Add regression tests for infinitely looping shrinking of allNull `E…

…rrors`.

When `allNull` holds for arbitrarily generated `Errors` and we start shrinking,
the shrinker will loop forever. The cause lies in the `genInfinite` function for
`Stream`s, which generates a finite list even though it is marked as infinite.
The next commit introduces a minimal bug fix.

fs-sim/test/Test/System/FS/Sim/Error.hs

@@ -140,6 +140,13 @@ tests = testGroup "Test.System.FS.Sim.Error" [
                 MockFS.fromBuffer mba 0 (fromIntegral $ BS.length bs) >>=
                   maybe (error "fromOutput: should not fail") pure
         in  propGetterGetsAll hGetBufSomeAtC get fromOutput p bs
+
+    -- Generators and shrinkers
+
+    , testProperty "prop_regression_shrinkNotAllNullErrors"
+        prop_regression_shrinkNotAllNullErrors
+    , testProperty "prop_regression_shrinkAllNullErrors"
+        prop_regression_shrinkAllNullErrors
     ]
 
 instance Arbitrary BS.ByteString where
@@ -242,3 +249,15 @@ propGetterGetsAll getCounter get fromOutput (SometimesPartialReads errStream) bs
       , hGetBufSomeE = errStream
       , hGetBufSomeAtE = errStream
       }
+
+{-------------------------------------------------------------------------------
+  Generators and shrinkers
+-------------------------------------------------------------------------------}
+
+prop_regression_shrinkNotAllNullErrors :: Errors -> Property
+prop_regression_shrinkNotAllNullErrors errs = expectFailure $
+    not (allNull errs) ==> property False
+
+prop_regression_shrinkAllNullErrors :: Errors -> Property
+prop_regression_shrinkAllNullErrors errs = expectFailure $
+    allNull errs ==> property False