(current) ] ;
        }

        if $(e) = $(added-requirements)
        {
            # If we got the same result, we have found the final properties.
            count = ;
            ok = true ;
        }
        else
        {
            # Oops, conditional evaluation results have changed. Also 'current'
            # contains leftovers from a previous evaluation. Recompute 'current'
            # using initial properties and conditional requirements.
            added-requirements = $(e) ;
            current = [ property.refine $(raw) : [ feature.expand $(e) ] ] ;
        }
        count = $(count[2-]) ;
    }
    if ! $(ok)
    {
        errors.error "Can not evaluate conditional properties " $(conditionals) ;
    }

    if $(what) = added
    {
        return [ property-set.create $(unconditional) $(added-requirements) ] ;
    }
    else if $(what) = refined
    {
        return [ property-set.create $(current) ] ;
    }
    else
    {
        errors.error "Invalid value of the 'what' parameter." ;
    }
}


rule common-properties2 ( build-request requirements )
{
    # This guarantees that default properties are present in the result, unless
    # they are overriden by some requirement. FIXME: There is possibility that
    # we have added <foo>bar, which is composite and expands to <foo2>bar2, but
    # default value of <foo2> is not bar2, in which case it is not clear what to
    # do.
    #
    build-request = [ $(build-request).add-defaults ] ;
    # Features added by 'add-default' can be composite and expand to features
    # without default values -- so they are not added yet. It could be clearer/
    # /faster to expand only newly added properties but that is not critical.
    build-request = [ $(build-request).expand ] ;

    return [ evaluate-requirements $(requirements) : $(build-request) :
        refined ] ;
}

rule push-target ( target )
{
    .targets = $(target) $(.targets) ;
}

rule pop-target ( )
{
    .targets = $(.targets[2-]) ;
}

# Return the metatarget that is currently being generated.
rule current ( )
{
    return $(.targets[1]) ;
}


# Implements the most standard way of constructing main target alternative from
# sources. Allows sources to be either file or other main target and handles
# generation of those dependency targets.
#
class basic-target : abstract-target
{
    import build-request ;
    import build-system ;
    import "class" : new ;
    import errors ;
    import feature ;
    import property ;
    import property-set ;
    import sequence ;
    import set ;
    import targets ;
    import virtual-target ;

    rule __init__ ( name : project : sources * : requirements *
        : default-build * : usage-requirements * )
    {
        abstract-target.__init__ $(name) : $(project) ;

        self.sources = $(sources) ;
        if ! $(requirements) {
            requirements = [ property-set.empty ] ;
        }
        self.requirements = $(requirements) ;
        if ! $(default-build)
        {
            default-build = [ property-set.empty ] ;
        }
        self.default-build = $(default-build) ;
        if ! $(usage-requirements)
        {
            usage-requirements = [ property-set.empty ] ;
        }
        self.usage-requirements = $(usage-requirements) ;

        if $(sources:G)
        {
            errors.user-error properties found in the 'sources' parameter for
                [ full-name ] ;
        }
    }
    
    rule always ( )
    {
        self.always = 1 ;
    }
    
    # Returns the list of abstract-targets which are used as sources. The extra
    # properties specified for sources are not represented. The only user for
    # this rule at the moment is the "--dump-tests" feature of the test system.
    #
    rule sources ( )
    {
        if ! $(self.source-targets)
        {
            for local s in $(self.sources)
            {
                self.source-targets +=
                  [ targets.resolve-reference $(s) : $(self.project) ] ;
            }
        }
        return $(self.source-targets) ;
    }

