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.
#include "fuzzutil.h"
#include <cstring>
#include <utility>
contrib::optional<two_inputs> SplitInputs(const uint8_t *Data, size_t Size)
{
if (!Size) {
return contrib::nullopt;
}
// figure out a random point in [0, Size] to split our input.
size_t left_size = (Data[0] / 255.0) * (Size - 1);
// Copy inputs to new allocations so if bdiff over-reads
// AddressSanitizer can detect it.
std::unique_ptr<char[]> left(new char[left_size]);
std::memcpy(left.get(), Data + 1, left_size);
// right starts at the next byte after left ends
size_t right_size = Size - (left_size + 1);
std::unique_ptr<char[]> right(new char[right_size]);
std::memcpy(right.get(), Data + 1 + left_size, right_size);
LOG(2) << "inputs are " << left_size << " and " << right_size
<< " bytes" << std::endl;
two_inputs result = {std::move(right), right_size, std::move(left),
left_size};
return result;
}