ghc-pkg: Add support for --target

This adds support to ghc-pkg to infer a package-db from a target name.

Author: angerman

utils/ghc-pkg/Main.hs

@@ -5,6 +5,7 @@
 {-# LANGUAGE TypeSynonymInstances #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE MultiWayIf #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TupleSections #-}
 {-# LANGUAGE ScopedTypeVariables #-}
@@ -151,6 +152,7 @@ data Flag
   | FlagVerbosity (Maybe String)
   | FlagUnitId
   | FlagShowUnitIds
+  | FlagTarget String
   deriving Eq
 
 flags :: [OptDescr Flag]
@@ -198,7 +200,9 @@ flags = [
   Option [] ["ipid", "unit-id"] (NoArg FlagUnitId)
         "interpret package arguments as unit IDs (e.g. installed package IDs)",
   Option ['v'] ["verbose"] (OptArg FlagVerbosity "Verbosity")
-        "verbosity level (0-2, default 1)"
+        "verbosity level (0-2, default 1)",
+  Option [] ["target"] (ReqArg FlagTarget "TARGET")
+        "run against the specified target (this has no effect if --global-package-db is specified)"
   ]
 
 data Verbosity = Silent | Normal | Verbose
@@ -587,6 +591,29 @@ readFromSettingsFile settingsFile f = do
       Right archOS -> Right archOS
       Left e -> Left e
 
+-- | Get the cross target.
+--
+-- This is either extracted from the '--target' flag or inferred
+-- from the current program name.
+getTarget :: [Flag] -> IO (Maybe String)
+getTarget my_flags = do
+  case [ t | FlagTarget t <- my_flags ] of
+    [] -> do
+      -- when no target is specified on the command line, infer it from the program name.
+      -- e.g. x86_64-unknown-linux-ghc-pkg
+      progN <- getProgName
+      if | "-ghc-pkg" `isSuffixOf` progN
+         , parts <- split '-' progN
+         , length parts > 3 -> pure (Just (take (length progN - 8) progN))
+         | otherwise -> pure Nothing
+    ts -> pure (Just (last ts))
+ where
+  split :: Char -> String -> [String]
+  split c s = case rest of
+                  []     -> [chunk]
+                  _:rest -> chunk : split c rest
+    where (chunk, rest) = break (==c) s
+
 getPkgDatabases :: Verbosity
                 -> GhcPkg.DbOpenMode mode DbModifySelector
                 -> Bool    -- use the user db
@@ -616,7 +643,12 @@ getPkgDatabases verbosity mode use_user use_cache expand_vars my_flags = do
         [] -> do mb_dir <- getBaseDir
                  case mb_dir of
                    Nothing  -> die err_msg
-                   Just dir -> do
+                   Just dir' -> do
+                     mt <- getTarget my_flags
+                     dir <- case mt of
+                              Nothing -> pure dir'
+                              Just target -> pure (dir' </> "targets" </> target </> "lib")
+
                      -- Look for where it is given in the settings file, if marked there.
                      let settingsFile = dir </> "settings"
                      exists_settings_file <- doesFileExist settingsFile