testsuite: Be explicit about runtime test dependencies

cabal-testsuite/README.md

@@ -96,6 +96,15 @@ Otherwise, here is a walkthrough:
  ...
  ```
 
+ The dependencies which your test is allowed to use are listed in the
+ cabal file under the `test-runtime-deps` executable. At compile-time there is
+ a custom Setup.hs script which inspects this list and records the versions of
+ each package in a generated file. These are then used when `cabal-tests` runs
+ when it invokes `runghc` to run each test.
+ We ensure they are built and available by listing `test-runtime-deps` in the
+ build-tool-depends section of the cabal-tests executable.
+
+
 3. Run your tests using `cabal-tests` (no need to rebuild when
  you add or modify a test; it is automatically picked up).
  The first time you run a test, assuming everything else is

cabal-testsuite/Setup.hs

@@ -73,7 +73,7 @@ canonicalizePackageDB x = return x
 -- non-Backpack.
 cabalTestsPackages :: LocalBuildInfo -> [(OpenUnitId, ModuleRenaming)]
 cabalTestsPackages lbi =
- case componentNameCLBIs lbi (CExeName (mkUnqualComponentName "cabal-tests")) of
+ case componentNameCLBIs lbi (CExeName (mkUnqualComponentName "test-runtime-deps")) of
  [clbi] -> -- [ (unUnitId $ unDefUnitId duid,rn) | (DefiniteUnitId duid, rn) <- componentIncludes clbi ]
  componentIncludes clbi
  _ -> error "cabalTestsPackages"

cabal-testsuite/cabal-testsuite.cabal

@@ -90,6 +90,8 @@ executable cabal-tests
  main-is: cabal-tests.hs
  hs-source-dirs: main
  ghc-options: -threaded
+ -- Make sure these are built before the executable is run
+ build-tool-depends: cabal-testsuite:test-runtime-deps
  build-depends:
  , cabal-testsuite
  -- constraints inherited via lib:cabal-testsuite component
@@ -101,18 +103,6 @@ executable cabal-tests
  , transformers
  -- dependencies specific to exe:cabal-tests
  , clock ^>= 0.7.2 || ^>=0.8
- -- Extra dependencies used by PackageTests.
- --
- -- The runner allows the tests to use extra dependencies and the custom Prelude
- -- from 'cabal-testsuite'.
- -- However, if the tests use a dependency, say 'directory', and there are two
- -- packages with the same unit id available in the store, the test fails since
- -- it doesn't know which one to pick.
- -- By including an extra dependency to directory, we force the test runner to
- -- use a specific version directory, fixing the test failure.
- --
- -- See issue description and discussion: https://github.com/haskell/cabal/issues/8356
- , directory
 
  build-tool-depends: cabal-testsuite:setup
  default-extensions: TypeOperators
@@ -122,6 +112,32 @@ executable setup
  import: shared
  main-is: Setup.simple.hs
 
+-- This executable component is used to describe the runtime dependencies of
+-- the tests. The Main.hs file and resulting executable are not useful in any way.
+
+-- Ideally this would be an empty library, but because build-type: Custom, we can't
+-- have sublibraries.
+
+-- If you require an external dependency for a test it must be listed here.
+executable test-runtime-deps
+ build-depends: cabal-testsuite,
+ base,
+ directory,
+ Cabal,
+ Cabal-syntax,
+ filepath,
+ transformers,
+ bytestring,
+ time,
+ process,
+ exceptions
+ main-is: static/Main.hs
+ if !os(windows)
+ build-depends: unix
+ else
+ build-depends:
+ , Win32
+
 custom-setup
  -- we only depend on even stable releases of lib:Cabal
  -- and due to Custom complexity and ConstraintSetupCabalMaxVersion

cabal-testsuite/src/Test/Cabal/Script.hs

@@ -93,6 +93,7 @@ runnerGhcArgs senv =
  where
  ghc_options = M.mempty { ghcOptPackageDBs = runnerPackageDbStack senv
  , ghcOptPackages = toNubListR (runnerPackages senv)
+ , ghcOptHideAllPackages = Flag True
  -- Avoid picking stray module files that look
  -- like our imports
  , ghcOptSourcePathClear = Flag True }

cabal-testsuite/static/Main.hs

@@ -0,0 +1,3 @@
+module Main where
+
+main = return ()