fdReadBuf001 fails non-deterministically on FreeBSD

With this patch to libraries/unix,

diff --git a/tests/fdReadBuf001.hs b/tests/fdReadBuf001.hs
index 4c121a2..f987c94 100644
--- a/tests/fdReadBuf001.hs
+++ b/tests/fdReadBuf001.hs
@@ -21,7 +21,7 @@ main = do
            r <- fdReadBuf rd p block
            let (chunk,rest) = splitAt (fromIntegral r) text
            chars <- peekArray (fromIntegral r) p
-           when (chars /= chunk) $ error "mismatch"
+           when (chars /= chunk) $ error $ "mismatch: expected="++show chunk++", found="++show chars
            when (null rest) $ exitWith ExitSuccess
            loop rest
     loop bytes

fdReadBuf001 sometimes fails under the ghci way on FreeBSD 11.0 with something like,

=====> fdReadBuf001(ghci) 1 of 1 [0, 0, 0]
cd "../../libraries/unix/tests/fdReadBuf001.run" && "/home/ben/ghc-8.2.1/bin/ghc" fdReadBuf001.hs -dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow-warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno-debug-output  --interactive -v0 -ignore-dot-ghci -fno-ghci-history +RTS -I0.1 -RTS -package unix< fdReadBuf001.genscript
Wrong exit code for fdReadBuf001(ghci) (expected 0 , actual 1 )
Stderr ( fdReadBuf001 ):
fdReadBuf001: mismatch: expected=[101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112], found=[168,149,144,14,8,0,0,0,58,1,0,0,0,0,0,0,32,228,123,10,8,0,0,0,200,149,144,14,8,0,0,0,0,150,144,14,8,0,0,0,96,127,174,10,8,0,0,0,33,152,144,14,8,0,0,0,0,150,144,14,8,0,0,0,42,0,0,0,0,0,0,0,96,127,174,10,8,0,0,0,162,21,74,13,8,0,0,0,56,150,144,14,8,0,0,0,96,127,174,10,8,0,0,0,10,152,144,14,8,0,0,0,56,150,144,14,8,0,0,0,32,228,123,10,8,0,0,0,200,149,144,14,8,0,0,0,112,150,144,14,8,0,0,0,112,209,123,10,8,0,0,0,138,150,144,14,8,0,0,0,88,150,144,14,8,0,0,0,96,127,174,10,8,0,0,0,249,150,144,14,8,0,0,0,48,187,247,9,8,0,0,0,129,223,208,10,8,0,0,0,96,127,174,10,8,0,0,0,9,151,144,14,8,0,0,0,16,46,250,9,8,0,0,0,184,150,144,14,8,0,0,0,224,227,123,10,8,0,0,0,111,0,0,0,0,0,0,0,8,246,255,9,8,0,0,0,111,0,0,0,0,0,0,0,224,0,194,9,8,0,0,0]
CallStack (from HasCallStack):
  error, called at fdReadBuf001.hs:24:36 in main:Main

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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