Fuzz the streaming input adapters against the contiguous one (#5827)

* Fuzz the streaming input adapters against the contiguous one

The fast paths for contiguous input (bulk string scanning, multi-byte
loads, lazy position tracking) are taken only when parsing from a
pointer range; streams and non-contiguous iterators still read byte by
byte. The fuzzer drivers parsed only contiguous input, so a difference
between the two would ship unnoticed.

check_input_adapters() in fuzzer_common.hpp reads the input through a
std::istringstream, a std::list iterator range, and (on POSIX) a FILE*
from fmemopen, and asserts the same value, or the same exception id,
message and byte position, as for the pointer range. All seven drivers
call it; the JSON driver checks parse and accept, each with and without
ignore_comments. It replaces the stream check of the BON8 driver.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Rename the reader parameter to avoid a flawfinder false positive

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/fuzzer-parse_cbor.cpp

@@ -17,12 +17,30 @@ array data, it performs the following steps:
 - j2 = from_cbor(vec)
 - assert(to_cbor(j2) == vec)
 
+Finally, it reads the data from a stream, a non-contiguous iterator range, and
+a FILE*, which read byte by byte, and checks that each gives the same value or
+error (id, message, byte position) as the pointer range, which takes the bulk
+fast paths (see check_input_adapters()).
+
 The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
 drivers.
 */
 
 #include "fuzzer_common.hpp"
 
+namespace
+{
+// json::from_cbor for check_input_adapters()
+struct read_cbor
+{
+    template<typename... Args>
+    json operator()(Args&& ... args) const
+    {
+        return json::from_cbor(std::forward<Args>(args)...);
+    }
+};
+} // namespace
+
 // see http://llvm.org/docs/LibFuzzer.html
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
@@ -77,6 +95,9 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
     }
 
+    // step 3: the other input adapters must read the data alike
+    check_input_adapters(data, size, read_cbor());
+
     // return 0 - non-zero return values are reserved for future use
     return 0;
 }