    rule requirements ( )
    {
        return $(self.requirements) ;
    }

    rule default-build ( )
    {
        return $(self.default-build) ;
    }

    # Returns the alternative condition for this alternative, if the condition
    # is satisfied by 'property-set'.
    #
    rule match ( property-set debug ? )
    {
        # The condition is composed of all base non-conditional properties. It
        # is not clear if we should expand 'self.requirements' or not. For one
        # thing, it would be nice to be able to put
        #    <toolset>msvc-6.0
        # in requirements. On the other hand, if we have <variant>release as a
        # condition it does not make sense to require <optimization>full to be
        # in the build request just to select this variant.
        local bcondition = [ $(self.requirements).base ] ;
        local ccondition = [ $(self.requirements).conditional ] ;
        local condition = [ set.difference $(bcondition) : $(ccondition) ] ;
        if $(debug)
        {
            ECHO "    next alternative: required properties:" $(condition:E=(empty)) ;
        }

        if $(condition) in [ $(property-set).raw ]
        {
            if $(debug)
            {
                ECHO "        matched" ;
            }
            return $(condition) ;
        }
        else
        {
            if $(debug)
            {
                ECHO "        not matched" ;
            }
            return no-match ;
        }
    }

    # Takes a target reference, which might be either target id or a dependency
    # property, and generates that target using 'property-set' as build request.
    #
    # The results are added to the variable called 'result-var'. Usage
    # requirements are added to the variable called 'usage-requirements-var'.
    #
    rule generate-dependencies ( dependencies * : property-set
        : result-var usage-requirements-var )
    {
        for local dependency in $(dependencies)
        {
            local grist = $(dependency:G) ;
            local id = $(dependency:G=) ;

            local result = [ targets.generate-from-reference $(id) :
                $(self.project) : $(property-set) ] ;

            $(result-var) += $(result[2-]:G=$(grist)) ;
            $(usage-requirements-var) += [ $(result[1]).raw ] ;
        }
    }

    # Determines final build properties, generates sources, and calls
    # 'construct'. This method should not be overridden.
    #
    rule generate ( property-set )
    {
        if [ modules.peek : .debug-building ]
        {
            ECHO ;
            local fn = [ full-name ] ;
            ECHO [ targets.indent ] "Building target '$(fn)'" ;
            targets.increase-indent ;
            ECHO [ targets.indent ] "Build request: " $(property-set) [ $(property-set).raw ] ;
            local cf = [ build-system.command-line-free-features ] ;
            ECHO [ targets.indent ] "Command line free features: " [ $(cf).raw ] ;
            ECHO [ targets.indent ] "Target requirements: " [ $(self.requirements).raw ] ;
        }
        targets.push-target $(__name__) ;

        if ! $(self.generated.$(property-set))
        {
            # Apply free features from the command line. If user said
            #   define=FOO
            # he most likely wants this define to be set for all compiles.
            property-set = [ $(property-set).refine
                [ build-system.command-line-free-features ] ] ;
            local rproperties = [ targets.common-properties $(property-set)
                $(self.requirements) ] ;

            if [ modules.peek : .debug-building ]
            {
                ECHO ;
                ECHO [ targets.indent ] "Common properties: " [ $(rproperties).raw ] ;
            }

            if ( $(rproperties[1]) != "@error" ) && ( [ $(rproperties).get
                <build> ] != no )
            {
                local source-targets ;
                local properties = [ $(rproperties).non-dependency ] ;
                local usage-requirements ;

                generate-dependencies [ $(rproperties).dependency ] :
                    $(rproperties) : properties usage-requirements ;

                generate-dependencies $(self.sources) : $(rproperties) :
                    source-targets usage-requirements ;

                if [ modules.peek : .debug-building ]
                {
                    ECHO ;
                    ECHO [ targets.indent ] "Usage requirements for"
                        $(self.name)": " $(usage-requirements) ;
                }

                rproperties = [ property-set.create $(properties)
                    $(usage-requirements) ] ;
                usage-requirements = [ property-set.create $(usage-requirements) ] ;

                if [ modules.peek : .debug-building ]
                {
                    ECHO [ targets.indent ] "Build properties: "
                        [ $(rproperties).raw ] ;
                }

