Add Semaphore#try_wait. Fix libpsem to convert relative timeout to absolute timeout.

lib/process_shared/abstract_semaphore.rb

@@ -36,6 +36,15 @@ module ProcessShared
 
     # private_class_method :new
 
+    def synchronize
+      wait
+      begin
+        yield
+      ensure
+        post
+      end
+    end
+
     protected
 
     attr_reader :sem, :err

lib/process_shared/psem.rb

@@ -90,7 +90,7 @@ module ProcessShared
     attach_function :psem_post, [:pointer, :pointer], :int
     attach_function :psem_wait, [:pointer, :pointer], :int
     attach_function :psem_trywait, [:pointer, :pointer], :int
-    attach_function :psem_timedwait, [:pointer, :pointer, :pointer], :int
+    attach_function :psem_timedwait, [:pointer, :float, :pointer], :int
     attach_function :psem_getvalue, [:pointer, :pointer, :pointer], :int
 
     psem_error_check(:psem_open, :psem_close, :psem_unlink, :psem_post,
@@ -104,7 +104,7 @@ module ProcessShared
     attach_function :bsem_post, [:pointer, :pointer], :int
     attach_function :bsem_wait, [:pointer, :pointer], :int
     attach_function :bsem_trywait, [:pointer, :pointer], :int
-    attach_function :bsem_timedwait, [:pointer, :pointer, :pointer], :int
+    attach_function :bsem_timedwait, [:pointer, :float, :pointer], :int
     attach_function :bsem_getvalue, [:pointer, :pointer, :pointer], :int
 
     psem_error_check(:bsem_open, :bsem_close, :bsem_unlink, :bsem_post,

lib/process_shared/semaphore.rb

@@ -38,6 +38,24 @@ module ProcessShared
       psem_wait(sem, err)
     end
 
+    # Decrement the value of the semaphore if it can be done
+    # immediately (i.e. if it was non-zero).  Otherwise, wait up to
+    # +timeout+ seconds until another process increments via {#post}.
+    #
+    # @param timeout [Numeric] the maximum seconds to wait, or nil to not wait
+    #
+    # @return If +timeout+ is nil and the semaphore cannot be
+    # decremented immediately, raise Errno::EAGAIN.  If +timeout+
+    # passed before the semaphore could be decremented, raise
+    # Errno::ETIMEDOUT.
+    def try_wait(timeout = nil)
+      if timeout
+        psem_timedwait(sem, timeout, err)
+      else
+        psem_trywait(sem, err)
+      end
+    end
+
     # Increment the value of the semaphore.  If other processes are
     # waiting on this semaphore, one will be woken.
     def post