new option --rand, randomize task prerequisites. Suggested by Thomas …

…Sawyer.

README

@@ -75,6 +75,42 @@ other tasks is rather contrary to this notion, throwing a monkey
 wrench into the system.  An exception will be raised when this is
 attempted in multi-threaded mode.
 
+=== Migrating to -j
+
+First of all, do you want to bother with <tt>-j</tt>?  If you are
+satisfied with your build time, then there is really no reason to use
+it.
+
+If on the other hand your build takes twenty minutes to complete, you
+may be interested in investing some time getting the full dependency
+tree correct in order to take advantage of multiple CPUs or cores.
+
+Though is no way for Drake to fathom what <em>you</em> mean by a
+correct dependency, there is a tool available which helps you get
+closer to saying what you mean.
+
+  % drake --rand[=SEED]
+
+This will randomize the order in which prerequisites are invoked.
+When given the optional SEED integer, it will call srand(SEED) to
+produce the same permutation each time.
+
+Though this may produce an error due to an unspecified dependency, at
+least it will be an error which is exactly the same on each run.  In
+addition, you'll have the major debugging advantage of using a single
+thread.
+
+The randomize option will also work in multi-threaded mode.  After
+all, once <tt>-jN</tt> is running smoothly there is <em>still</em> no
+guarantee that you have it right.  However with each successful
+execution of <tt>-jN --rand</tt>, the probability of correctness gets
+closer to 1 (though asymptotically so).
+
+(The only way to <em>prove</em> correctness is to test <em>all</em>
+such permutations, which for any non-trivial project would be
+prohibitively large, especially those which meaningfully benefit from
+<tt>-j</tt>.)
+
 == Links
 
 * Download: http://rubyforge.org/frs/?group_id=6530

lib/rake.rb

@@ -595,13 +595,19 @@ def invoke_with_call_chain(task_args, invocation_chain) # :nodoc:
         end
         return if @already_invoked
         @already_invoked = true
+
+        if application.options.randomize
+          @prerequisites = @prerequisites.sort_by { rand }
+        end
+
         prereqs = 
           if application.num_threads == 1
             invoke_prerequisites(task_args, new_chain)
             nil
           else
             invoke_prerequisites_parallel(task_args, new_chain)
           end
+
         if needed?
           if application.num_threads == 1
             execute(task_args) 
@@ -2276,9 +2282,15 @@ def standard_rake_options
         ['--threads', '-j N', "Specifies the number of threads to run simultaneously.",
           lambda { |value| self.num_threads = value.to_i }
         ],
-        #['--fork', '-k', "When --threads=N given, run each thread in a separate process.",
-        #  lambda { options.fork = true }
-        #],
+        ['--rand[=SEED]', "Randomize task prerequisite orders",
+          lambda { |value|
+            MultiTask.class_eval { remove_method(:invoke_prerequisites) }
+            options.randomize = true
+            if value
+              srand(value.to_i)
+            end
+          }
+        ],
         ['--libdir', '-I LIBDIR', "Include LIBDIR in the search path for required modules.",
           lambda { |value| $:.push(value) }
         ],