                local extra = [ $(rproperties).get <source> ] ;
                source-targets += $(extra:G=) ;
                # We might get duplicate sources, for example if we link to two
                # libraries having the same <library> usage requirement.
                # Use stable sort, since for some targets the order is
                # important. E.g. RUN_PY target need python source to come
                # first.
                source-targets = [ sequence.unique $(source-targets) : stable ] ;

                local result = [ construct $(self.name) : $(source-targets) :
                    $(rproperties) ] ;

                if $(result)
                {
                    local gur = $(result[1]) ;
                    result = $(result[2-]) ;
                    
                    if $(self.always)
                    {
                        for local t in $(result)
                        {
                            $(t).always ;
                        }                        
                    }
                    
                    local s = [ create-subvariant $(result)
                        : [ virtual-target.recent-targets ]
                        : $(property-set) : $(source-targets)
                        : $(rproperties) : $(usage-requirements) ] ;
                    virtual-target.clear-recent-targets ;

                    local ur = [ compute-usage-requirements $(s) ] ;
                    ur = [ $(ur).add $(gur) ] ;
                    $(s).set-usage-requirements $(ur) ;
                    if [ modules.peek : .debug-building ]
                    {
                        ECHO [ targets.indent ] "Usage requirements from"
                            $(self.name)": " [ $(ur).raw ] ;
                    }

                    self.generated.$(property-set) = $(ur) $(result) ;
                }
            }
            else
            {
                if $(rproperties[1]) = "@error"
                {
                    ECHO [ targets.indent ] "Skipping build of:" [ full-name ]
                        "cannot compute common properties" ;
                }
                else if [ $(rproperties).get <build> ] = no
                {
                    # If we just see <build>no, we cannot produce any reasonable
                    # diagnostics. The code that adds this property is expected
                    # to explain why a target is not built, for example using
                    # the configure.log-component-configuration function.
                }
                else
                {
                    ECHO [ targets.indent ] "Skipping build of: " [ full-name ]
                        " unknown reason" ;
                }

                # We are here either because there has been an error computing
                # properties or there is <build>no in properties. In the latter
                # case we do not want any diagnostic. In the former case, we
                # need diagnostics. FIXME

                # If this target fails to build, add <build>no to properties to
                # cause any parent target to fail to build. Except that it
                # - does not work now, since we check for <build>no only in
                #   common properties, but not in properties that came from
                #   dependencies
                # - it is not clear if that is a good idea anyway. The alias
                #   target, for example, should not fail to build if a
                #   dependency fails.
                self.generated.$(property-set) = [ property-set.create <build>no ] ;
            }
        }
        else
        {
            if [ modules.peek : .debug-building ]
            {
                ECHO [ targets.indent ] "Already built" ;
                local ur = $(self.generated.$(property-set)) ;
                ur = $(ur[0]) ;
                targets.increase-indent ;
                ECHO [ targets.indent ] "Usage requirements from"
                    $(self.name)": " [ $(ur).raw ] ;
                targets.decrease-indent ;
            }
        }

        targets.pop-target ;
        targets.decrease-indent ;
        return $(self.generated.$(property-set)) ;
    }

    # Given the set of generated targets, and refined build properties,
    # determines and sets appropriate usage requirements on those targets.
    #
    rule compute-usage-requirements ( subvariant )
    {
        local rproperties = [ $(subvariant).build-properties ] ;
        xusage-requirements = [ targets.evaluate-requirements
            $(self.usage-requirements) : $(rproperties) : added ] ;

        # We generate all dependency properties and add them, as well as their
        # usage requirements, to the result.
        local extra ;
        generate-dependencies [ $(xusage-requirements).dependency ] :
            $(rproperties) : extra extra ;

        local result = [ property-set.create
            [ $(xusage-requirements).non-dependency ] $(extra) ] ;

