Fail on errors when building fuzzers

Fails on errors when building fuzzers with `fuzz.py` (adds `Werror`).
Currently allows `declaration-after-statement`, `c++-compat` and
`deprecated` as they are abundant in code (some fixes to
`declaration-after-statement` are presented in this commit).

tests/fuzz/fuzz.py

@@ -405,7 +405,12 @@ def build(args):
     cxxflags = shlex.split(args.cxxflags)
     mflags = shlex.split(args.mflags)
     # Flags to be added to both cflags and cxxflags
-    common_flags = []
+    common_flags = [
+        '-Werror',
+        '-Wno-error=declaration-after-statement',
+        '-Wno-error=c++-compat',
+        '-Wno-error=deprecated' # C files are sometimes compiled with CXX
+    ]
 
     cppflags += [
         '-DDEBUGLEVEL={}'.format(args.debug),

tests/fuzz/fuzz_data_producer.c

@@ -28,12 +28,12 @@ void FUZZ_dataProducer_free(FUZZ_dataProducer_t *producer) { free(producer); }
 
 uint32_t FUZZ_dataProducer_uint32Range(FUZZ_dataProducer_t *producer, uint32_t min,
                                   uint32_t max) {
-    FUZZ_ASSERT(min <= max);
-
     uint32_t range = max - min;
     uint32_t rolling = range;
     uint32_t result = 0;
 
+    FUZZ_ASSERT(min <= max);
+
     while (rolling > 0 && producer->size > 0) {
       uint8_t next = *(producer->data + producer->size - 1);
       producer->size -= 1;
@@ -79,11 +79,11 @@ int FUZZ_dataProducer_empty(FUZZ_dataProducer_t *producer) {
 
 size_t FUZZ_dataProducer_contract(FUZZ_dataProducer_t *producer, size_t newSize)
 {
-    newSize = newSize > producer->size ? producer->size : newSize;
+    const size_t effectiveNewSize = newSize > producer->size ? producer->size : newSize;
 
-    size_t remaining = producer->size - newSize;
+    size_t remaining = producer->size - effectiveNewSize;
     producer->data = producer->data + remaining;
-    producer->size = newSize;
+    producer->size = effectiveNewSize;
     return remaining;
 }
 

tests/fuzz/regression_driver.c

@@ -44,11 +44,12 @@ int main(int argc, char const **argv) {
     fprintf(stderr, "WARNING: No files passed to %s\n", argv[0]);
   for (i = 0; i < files->tableSize; ++i) {
     char const *fileName = files->fileNames[i];
-    DEBUGLOG(3, "Running %s", fileName);
     size_t const fileSize = UTIL_getFileSize(fileName);
     size_t readSize;
     FILE *file;
 
+    DEBUGLOG(3, "Running %s", fileName);
+
     /* Check that it is a regular file, and that the fileSize is valid.
      * If it is not a regular file, then it may have been deleted since we
      * constructed the list, so just skip it, but return an error exit code.

tests/fuzz/simple_decompress.c

@@ -37,9 +37,9 @@ int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size)
         FUZZ_ASSERT(dctx);
     }
 
+    {
         size_t const bufSize = FUZZ_dataProducer_uint32Range(producer, 0, 10 * size);
         void *rBuf = FUZZ_malloc(bufSize);
-
         size_t const dSize = ZSTD_decompressDCtx(dctx, rBuf, bufSize, src, size);
         if (!ZSTD_isError(dSize)) {
             /* If decompression was successful, the content size from the frame header(s) should be valid. */
@@ -48,6 +48,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size)
             FUZZ_ASSERT(expectedSize == ZSTD_CONTENTSIZE_UNKNOWN || expectedSize == dSize);
         }
         free(rBuf);
+    }
 
     FUZZ_dataProducer_free(producer);
 

tests/fuzz/stream_round_trip.c

@@ -136,7 +136,7 @@ static size_t compress(uint8_t *dst, size_t capacity,
     return dstSize;
 }
 
-size_t decompress(void* dst, size_t dstCapacity, void const* src, size_t srcSize, FUZZ_dataProducer_t* producer)
+static size_t decompress(void* dst, size_t dstCapacity, void const* src, size_t srcSize, FUZZ_dataProducer_t* producer)
 {
     ZSTD_inBuffer in = {src, srcSize, 0};
     ZSTD_outBuffer out = {dst, dstCapacity, 0};