Add BinarySemaphore (error checking semaphore with max value 1).

lib/process_shared/binary_semaphore.rb

@@ -0,0 +1,48 @@
+require 'process_shared/psem'
+require 'process_shared/semaphore'
+require 'process_shared/process_error'
+
+module ProcessShared
+  # BinarySemaphore is identical to Semaphore except that its value is
+  # not permitted to rise above one (it may be either zero or one).
+  # When the value is at the maximum, calls to #post will raise an
+  # exception.
+  #
+  # This is identical to a Semaphore but with extra error checking.
+  class BinarySemaphore < Semaphore
+    # Create a new semaphore with initial value +value+.  After
+    # {Kernel#fork}, the semaphore will be shared across two (or more)
+    # processes. The semaphore must be closed with {#close} in each
+    # process that no longer needs the semaphore.
+    #
+    # (An object finalizer is registered that will close the semaphore
+    # to avoid memory leaks, but this should be considered a last
+    # resort).
+    #
+    # @param [Integer] value the initial semaphore value
+    # @param [String] name not currently supported
+    def initialize(value = 1, name = nil)
+      raise ArgumentErrror 'value must be 0 or 1' if (value < 0 or value > 1)
+      super(value, name)
+    end
+
+    # Increment from zero to one.
+    #
+    # First, attempt to decrement.  If this fails with EAGAIN, the
+    # semaphore was at zero, so continue with the post. If this
+    # succeeds, the semaphore was not at zero, so increment back to
+    # one and raise {ProcesError} (multiple workers may have acquired
+    # the semaphore at this point).
+    def post
+      begin
+        try_wait
+        # oops, value was not zero...
+        psem_post(sem, err)
+        raise ProcessError, 'post would raise value over bound'
+      rescue Errno::EAGAIN
+        # ok, value was zero
+        psem_post(sem, err)
+      end
+    end
+  end
+end

spec/process_shared/binary_semaphore_spec.rb

@@ -0,0 +1,13 @@
+require 'spec_helper'
+require 'process_shared/binary_semaphore'
+
+module ProcessShared
+  describe BinarySemaphore do
+    it 'raises exception with double post' do
+      BinarySemaphore.open(0) do |sem|
+        sem.post
+        proc { sem.post }.must_raise(ProcessError)
+      end
+    end
+  end
+end