add Rscons.n_threads

lib/rscons.rb

@@ -41,6 +41,10 @@ module Rscons
 
   class << self
 
+    # @return [Integer]
+    #   The number of threads to use when scheduling subprocesses.
+    attr_accessor :n_threads
+
     # Remove all generated files.
     #
     # @return [void]
@@ -147,7 +151,45 @@ module Rscons
       @command_executer = val
     end
 
+    private
+
+    # Determine the number of threads to use by default.
+    #
+    # @return [Integer]
+    #   The number of threads to use by default.
+    def determine_n_threads
+      # If the user specifies the number of threads in the environment, then
+      # respect that.
+      if ENV["RSCONS_NTHREADS"] =~ /^(\d+)$/
+        return $1.to_i
+      end
+
+      # Otherwise try to figure out how many threads are available on the
+      # host hardware.
+      begin
+        case RbConfig::CONFIG["host_os"]
+        when /linux/
+          return File.read("/proc/cpuinfo").scan(/^processor\s*:/).size
+        when /mswin|mingw/
+          if `wmic cpu get NumberOfLogicalProcessors /value` =~ /NumberOfLogicalProcessors=(\d+)/
+            return $1.to_i
+          end
+        when /darwin/
+          if `sysctl -n hw.ncpu` =~ /(\d+)/
+            return $1.to_i
+          end
+        end
+      rescue
+      end
+
+      # If we can't figure it out, default to 1.
+      1
+    end
+
   end
+
+  @n_threads = determine_n_threads
+
 end
 
 # Unbuffer $stdout

spec/rscons_spec.rb

@@ -122,4 +122,70 @@ describe Rscons do
       end
     end
   end
+
+  describe ".determine_n_threads" do
+    context "when specified by environment variable" do
+      before(:each) do
+        expect(ENV).to receive(:[]).with("RSCONS_NTHREADS").and_return("3")
+      end
+      it "returns the user-specified number of threads to use" do
+        expect(Rscons.__send__(:determine_n_threads)).to eq(3)
+      end
+    end
+
+    context "when not specified by environment variable" do
+      before(:each) do
+        expect(ENV).to receive(:[]).with("RSCONS_NTHREADS").and_return(nil)
+      end
+
+      context "on Linux" do
+        before(:each) do
+          expect(RbConfig::CONFIG).to receive(:[]).with("host_os").and_return("linux")
+        end
+        it "returns the number of processors from /proc/cpuinfo" do
+          expect(File).to receive(:read).with("/proc/cpuinfo").and_return(<<EOF)
+processor : 0
+processor   : 1
+EOF
+          expect(Rscons.__send__(:determine_n_threads)).to eq(2)
+        end
+      end
+
+      context "on Windows" do
+        before(:each) do
+          expect(RbConfig::CONFIG).to receive(:[]).with("host_os").and_return("mingw")
+        end
+        it "returns the number of logical processors that wmic reports" do
+          expect(Rscons).to receive(:`).with("wmic cpu get NumberOfLogicalProcessors /value").and_return("NumberOfLogicalProcessors=7")
+          expect(Rscons.__send__(:determine_n_threads)).to eq(7)
+        end
+      end
+
+      context "on Darwin" do
+        before(:each) do
+          expect(RbConfig::CONFIG).to receive(:[]).with("host_os").and_return("darwin")
+        end
+        it "returns the number of threads that sysctl reports" do
+          expect(Rscons).to receive(:`).with("sysctl -n hw.ncpu").and_return("6")
+          expect(Rscons.__send__(:determine_n_threads)).to eq(6)
+        end
+      end
+
+      context "on an unknown platform" do
+        before(:each) do
+          expect(RbConfig::CONFIG).to receive(:[]).with("host_os").and_return("other")
+        end
+        it "returns 1" do
+          expect(Rscons.__send__(:determine_n_threads)).to eq(1)
+        end
+      end
+
+      context "when an error occurs" do
+        it "returns 1" do
+          expect(RbConfig::CONFIG).to receive(:[]).with("host_os").and_raise("foo")
+          expect(Rscons.__send__(:determine_n_threads)).to eq(1)
+        end
+      end
+    end
+  end
 end