Fix cabal on Windows 7 (backport #7844)

Cabal/src/Distribution/Compat/Process.hs

@@ -18,23 +18,44 @@ import qualified System.Process as Process
 import           System.Process (waitForProcess)
 #endif
 
+#if defined(mingw32_HOST_OS) && MIN_VERSION_process(1,6,9)
+import           System.IO.Unsafe (unsafePerformIO)
+import           System.Win32.Info.Version (dwMajorVersion, dwMinorVersion, getVersionEx)
+#endif
+
 -------------------------------------------------------------------------------
 -- enableProcessJobs
 -------------------------------------------------------------------------------
 
+#if defined(mingw32_HOST_OS) && MIN_VERSION_process(1,6,9)
+-- This exception, needed to support Windows 7, could be removed when
+-- the lowest GHC version cabal supports is a GHC that doesn’t support
+-- Windows 7 any more.
+{-# NOINLINE isWindows8OrLater #-}
+isWindows8OrLater :: Bool
+isWindows8OrLater = unsafePerformIO $ do
+  v <- getVersionEx
+  pure $ (dwMajorVersion v, dwMinorVersion v) >= (6, 2)
+#endif
+
 -- | Enable process jobs to ensure accurate determination of process completion
 -- in the presence of @exec(3)@ on Windows.
 --
 -- Unfortunately the process job support is badly broken in @process@ releases
 -- prior to 1.6.9, so we disable it in these versions, despite the fact that
--- this means we may see sporatic build failures without jobs.
+-- this means we may see sporadic build failures without jobs.
+--
+-- On Windows 7 or before the jobs are disabled due to the fact that
+-- processes on these systems can only have one job. This prevents
+-- spawned process from assigning jobs to its own children. Suppose
+-- processs A spawns process B. The B process has a job assigned (call
+-- it J1) and when it tries to spawn a new process C the C
+-- automatically inherits the job. But at it also tries to assign a
+-- new job J2 to C since it doesn’t have access J1. This fails on
+-- Windows 7 or before.
 enableProcessJobs :: CreateProcess -> CreateProcess
-#ifdef MIN_VERSION_process
-#if MIN_VERSION_process(1,6,9)
-enableProcessJobs cp = cp {Process.use_process_jobs = True}
-#else
-enableProcessJobs cp = cp
-#endif
+#if defined(mingw32_HOST_OS) && MIN_VERSION_process(1,6,9)
+enableProcessJobs cp = cp {Process.use_process_jobs = isWindows8OrLater}
 #else
 enableProcessJobs cp = cp
 #endif

changelog.d/pr-7844

@@ -0,0 +1,8 @@
+synopsis: Disable job management on Windows 7
+packages: Cabal
+prs: #7844
+significance: significant
+
+description: {
+- cabal now works on Windows 7
+}