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_json.cpp

@@ -17,6 +17,12 @@ array data, it performs the following steps:
 - j2 = parse(s1)
 - s2 = serialize(j2)
 - assert(s1 == s2)
+- for ignore_comments = false and true: assert(parse(data) and accept(data)
+  give the same result for a stream, a non-contiguous iterator range, and a
+  FILE* as for the pointer range)
+
+The pointer range takes the bulk fast paths, the other input adapters read byte
+by byte; see check_input_adapters().
 
 The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
 drivers.
@@ -24,6 +30,32 @@ drivers.
 
 #include "fuzzer_common.hpp"
 
+namespace
+{
+// json::parse and json::accept for check_input_adapters()
+struct parse_json
+{
+    bool ignore_comments;
+
+    template<typename... Args>
+    json operator()(Args&& ... args) const
+    {
+        return json::parse(std::forward<Args>(args)..., nullptr, true, ignore_comments);
+    }
+};
+
+struct accept_json
+{
+    bool ignore_comments;
+
+    template<typename... Args>
+    bool operator()(Args&& ... args) const
+    {
+        return json::accept(std::forward<Args>(args)..., ignore_comments);
+    }
+};
+} // namespace
+
 // see http://llvm.org/docs/LibFuzzer.html
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
@@ -76,6 +108,16 @@ 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
+    for (const bool ignore_comments :
+            {
+                false, true
+            })
+    {
+        check_input_adapters(data, size, parse_json{ignore_comments});
+        check_input_adapters(data, size, accept_json{ignore_comments});
+    }
+
     // return 0 - non-zero return values are reserved for future use
     return 0;
 }