Change behaviour in corner case of empty decompression input

The incremental decompressor would ask for input again after being
given empty initially. It should just ask once. This case is always
an error anyway, but the decompressor should never ask for more input
after having been given the end of input indication.

Author: dcoutts

Codec/Compression/Zlib/Internal.hs

@@ -595,8 +595,14 @@ decompressStream format (DecompressParams bits initChunkSize mdict allMembers)
                   else assert outputBufferFull $
                        Stream.inflateInit format bits
       case chunk of
-        _ | S.null chunk ->
-          fillBuffers 4  --always an error anyway
+        _ | S.null chunk -> do
+          -- special case to avoid demanding more input again
+          -- always an error anyway
+          when outputBufferFull $ do
+            let outChunkSize = 1
+            outFPtr <- Stream.unsafeLiftIO (S.mallocByteString outChunkSize)
+            Stream.pushOutputBuffer outFPtr 0 outChunkSize
+          drainBuffers True
 
         S.PS inFPtr offset length -> do
           Stream.pushInputBuffer inFPtr offset length

test/Test.hs

@@ -50,6 +50,7 @@ main = defaultMain $
       testCase "handle trailing data"      test_trailing_data,
       testCase "multiple gzip members"     test_multiple_members,
       testCase "check small input chunks"  test_small_chunks,
+      testCase "check empty input"         test_empty,
       testCase "check exception raised"    test_exception
     ]
   ]
@@ -244,6 +245,20 @@ test_small_chunks = do
   compressedFile   <- readSampleData "hello.gz"
   (GZip.decompress . smallChunks) compressedFile @?= GZip.decompress compressedFile
 
+test_empty :: Assertion
+test_empty = do
+  -- Regression test to make sure we only ask for input once in the case of
+  -- initially empty input. We previously asked for input twice before
+  -- returning the error.
+  let decomp = decompressIO zlibFormat defaultDecompressParams
+  case decomp of
+    DecompressInputRequired next -> do
+      decomp' <- next BS.empty
+      case decomp' of
+        DecompressStreamError TruncatedInput -> return ()
+        _ -> assertFailure "expected truncated error"
+
+    _ -> assertFailure "expected input"
 
 test_exception :: Assertion
 test_exception =