lualoader: config: add a table for restricted environment vars

This new table should be used for transient values that don't need to end up
in the loader environment. Generally, these will be things that are internal
details that really aren't needed or interesting outside of the config
module (e.g. if we changed how ${module}_* directives work, they might use
this instead).

To start, populate it with loader_conf_files. Any specific value of
loader_conf_files isn't all that interesting; if we're going to export it,
we should really instead export a loader_conf_files that indicates all of
the configuration files we processed. This will be used to reduce
bookkeeping overhead in a future commit that cleans up readConfFiles.

stand/lua/config.lua

@@ -61,6 +61,17 @@ local QVALREPL = QVALEXPR:gsub('%%', '%%%%')
 local WORDEXPR = "([%w]+)"
 local WORDREPL = WORDEXPR:gsub('%%', '%%%%')
 
+-- Entries that should never make it into the environment; each one should have
+-- a documented reason for its existence, and these should all be implementation
+-- details of the config module.
+local loader_env_restricted_table = {
+	-- loader_conf_files should be considered write-only, and consumers
+	-- should not rely on any particular value; it's a loader implementation
+	-- detail.  Moreover, it's not a particularly useful variable to have in
+	-- the kenv.  Save the overhead, let it get fetched other ways.
+	loader_conf_files = true,
+}
+
 local function restoreEnv()
 	-- Examine changed environment variables
 	for k, v in pairs(env_changed) do
@@ -88,14 +99,31 @@ local function restoreEnv()
 	env_restore = {}
 end
 
+-- XXX This getEnv/setEnv should likely be exported at some point.  We can save
+-- the call back into loader.getenv for any variable that's been set or
+-- overridden by any loader.conf using this implementation with little overhead
+-- since we're already tracking the values.
+local function getEnv(key)
+	if loader_env_restricted_table[key] ~= nil or
+	    env_changed[key] ~= nil then
+		return env_changed[key]
+	end
+
+	return loader.getenv(key)
+end
+
 local function setEnv(key, value)
+	env_changed[key] = value
+
+	if loader_env_restricted_table[key] ~= nil then
+		return 0
+	end
+
 	-- Track the original value for this if we haven't already
 	if env_restore[key] == nil then
 		env_restore[key] = {value = loader.getenv(key)}
 	end
 
-	env_changed[key] = value
-
 	return loader.setenv(key, value)
 end
 
@@ -465,7 +493,7 @@ function config.readConfFiles(files, loaded_files)
 		-- The caller may not have passed in loader_conf_files; we could
 		-- have instead gotten some other string of files.  We don't
 		-- want to trigger any redundant re-read/loads based on this.
-		local prefiles = loader.getenv("loader_conf_files")
+		local prefiles = getEnv("loader_conf_files")
 		for name in files:gmatch("([%w%p]+)%s*") do
 			if loaded_files[name] ~= nil then
 				goto continue
@@ -480,7 +508,7 @@ function config.readConfFiles(files, loaded_files)
 			end
 
 			loaded_files[name] = true
-			local newfiles = loader.getenv("loader_conf_files")
+			local newfiles = getEnv("loader_conf_files")
 			if prefiles ~= newfiles then
 				-- Recurse; process the new files immediately.
 				-- If we come back and it turns out we've
@@ -607,7 +635,7 @@ function config.load(file, reloading)
 	end
 
 	local loaded_files = {file = true}
-	config.readConfFiles(loader.getenv("loader_conf_files"), loaded_files)
+	config.readConfFiles(getEnv("loader_conf_files"), loaded_files)
 
 	checkNextboot()