base: Validate input in setNumCapabilities

Test Plan: validate

Reviewers: austin, hvr, erikd, simonmar

Subscribers: rwbarton, thomie

GHC Trac Issues: #13832

Differential Revision: https://phabricator.haskell.org/D3652

libraries/base/GHC/Conc/Sync.hs

@@ -379,7 +379,9 @@ to avoid contention with other processes in the machine.
 @since 4.5.0.0
 -}
 setNumCapabilities :: Int -> IO ()
-setNumCapabilities i = c_setNumCapabilities (fromIntegral i)
+setNumCapabilities i
+  | i <= 0    = fail $ "setNumCapabilities: Capability count ("++show i++") must be positive"
+  | otherwise = c_setNumCapabilities (fromIntegral i)
 
 foreign import ccall safe "setNumCapabilities"
   c_setNumCapabilities :: CUInt -> IO ()

testsuite/tests/rts/T13832.hs

+import GHC.Conc
+
+main :: IO ()
+main = setNumCapabilities 0

testsuite/tests/rts/T13832.stderr

+T13832: user error (setNumCapabilities: Capability count (0) must be positive)

testsuite/tests/rts/all.T

@@ -372,4 +372,4 @@ test('T12497', [ unless(opsys('mingw32'), skip)
                run_command, ['$MAKE -s --no-print-directory T12497'])
 
 test('T12903', [when(opsys('mingw32'), skip)], compile_and_run, [''])
-
+test('T13832', exit_code(1), compile_and_run, ['-threaded'])