Test suite: create symlinks as needed.

Fixes #3190.

Cabal/Cabal.cabal

@@ -414,7 +414,7 @@ test-suite package-tests
     regex-posix,
     old-time
   if !os(windows)
-    build-depends: unix
+    build-depends: unix, exceptions
   ghc-options: -Wall -rtsopts
   default-extensions: CPP
   default-language: Haskell98

Cabal/tests/PackageTests/GhcPkgGuess/Symlink/ghc

-bin/ghc
\ No newline at end of file

Cabal/tests/PackageTests/GhcPkgGuess/SymlinkGhcVersion/ghc

-bin/ghc-7.10
\ No newline at end of file

Cabal/tests/PackageTests/GhcPkgGuess/SymlinkVersion/ghc

-bin/ghc-7.10
\ No newline at end of file

Cabal/tests/PackageTests/PackageTester.hs

@@ -57,6 +57,7 @@ module PackageTests.PackageTester
     , assertFindInFile
     , concatOutput
     , ghcFileModDelay
+    , withSymlink
 
     -- * Test trees
     , TestTreeM
@@ -126,6 +127,12 @@ import System.Process (runProcess, waitForProcess, showCommandForUser)
 import Control.Concurrent (threadDelay)
 import Test.Tasty (TestTree, askOption, testGroup)
 
+#ifndef mingw32_HOST_OS
+import Control.Monad.Catch ( bracket_ )
+import System.Directory    ( removeFile )
+import System.Posix.Files  ( createSymbolicLink )
+#endif
+
 -- | Our test monad maintains an environment recording the global test
 -- suite configuration 'SuiteConfig', and the local per-test
 -- configuration 'TestConfig'.
@@ -741,6 +748,20 @@ ghcFileModDelay = do
               = mtimeChangeDelay suite
     liftIO $ threadDelay delay
 
+-- | Create a symlink for the duration of the provided action. If the symlink
+-- already exists, it is deleted. Does not work on Windows.
+withSymlink :: FilePath -> FilePath -> TestM a -> TestM a
+#ifdef mingw32_HOST_OS
+withSymlink _oldpath _newpath _act =
+  error "PackageTests.PackageTester.withSymlink: does not work on Windows!"
+#else
+withSymlink oldpath newpath act = do
+  symlinkExists <- liftIO $ doesFileExist newpath
+  when symlinkExists $ liftIO $ removeFile newpath
+  bracket_ (liftIO $ createSymbolicLink oldpath newpath)
+           (liftIO $ removeFile newpath) act
+#endif
+
 ------------------------------------------------------------------------
 -- * Test trees
 

Cabal/tests/PackageTests/Tests.hs

@@ -270,9 +270,21 @@ tests config = do
       tc "GhcPkgGuess/SameDirectoryGhcVersion" $ ghc_pkg_guess "ghc-7.10"
 
   unlessWindows $ do
-      tc "GhcPkgGuess/Symlink" $ ghc_pkg_guess "ghc"
-      tc "GhcPkgGuess/SymlinkVersion" $ ghc_pkg_guess "ghc"
-      tc "GhcPkgGuess/SymlinkGhcVersion" $ ghc_pkg_guess "ghc"
+      tc "GhcPkgGuess/Symlink" $ do
+        -- We don't want to distribute a tarball with symlinks. See #3190.
+        withSymlink "bin/ghc"
+                    "tests/PackageTests/GhcPkgGuess/Symlink/ghc" $
+                    ghc_pkg_guess "ghc"
+
+      tc "GhcPkgGuess/SymlinkVersion" $ do
+        withSymlink "bin/ghc-7.10"
+                    "tests/PackageTests/GhcPkgGuess/SymlinkVersion/ghc" $
+                    ghc_pkg_guess "ghc"
+
+      tc "GhcPkgGuess/SymlinkGhcVersion" $ do
+        withSymlink "bin/ghc-7.10"
+                    "tests/PackageTests/GhcPkgGuess/SymlinkGhcVersion/ghc" $
+                    ghc_pkg_guess "ghc"
 
   -- Basic test for internal libraries (in p); package q is to make
   -- sure that the internal library correctly is used, not the