Do not run tasks when their prerequisites fail

If tasks A and B both depend on C, they race to grab the invocation
lock on C. If A wins, it sets @already_invoked and executes C while
holding the lock. When C completes, A releases the lock, B acquires it
and immediately returns since C is @already_invoked. Unfortunately,
this does not distinguish failed execution of C. Instead, if executing
C raises an exception for A, we want to raise the same exception in B
so it is aborted.

lib/rake/multi_task.rb

@@ -4,6 +4,42 @@ module Rake
   # parallel using Ruby threads.
   #
   class MultiTask < Task
+
+    # Same as invoke, but explicitly pass a call chain to detect
+    # circular dependencies. This is largely copied from Rake::Task
+    # but has been updated such that if multiple tasks depend on this
+    # one in parallel, they will all fail if the first execution of
+    # this task fails.
+    def invoke_with_call_chain(task_args, invocation_chain)
+      new_chain = Rake::InvocationChain.append(self, invocation_chain)
+      @lock.synchronize do
+        begin
+          if @already_invoked
+            if @invocation_exception
+              if application.options.trace
+                application.trace "** Previous invocation of #{name} failed #{format_trace_flags}"
+              end
+              raise @invocation_exception
+            else
+              return
+            end
+          end
+
+          if application.options.trace
+            application.trace "** Invoke #{name} #{format_trace_flags}"
+          end
+          @already_invoked = true
+
+          invoke_prerequisites(task_args, new_chain)
+          execute(task_args) if needed?
+        rescue Exception => ex
+          add_chain_to(ex, new_chain)
+          @invocation_exception = ex
+          raise
+        end
+      end
+    end
+
     private
     def invoke_prerequisites(task_args, invocation_chain) # :nodoc:
       invoke_prerequisites_concurrently(task_args, invocation_chain)

test/test_rake_multi_task.rb

@@ -61,4 +61,25 @@ def test_multitasks_with_parameters
     assert @runs[0] == "b"
     assert @runs[1] == "bmt"
   end
+
+  def test_cross_thread_prerequisite_failures
+    failed = false
+
+    multitask :fail_once do
+      fail_now = !failed
+      failed = true
+      raise 'failing once' if fail_now
+    end
+
+    task a: :fail_once
+    task b: :fail_once
+
+    assert_raises RuntimeError do
+      Rake::Task[:a].invoke
+    end
+
+    assert_raises RuntimeError do
+      Rake::Task[:b].invoke
+    end
+  end
 end