propellor spin

Propellor/Property.hs

@@ -92,6 +92,19 @@ check c property = Property (propertyDesc property) $ ifM (liftIO c)
 	, return NoChange
 	)
 
+-- | Marks a Property as trivial. It can only return FailedChange or
+-- NoChange. 
+--
+-- Useful when it's just as expensive to check if a change needs
+-- to be made as it is to just idempotently assure the property is
+-- satisfied. For example, chmodding a file.
+trivial :: Property -> Property
+trivial p = Property (propertyDesc p) $ do
+	r <- ensureProperty p
+	if r == MadeChange
+		then return NoChange
+		else return r
+
 -- | Makes a property that is satisfied differently depending on the host's
 -- operating system. 
 --

Propellor/Property/Apache.hs

@@ -10,12 +10,12 @@ type ConfigFile = [String]
 siteEnabled :: HostName -> ConfigFile -> RevertableProperty
 siteEnabled hn cf = RevertableProperty enable disable
   where
-	enable = cmdProperty "a2ensite" ["--quiet", hn]
+	enable = trivial $ cmdProperty "a2ensite" ["--quiet", hn]
 		`describe` ("apache site enabled " ++ hn)
 		`requires` siteAvailable hn cf
 		`requires` installed
 		`onChange` reloaded
-	disable = File.notPresent (siteCfg hn)
+	disable = trivial $ File.notPresent (siteCfg hn)
 		`describe` ("apache site disabled " ++ hn)
 		`onChange` cmdProperty "a2dissite" ["--quiet", hn]
 		`requires` installed
@@ -30,11 +30,11 @@ siteAvailable hn cf = siteCfg hn `File.hasContent` (comment:cf)
 modEnabled :: String -> RevertableProperty
 modEnabled modname = RevertableProperty enable disable
   where
-	enable = cmdProperty "a2enmod" ["--quiet", modname]
+	enable = trivial $ cmdProperty "a2enmod" ["--quiet", modname]
 		`describe` ("apache module enabled " ++ modname)
 		`requires` installed
 		`onChange` reloaded
-	disable = cmdProperty "a2dismod" ["--quiet", modname]
+	disable = trivial $ cmdProperty "a2dismod" ["--quiet", modname]
 		`describe` ("apache module disabled " ++ modname)
 		`requires` installed
 		`onChange` reloaded

Propellor/Property/Apt.hs

@@ -103,7 +103,7 @@ installed' params ps = robustly $ check (isInstallable ps) go
 	go = runApt $ params ++ ["install"] ++ ps
 
 installedBackport :: [Package] -> Property
-installedBackport ps = withOS desc $ \o -> case o of
+installedBackport ps = trivial $ withOS desc $ \o -> case o of
 	Nothing -> error "cannot install backports; os not declared"
 	(Just (System (Debian suite) _))
 		| isStable suite ->