add Environment#build_after

build_tests/custom_builder/build_after.rb

@@ -0,0 +1,8 @@
+Rscons::Environment.new do |env|
+  env.Command("inc.c",
+              [],
+              "CMD" => %w[ruby gen.rb ${_TARGET}],
+              "CMD_DESC" => "Generating")
+  env.build_after("program.o", "inc.c")
+  env.Program("program.exe", ["program.c", "inc.c"])
+end

build_tests/custom_builder/gen.rb

@@ -0,0 +1,9 @@
+sleep 1.0
+c_file = ARGV.first
+h_file = File.basename(c_file, ".c") + ".h"
+File.open(c_file, "w") do |fh|
+  fh.puts
+end
+File.open(h_file, "w") do |fh|
+  fh.puts("#define THE_VALUE 191")
+end

lib/rscons/environment.rb

@@ -459,6 +459,42 @@ module Rscons
       @user_deps[target] = (@user_deps[target] + user_deps).uniq
     end
 
+    # Manually record the given target(s) as needing to be built after the
+    # given prerequisite(s).
+    #
+    # For example, consider a builder registered to generate gen.c which also
+    # generates gen.h as a side-effect. If program.c includes gen.h, then it
+    # should not be compiled before gen.h has been generated. When using
+    # multiple threads to build, Rscons may attempt to compile program.c before
+    # gen.h has been generated because it does not know that gen.h will be
+    # generated along with gen.c. One way to prevent that situation would be
+    # to first process the Environment with just the code-generation builders
+    # in place and then register the compilation builders. Another way is to
+    # use this method to record that a certain target should not be built until
+    # another has completed. For example, for the situation previously
+    # described:
+    #   env.build_after("program.o", "gen.c")
+    #
+    # @since 1.10.0
+    #
+    # @param targets [String, Array<String>]
+    #   Target files to wait to build until the prerequisites are finished
+    #   building.
+    # @param prerequisites [String, Array<String>]
+    #   Files that must be built before building the specified targets.
+    #
+    # @return [void]
+    def build_after(targets, prerequisites)
+      targets = Array(targets)
+      prerequisites = Array(prerequisites)
+      targets.each do |target|
+        @registered_build_dependencies[target] ||= Set.new
+        prerequisites.each do |prerequisite|
+          @registered_build_dependencies[target] << prerequisite
+        end
+      end
+    end
+
     # Return the list of user dependencies for a given target.
     #
     # @param target [String] Target file name.

spec/build_tests_spec.rb

@@ -1090,6 +1090,12 @@ EOF
       elapsed = Time.new - start_time
       expect(elapsed).to be < 4
     end
+
+    it "allows the user to specify that a target be built after another" do
+      test_dir("custom_builder")
+      result = run_test(rsconsfile: "build_after.rb", rscons_args: %w[-j 4])
+      expect(result.stderr).to eq ""
+    end
   end
 
   context "CLI" do