contrib/fuzz/fuzzutil.h

posix: always seek to EOF when opening a file in append mode

Python 3 already does this, so skip it there.

Consider the program:

   #include <stdio.h>

   int main() {
     FILE *f = fopen("narf", "w");
     fprintf(f, "narf\n");
     fclose(f);

     f = fopen("narf", "a");
     printf("%ld\n", ftell(f));
     fprintf(f, "troz\n");
     printf("%ld\n", ftell(f));

     return 0;
   }

on macOS, FreeBSD, and Linux with glibc, this program prints

   5
   10

but on musl libc (Alpine Linux and probably others) this prints

   0
   10

By my reading of
https://pubs.opengroup.org/onlinepubs/009695399/functions/fopen.html
this is technically correct, specifically:

> Opening a file with append mode (a as the first character in the
> mode argument) shall cause all subsequent writes to the file to be
> forced to the then current end-of-file, regardless of intervening
> calls to fseek().

in other words, the file position doesn't really matter in append-mode
files, and we can't depend on it being at all meaningful unless we
perform a seek() before tell() after open(..., 'a'). Experimentally
after a .write() we can do a .tell() and it'll always be reasonable,
but I'm unclear from reading the specification if that's a smart thing
to rely on. This matches what we do on Windows and what Python 3 does
for free, so let's just be consistent. Thanks to Yuya for the idea.

#ifndef CONTRIB_FUZZ_FUZZUTIL_H
#define CONTRIB_FUZZ_FUZZUTIL_H
#include <iostream>
#include <memory>
#include <stdint.h>

/* Try and use std::optional, but failing that assume we'll have a
 * workable https://abseil.io/ install on the include path to get
 * their backport of std::optional. */
#ifdef __has_include
#if __has_include(<optional>) && __cplusplus >= 201703L
#include <optional>
#define CONTRIB_FUZZ_HAVE_STD_OPTIONAL
#endif
#endif
#ifdef CONTRIB_FUZZ_HAVE_STD_OPTIONAL
namespace contrib
{
using std::nullopt;
using std::optional;
} /* namespace contrib */
#else
#include "third_party/absl/types/optional.h"
namespace contrib
{
using absl::nullopt;
using absl::optional;
} /* namespace contrib */
#endif

/* set DEBUG to 1 for a few debugging prints, or 2 for a lot */
#define DEBUG 0
#define LOG(level)                                                             \
	if (level <= DEBUG)                                                    \
	std::cout

struct two_inputs {
	std::unique_ptr<char[]> right;
	size_t right_size;
	std::unique_ptr<char[]> left;
	size_t left_size;
};

/* Split a non-zero-length input into two inputs. */
contrib::optional<two_inputs> SplitInputs(const uint8_t *Data, size_t Size);

#endif /* CONTRIB_FUZZ_FUZZUTIL_H */