All tests should give some portion of data to the producer and use the rest.

tests/fuzz/block_round_trip.c

@@ -52,9 +52,13 @@ static size_t roundTripTest(void *result, size_t resultCapacity,
 
 int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size)
 {
+    /* Give a random portion of src data to the producer, to use for
+    parameter generation. The rest will be used for (de)compression */
     FUZZ_dataProducer_t *producer = FUZZ_dataProducer_create(src, size);
+    size_t producerSliceSize = FUZZ_dataProducer_uint32Range(producer, 0, size);
+    size = FUZZ_dataProducer_contract(producer, producerSliceSize);
+
     int cLevel = FUZZ_dataProducer_uint32(producer) % kMaxClevel;
-    size = FUZZ_dataProducer_remainingBytes(producer);
 
     size_t neededBufSize = size;
     if (size > ZSTD_BLOCKSIZE_MAX)