add "tweakers" functionality

tweakers allow the user to modify construction variables before any build is performed

build_tests/build_dir/tweaker_build.rb

@@ -0,0 +1,12 @@
+Rscons::Environment.new do |env|
+  env.append('CPPPATH' => Dir['src/**/*/'])
+  env.build_dir(%r{^src/([^/]+)/}, 'build_\\1/')
+  env.add_tweaker do |build_op|
+    if build_op[:target] =~ %r{build_one/.*\.o}
+      build_op[:vars]["CFLAGS"] << "-O1"
+    elsif build_op[:target] =~ %r{build_two/.*\.o}
+      build_op[:vars]["CFLAGS"] << "-O2"
+    end
+  end
+  env.Program('tweaker', Dir['src/**/*.c'])
+end

lib/rscons/environment.rb

@@ -22,6 +22,7 @@ module Rscons
       @targets = {}
       @builders = {}
       @build_dirs = []
+      @tweakers = []
       @varset[:exclude_builders] ||= []
       unless @varset[:exclude_builders] == :all
         exclude_builders = Set.new(@varset[:exclude_builders] || [])
@@ -45,7 +46,7 @@ module Rscons
     # Make a copy of the Environment object.
     # The cloned environment will contain a copy of all environment options,
     # construction variables, builders, and build directories. It will not
-    # contain a copy of the targets.
+    # contain a copy of the targets or tweakers.
     # If a block is given, the Environment object is yielded to the block and
     # when the block returns, the {#process} method is automatically called.
     def clone(variables = {})
@@ -74,6 +75,11 @@ module Rscons
       end
     end
 
+    # Add a tweaker block to the Environment.
+    def add_tweaker(&block)
+      @tweakers << block
+    end
+
     # Specify a build directory for this Environment.
     # Source files from src_dir will produce object files under obj_dir.
     def build_dir(src_dir, obj_dir)
@@ -268,6 +274,16 @@ module Rscons
     # @param vars [Hash] Extra variables to pass to the builder.
     # Return the result of the builder's run() method.
     def run_builder(builder, target, sources, cache, vars)
+      vars = @varset.merge(vars)
+      @tweakers.each do |tweaker_block|
+        build_operation = {
+          builder: builder,
+          target: target,
+          sources: sources,
+          vars: vars,
+        }
+        tweaker_block.call(build_operation)
+      end
       builder.run(target, sources, cache, self, vars)
     end
   end

spec/build_tests_spec.rb

@@ -11,10 +11,10 @@ describe Rscons do
     FileUtils.rm_rf('build_tests_run')
   end
 
-  def build_testdir
-    if File.exists?("build.rb")
-      build_rb = File.read("build.rb")
-      File.open("build.rb", "w") do |fh|
+  def build_testdir(build_script = "build.rb")
+    if File.exists?(build_script)
+      build_rb = File.read(build_script)
+      File.open(build_script, "w") do |fh|
         fh.puts(<<EOF + build_rb)
 require "simplecov"
 
@@ -27,7 +27,7 @@ end
 require "rscons"
 EOF
       end
-      IO.popen(%{ruby -I #{@owd}/lib build.rb}) do |io|
+      IO.popen(%{ruby -I #{@owd}/lib #{build_script}}) do |io|
         io.readlines.reject do |line|
           line =~ /^Coverage report/
         end
@@ -35,11 +35,11 @@ EOF
     end
   end
 
-  def test_dir(build_test_directory)
+  def test_dir(build_test_directory, build_script = "build.rb")
     @build_test_name = build_test_directory
     FileUtils.cp_r("build_tests/#{build_test_directory}", 'build_tests_run')
     Dir.chdir("build_tests_run")
-    build_testdir
+    build_testdir(build_script)
   end
 
   def file_sub(fname)
@@ -207,4 +207,14 @@ EOF
     File.exists?('library').should be_true
     `ar t lib.a`.should == "one.o\ntwo.o\n"
   end
+
+  it 'supports tweakers to override construction variables' do
+    lines = test_dir("build_dir", "tweaker_build.rb")
+    `./tweaker`.should == "Hello from two()\n"
+    lines.should =~ [
+      'gcc -c -o build_one/one.o -MMD -MF build_one/one.mf -Isrc/one/ -Isrc/two/ -O1 src/one/one.c',
+      'gcc -c -o build_two/two.o -MMD -MF build_two/two.mf -Isrc/one/ -Isrc/two/ -O2 src/two/two.c',
+      'gcc -o tweaker build_one/one.o build_two/two.o',
+    ]
+  end
 end