Restore environment_value after source layout change

_/libs/base/System.idr

@@ -151,6 +151,23 @@
            then pure Nothing
            else pure (Just (prim__getString env))
 
+||| Retrieve an environment variable into Idriç-owned String storage.
+||| Invalid environment names (those containing `=` or NUL) are rejected as
+||| `Nothing`; this is distinct from a valid variable whose value is empty.
+export
+environment_value : String -> IO (Maybe String)
+environment_value var =
+  if any invalidNameCharacter (unpack var)
+     then pure Nothing
+     else do
+       env <- primIO $ prim__getEnv var
+       if prim__nullPtr env /= 0
+          then pure Nothing
+          else pure $ Just $ fastPack $ unpack $ prim__getString env
+  where
+    invalidNameCharacter : Char -> Bool
+    invalidNameCharacter c = c == '=' || c == '\0'
+
 ||| Retrieve all the key-value pairs of the environment variables, and return a
 ||| list containing them.
 export

_/tests/base/system_environment_value/InvalidNames.idric

@@ -0,0 +1,10 @@
+import System
+
+showEnvironmentValue : Maybe String -> String
+showEnvironmentValue Nothing = "missing"
+showEnvironmentValue (Just value) = value
+
+main : IO ()
+main = do
+  putStrLn $ showEnvironmentValue !(environment_value "INVALID=NAME")
+  putStrLn $ showEnvironmentValue !(environment_value (pack ['N', '\0', 'X']))

_/tests/base/system_environment_value/Test.idric

@@ -0,0 +1,9 @@
+import System
+
+showEnvironmentValue : Maybe String -> String
+showEnvironmentValue Nothing = "missing"
+showEnvironmentValue (Just "") = "empty"
+showEnvironmentValue (Just value) = value
+
+main : IO ()
+main = putStrLn $ showEnvironmentValue !(environment_value "IDRIC_ENV_TEST")

_/tests/base/system_environment_value/expected

@@ -0,0 +1,5 @@
+hello
+empty
+missing
+missing
+missing

_/tests/base/system_environment_value/run

@@ -0,0 +1,9 @@
+. ../../testutils.sh
+
+idris2 --cg chez -o test Test.idric
+IDRIC_ENV_TEST=hello ./build/exec/test
+IDRIC_ENV_TEST= ./build/exec/test
+env -u IDRIC_ENV_TEST ./build/exec/test
+
+idris2 --cg chez -o invalid-names InvalidNames.idric
+./build/exec/invalid-names