        # Propagate usage requirements we got from sources, except for the
        # <pch-header> and <pch-file> features.
        #
        # That feature specifies which pch file to use, and should apply only to
        # direct dependents. Consider:
        #
        #   pch pch1 : ...
        #   lib lib1 : ..... pch1 ;
        #   pch pch2 :
        #   lib lib2 : pch2 lib1 ;
        #
        # Here, lib2 should not get <pch-header> property from pch1.
        #
        # Essentially, when those two features are in usage requirements, they
        # are propagated only to direct dependents. We might need a more general
        # mechanism, but for now, only those two features are special.
        #
        # TODO - Actually there are more possible candidates like for instance
        # when listing static library X as a source for another static library.
        # Then static library X will be added as a <source> property to the
        # second library's usage requirements but those requirements should last
        # only up to the first executable or shared library that actually links
        # to it.
        local raw = [ $(subvariant).sources-usage-requirements ] ;
        raw = [ $(raw).raw ] ;
        raw = [ property.change $(raw) : <pch-header> ] ;
        raw = [ property.change $(raw) : <pch-file> ] ;
        return [ $(result).add [ property-set.create $(raw) ] ] ;
    }

    # Creates new subvariant instances for 'targets'.
    # 'root-targets'  - virtual targets to be returned to dependants
    # 'all-targets'   - virtual targets created while building this main target
    # 'build-request' - property-set instance with requested build properties
    #
    local rule create-subvariant ( root-targets * : all-targets * :
        build-request : sources * : rproperties : usage-requirements )
    {
        for local e in $(root-targets)
        {
            $(e).root true ;
        }

        # Process all virtual targets that will be created if this main target
        # is created.
        local s = [ new subvariant $(__name__) : $(build-request) : $(sources) :
            $(rproperties) : $(usage-requirements) : $(all-targets) ] ;
        for local v in $(all-targets)
        {
            if ! [ $(v).creating-subvariant ]
            {
                $(v).creating-subvariant $(s) ;
            }
        }
        return $(s) ;
    }

    # Constructs virtual targets for this abstract target and the dependency
    # graph. Returns a usage-requirements property-set and a list of virtual
    # targets. Should be overriden in derived classes.
    #
    rule construct ( name : source-targets * : properties * )
    {
        errors.error "method should be defined in derived classes" ;
    }
}


class typed-target : basic-target
{
    import generators ;

    rule __init__ ( name : project : type : sources * : requirements * :
         default-build * : usage-requirements * )
    {
        basic-target.__init__ $(name) : $(project) : $(sources) :
            $(requirements) : $(default-build) : $(usage-requirements) ;

        self.type = $(type) ;
    }

    rule type ( )
    {
        return $(self.type) ;
    }

    rule construct ( name : source-targets * : property-set )
    {
        local r = [ generators.construct $(self.project) $(name:S=) : $(self.type)
            : [ property-set.create [ $(property-set).raw ]
                <main-target-type>$(self.type) ]
            : $(source-targets) : true ] ;
        if ! $(r)
        {
            ECHO "warn: Unable to construct" [ full-name ] ;

            # Are there any top-level generators for this type/property set.
            if ! [ generators.find-viable-generators $(self.type)
                : $(property-set) ]
            {
                ECHO "error: no generators were found for type '$(self.type)'" ;
                ECHO "error: and the requested properties" ;
                ECHO "error: make sure you've configured the needed tools" ;
                ECHO "See http://boost.org/boost-build2/doc/html/bbv2/advanced/configuration.html" ;
                ECHO "To debug this problem, try the --debug-generators option." ;
                EXIT ;
            }
        }
        return $(r) ;
    }
}


# Return the list of sources to use, if main target rule is invoked with
# 'sources'. If there are any objects in 'sources', they are treated as main
# target instances, and the name of such targets are adjusted to be
# '<name_of_this_target>__<name_of_source_target>'. Such renaming is disabled if
# a non-empty value is passed as the 'no-renaming' parameter.
#
rule main-target-sources ( sources * : main-target-name : no-renaming ? )
{
    local result ;
    for local t in $(sources)
    {
        if [ class.is-instance $(t) ]
        {
            local name = [ $(t).name ] ;
            if ! $(no-renaming)
            {
                name = $(main-target-name)__$(name) ;
                $(t).rename $(name) ;
            }
            # Inline targets are not built by default.
            local p = [ $(t).project ] ;
            $(p).mark-target-as-explicit $(name) ;
            result += $(name) ;
        }
        else
        {
            result += $(t) ;
        }
    }
    return $(result) ;
}


