No way to create empty + appendable files via file_descriptor c-tor

[justinasvd]

This is perhaps some corner case, but consider that you want to use file_descriptor to manage some log file. Writes to log files are typically write-only-to-the-end, but you may also want to do random reads on logs. Random reads via pread are no problem in Linux (they don't move file pointer), but random reads via ReadFile with offsets specified in OVERLAPPED struct actually move file pointer, so subsequent writes will not be at the end of file.

The problem is fixed by opening a file with O_APPEND/FILE_APPEND_DATA when creating a new file. It would be nice to be able to forcefully open a "fresh" file with std::ios_base::app flag. I imagine this could be done by allowing std::ios_base::trunc | std::ios_base::app combo. The effect should be as follows:

#if defined(BOOST_IOSTREAMS_WINDOWS)
    DWORD dwDesiredAccess;
    if (enable_append) {
        dwDesiredAccess = GENERIC_READ | FILE_APPEND_DATA | FILE_WRITE_ATTRIBUTES | FILE_WRITE_EA |
                          STANDARD_RIGHTS_WRITE | SYNCHRONIZE;
    } else {
        dwDesiredAccess = GENERIC_READ | GENERIC_WRITE;
    }

    HANDLE file_handle =
        ::CreateFileA(file_name.c_str(), dwDesiredAccess, FILE_SHARE_READ | FILE_SHARE_WRITE,
                      NULL, // lpSecurityAttributes
                      CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL,
                      NULL); // hTemplateFile
#else
    // Calculate oflag argument to open.
    int oflag = O_CREAT | O_TRUNC | O_RDWR;
    if (enable_append) {
        oflag |= O_APPEND;
    }
#ifdef _LARGEFILE64_SOURCE
    oflag |= O_LARGEFILE;
#endif

    // Calculate pmode argument to open.
    mode_t pmode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH;

    // Open file.
    int file_handle = BOOST_IOSTREAMS_FD_OPEN(file_name.c_str(), oflag, pmode);
#endif