hadrian: Persist the used flavour name in the build rootwip/hadrian-persistent-flavour

This executes the idea proposed in #20650, teaching Hadrian to persist
the flavour used to build a build root. This both makes use of multiple
build roots more ergonomic (as the user no longer needs to repeat
themselves) and avoids a tiresome foot-gun (forgetting to specify a
flavour, often wholly invalidating a build root).

Closes #20650.

4 files changed, 55 insertions, 5 deletions

diff --git a/hadrian/doc/flavours.md b/hadrian/doc/flavours.md
index b088b8ab92..6118272aba 100644
--- a/hadrian/doc/flavours.md
+++ b/hadrian/doc/flavours.md
@@ -3,7 +3,12 @@
 Hadrian supports a few predefined _build flavours_, i.e. collections of build
 settings that fully define a GHC build (see `src/Flavour.hs`). Users can add their
 own build flavours if need be, as described
-[here](https://gitlab.haskell.org/ghc/ghc/blob/master/hadrian/doc/user-settings.md#build-flavour).
+[here](https://gitlab.haskell.org/ghc/ghc/blob/master/hadrian/doc/user-settings.md#build-flavour) or modify existing flavours using the flavour transformers notion seem below.
+
+The flavour to be used for a build root can be specified on the command-line
+via the `--flavour` flag. Moreover, this flavour name is saved in the build
+root; future invocations of Hadrian without a `--flavour` flag will default to
+the last-used flavour.
 
 ## Arguments
 
diff --git a/hadrian/src/Main.hs b/hadrian/src/Main.hs
index d2e0ace795..5e27d88846 100644
--- a/hadrian/src/Main.hs
+++ b/hadrian/src/Main.hs
@@ -116,7 +116,7 @@ main = do
         Environment.setupEnvironment
         return . Just $ (shake_opts, if null targets'
                                       then rules
-                                      else want targets' >> withoutActions rules)
+                                      else want targets' >> rules)
 
 handleShakeException :: IORef ShakeOptions -> IO a -> IO a
 handleShakeException shake_opts_var shake_run = do
diff --git a/hadrian/src/Rules.hs b/hadrian/src/Rules.hs
index 67646a8689..f427da5be5 100644
--- a/hadrian/src/Rules.hs
+++ b/hadrian/src/Rules.hs
@@ -128,6 +128,7 @@ packageRules = do
 
 buildRules :: Rules ()
 buildRules = do
+    Settings.flavourFileRules
     Rules.BinaryDist.bindistRules
     Rules.Generate.copyRules
     Rules.Generate.generateRules
diff --git a/hadrian/src/Settings.hs b/hadrian/src/Settings.hs
index 50ef988036..17f2f41906 100755
--- a/hadrian/src/Settings.hs
+++ b/hadrian/src/Settings.hs
@@ -3,7 +3,8 @@
 module Settings (
     getArgs, getLibraryWays, getRtsWays, flavour, knownPackages,
     findPackageByName, unsafeFindPackageByName, unsafeFindPackageByPath,
-    isLibrary, stagePackages, getBignumBackend, getBignumCheck, completeSetting
+    isLibrary, stagePackages, getBignumBackend, getBignumCheck, completeSetting,
+    flavourFileRules
     ) where
 
 import CommandLine
@@ -58,6 +59,49 @@ hadrianFlavours =
     , ghcInGhciFlavour, validateFlavour, slowValidateFlavour
     ]
 
+{-
+Note [Persisting the flavour]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+For a long time Hadrian relied on UserSettings.hs for all configuration.
+However, this approach to configuration is both inconvenient and interacts
+poorly with multiple build roots. For this reason we introduced flavour
+transformers, making it easier to modify the flavour from the command-line.
+However, passing the flavour on the command-line introduces another problem:
+the user must consistently pass the same --flavour flag to all Hadrian
+invocations on a particular build root. Forgetting even once can result in a
+full rebuild of the tree.
+
+To eliminate the inconvenience of needing to always pass --flavour arguments we
+persist the flavour name used to build a particular build root in the build
+root itself (specifically, in `<build root>/flavour`). We refer to this file
+when looking up the active flavour, defaulting to it if the user does not
+specify a flavour name explicitly.
+-}
+
+flavourFileRules :: Rules ()
+flavourFileRules = do
+    root <- buildRootRules
+    root -/- "flavour" %> \out -> do
+        flavourName <- getFlavourName
+        writeFile' out flavourName
+
+    -- Ensure that the persistent flavour file is always updated.
+    want [root -/- "flavour"]
+
+-- | Lookup the requested flavour name, referring to the persistent flavour
+-- file saved from the previous Hadrian execution if not specified.
+getFlavourName :: Action String
+getFlavourName = do
+    root <- buildRoot
+    let flavourFile = root -/- "flavour"
+    mbFlavourName <- cmdFlavour
+    case mbFlavourName of
+      Nothing -> do exists <- doesFileExist flavourFile
+                    if exists
+                      then liftIO $ readFile flavourFile
+                      else return userDefaultFlavour
+      Just nm -> return nm
+
 -- | This action looks up a flavour with the name given on the
 --   command line with @--flavour@, defaulting to 'userDefaultFlavour'
 --   when no explicit @--flavour@ is passed. It then applies any
@@ -66,7 +110,7 @@ hadrianFlavours =
 --   syntax. See Note [Hadrian settings] at the bottom of this file.
 flavour :: Action Flavour
 flavour = do
-    flavourName <- fromMaybe userDefaultFlavour <$> cmdFlavour
+    flavourName <- getFlavourName
     kvs <- userSetting ([] :: [KeyVal])
     let flavours = hadrianFlavours ++ userFlavours
         (settingErrs, tweak) = applySettings kvs
@@ -96,4 +140,4 @@ unsafeFindPackageByName name = fromMaybe (error msg) $ findPackageByName name
 unsafeFindPackageByPath :: FilePath -> Package
 unsafeFindPackageByPath path = err $ find (\pkg -> pkgPath pkg == path) knownPackages
   where
-    err = fromMaybe $ error ("findPackageByPath: No package for path " ++ path)
\ No newline at end of file
+    err = fromMaybe $ error ("findPackageByPath: No package for path " ++ path)