# Returns the requirements to use when declaring a main target, obtained by
# translating all specified property paths and refining project requirements
# with the ones specified for the target.
#
rule main-target-requirements (
    specification *  # Properties explicitly specified for the main target.
    : project        # Project where the main target is to be declared.
)
{
    specification += [ toolset.requirements ] ;

    local requirements = [ property-set.refine-from-user-input
        [ $(project).get requirements ] : $(specification) :
        [ $(project).project-module ] : [ $(project).get location ] ] ;
    if $(requirements[1]) = "@error"
    {
        errors.error "Conflicting requirements for target:" $(requirements) ;
    }
    return $(requirements) ;
}


# Returns the usage requirements to use when declaring a main target, which are
# obtained by translating all specified property paths and adding project's
# usage requirements.
#
rule main-target-usage-requirements (
    specification *  # Use-properties explicitly specified for a main target.
    : project        # Project where the main target is to be declared.
)
{
    local project-usage-requirements = [ $(project).get usage-requirements ] ;

    # We do not use 'refine-from-user-input' because:
    # - I am not sure if removing parent's usage requirements makes sense
    # - refining usage requirements is not needed, since usage requirements are
    #   always free.
    local usage-requirements = [ property-set.create-from-user-input
        $(specification)
        : [ $(project).project-module ] [ $(project).get location ] ] ;

    return [ $(project-usage-requirements).add $(usage-requirements) ] ;
}


# Return the default build value to use when declaring a main target, which is
# obtained by using the specified value if not empty and parent's default build
# attribute otherwise.
#
rule main-target-default-build (
    specification *  # Default build explicitly specified for a main target.
    : project        # Project where the main target is to be declared.
)
{
    local result ;
    if $(specification)
    {
        result = $(specification) ;
    }
    else
    {
        result = [ $(project).get default-build ] ;
    }
    return [ property-set.create-with-validation $(result) ] ;
}


# Registers the specified target as a main target alternative and returns it.
#
rule main-target-alternative ( target )
{
    local ptarget = [ $(target).project ] ;
    $(ptarget).add-alternative $(target) ;
    return $(target) ;
}

# Creates a new metargets with the specified properties, using 'klass' as
# the class. The 'name', 'sources',
# 'requirements', 'default-build' and 'usage-requirements' are assumed to be in
# the form specified by the user in Jamfile corresponding to 'project'.
#
rule create-metatarget ( klass : project : name : sources * : requirements * :
    default-build * : usage-requirements * )
{
    return [
        targets.main-target-alternative
        [ new $(klass) $(name) : $(project)
            : [ targets.main-target-sources $(sources) : $(name) ]
            : [ targets.main-target-requirements $(requirements) : $(project) ]
            : [ targets.main-target-default-build $(default-build) : $(project) ]
            : [ targets.main-target-usage-requirements $(usage-requirements) : $(project) ]
        ] ] ;
}

# Creates a typed-target with the specified properties. The 'name', 'sources',
# 'requirements', 'default-build' and 'usage-requirements' are assumed to be in
# the form specified by the user in Jamfile corresponding to 'project'.
#
rule create-typed-target ( type : project : name : sources * : requirements * :
    default-build * : usage-requirements * )
{
    return [
        targets.main-target-alternative
        [ new typed-target $(name) : $(project) : $(type)
            : [ targets.main-target-sources $(sources) : $(name) ]
            : [ targets.main-target-requirements $(requirements) : $(project) ]
            : [ targets.main-target-default-build $(default-build) : $(project) ]
            : [ targets.main-target-usage-requirements $(usage-requirements) : $(project) ]
        ] ] ;
}