Fix race condition

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

@@ -55,19 +55,13 @@ run _verbosity mb_cwd env_overrides path0 args input = do
     (readstdout, writestdout) <- Compat.createPipe
     (readstderr, writestderr) <- Compat.createPipe
     (readall, writeall) <- Compat.createPipe
-    traverse_ (`hSetBuffering` LineBuffering) [ readstdout, writestdout, readstderr, writestderr, readall, writeall ]
+    traverse_ (`hSetBuffering` LineBuffering) [ stdout, readstdout, writestdout, readstderr, writestderr, readall, writeall ]
     let mkDrain h = do
-            r <- hGetContents h
-            length r `seq` hClose h
+            r <- hGetContents' h
             hPutStr writeall r
             return r
-    let mkDrain' h = do
-            r <- hGetContents h
-            length r `seq` hClose h
-            return r
     withAsync (mkDrain readstdout) $ \syncstdout -> do
     withAsync (mkDrain readstderr) $ \syncstderr -> do
-    withAsync (mkDrain' readall)   $ \syncall -> do
 
     let prc = (proc path args)
           { cwd = mb_cwd
@@ -90,7 +84,8 @@ run _verbosity mb_cwd env_overrides path0 args input = do
     rStdout <- wait syncstdout
     rStderr <- wait syncstderr
     hClose writeall
-    rAll <- wait syncall
+
+    rAll <- hGetContents' readall
 
     return Result {
             resultExitCode = exitcode,
@@ -99,3 +94,10 @@ run _verbosity mb_cwd env_overrides path0 args input = do
             resultStdout = rStdout,
             resultStderr = rStderr
         }
+
+-- `hGetContents'` is in since base-4.15.0.0 -- which we don't have.
+hGetContents' :: Handle -> IO String
+hGetContents' h = do
+  v <- hGetContents h
+  length v `seq` hClose h
+  pure v