Fix hd-view package-qualified imports bypassing home-unit

haskell-debugger/GHC/Debugger/Monad.hs

@@ -94,6 +94,8 @@ import GHC.Platform.Ways
 import GHC.Data.FastString.Env (emptyFsEnv)
 #endif
 
+import GHC.Unit.Home.Graph
+
 -- | A debugger action.
 newtype Debugger a = Debugger { unDebugger :: ReaderT DebuggerState GHC.Ghc a }
   deriving ( Functor, Applicative, Monad, MonadIO
@@ -428,11 +430,13 @@ runDebugger l rootDir compDir libdir units ghcInvocation' extraGhcArgs mainFp co
         let preludeImp = GHC.IIDecl . GHC.simpleImportDecl $ GHC.mkModuleName "Prelude"
         -- dbgView should always be available, either because we manually loaded it
         -- or because it's in the transitive closure.
+        hug <- hsc_HUG <$> getSession
         let dbgViewImps
-              -- Using in-memory hs-dbg-view. It's a home-unit, so refer to it directly
-              | hdv_uid == hsDebuggerViewInMemoryUnitId
+              -- If hs-dbg-view is a home-unit, refer to it directly
+              -- See Note [Do not package-qualify imports for home units]
+              | memberHugUnitId hdv_uid hug
               = map (GHC.IIModule . mkModule (RealUnit (Definite hdv_uid))) loadedBuiltinModNames
-              -- It's available in a unit in the transitive closure. Resolve it.
+              -- It's available in an exposed unit in the transitive closure. Resolve it
               | otherwise
               = map (\mn ->
                   GHC.IIDecl (GHC.simpleImportDecl mn)
@@ -507,9 +511,7 @@ add our own `MC.finally cleanupInterp` call which sends the `Shutdown` message
 to the external interpreter before propagating the exception further (to GHC's
 `withCleanupSession`, which will now do Nothing because we set `InterpPending`,
 and beyond).
--}
 
-{-
 Note [Must explicitly expose module graph units]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 `interactiveGhcDebugger` is our "current home unit", so its
@@ -533,6 +535,33 @@ An alternative, closer to what ghci does, would be to copy the `packageFlags`
 from the debuggee units, however doing so doesn't take care of fixing a unitId
 for dependencies of dependencies.
 
+Note [Do not package-qualify imports for home units]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A package-qualified module import will be looked up directly in the exposed
+packages, IGNORING the home units modules.
+
+This can lead to two scenarios:
+
+  1) a package-import of a loaded unit fails, because that unit, despite being
+  loaded, is not installed
+
+  2) a package-import of a loaded unit succeeds, because a unit with the same
+  name (but not necessarily the same unit-id!), is installed.
+
+The second case can result in subtly wrong interactive sessions, where the
+package-qualified imported module shadows the loaded module. Perhaps GHC could
+warn about this. Cabal-repl and ghci also suffer from this subtle interaction.
+
+In light of this, when the debugger imports the `haskell-debugger-view` modules,
+it is imperative that if the `haskell-debugger-view` unit is in the home units
+(e.g. if `haskell-debugger-view` is listed in the cabal.project, like it is in
+the debugger tree), we do not use a package-qualified import.
+
+On the other hand, if the `haskell-debugger-view` package is not in the
+home-units, we *should* package-qualify it to make sure we reference the right
+one.
+
+See also #283
 -}
 
 -- | See Note [Shutting down the external interpreter]

test/golden/T283/T283.cabal

@@ -0,0 +1,18 @@
+cabal-version:   3.16
+name:            T283
+version:         0.1.0.0
+license:         NONE
+author:          Rodrigo Mesquita
+maintainer:      rodrigo.m.mesquita@gmail.com
+build-type:      Simple
+extra-doc-files: CHANGELOG.md
+
+common warnings
+    ghc-options: -Wall
+
+executable T283
+    import:           warnings
+    main-is:          Main.hs
+    build-depends:    base
+    hs-source-dirs:   app
+    default-language: Haskell2010

test/golden/T283/T283.ghc-914.hdb-stdout

@@ -0,0 +1,10 @@
+[1 of 3] Compiling GHC.Debugger.View.Class ( in-memory:GHC.Debugger.View.Class, interpreted )[haskell-debugger-view-in-memory]
+[2 of 3] Compiling Main             ( <TEMPORARY-DIRECTORY>/app/Main.hs, interpreted )[T283-0.1.0.0-inplace-T283]
+(hdb) 
+(hdb) THIS IS A PACKAGE
+()
+(hdb) Aborted: <interactive>:1:1: error: [GHC-35235]
+    Could not find module `Main'.
+    It is not a module in the current program, or in any known package.
+(hdb) "If an older version of T283 was installed, we'd now shadow the home-unit Main with the outdated installed Main!"
+(hdb) Exiting...

test/golden/T283/T283.hdb-stdin

@@ -0,0 +1,4 @@
+p import Main
+p main
+p import "T283" Main
+p "If an older version of T283 was installed, we'd now shadow the home-unit Main with the outdated installed Main!"

test/golden/T283/T283.hdb-test

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+# Witnesses that package-qualified imports bypass home units (#283)
+$HDB -v0 app/Main.hs 2>&1 < T283.hdb-stdin

test/golden/T283/app/Main.hs

@@ -0,0 +1,4 @@
+module Main (main) where
+
+main :: IO ()
+main = putStrLn "THIS IS A PACKAGE"

test/haskell/Main.hs

@@ -44,7 +44,7 @@ import Test.Utils
 main :: IO ()
 main = do
   env <- getEnvironment
-  let mkTest = mkGoldenTest False env
+  let mkTest = mkGoldenTest (maybe False read (lookup "KEEP_TEMP_DIRS" env)) env
   golden_tests_paths <- findByExtension [".hdb-test"] "test/golden"
     `catch` \(e::IOException) -> do
        hPutStrLn stderr "-- !! ERROR !! -----------------------------------------------------------------"