Method impl

src/operations.cpp:881–953

copy_file implementation that uses copy_file_range loop. Requires copy_file_range to support cross-filesystem copying.

{
    //! copy_file implementation that uses copy_file_range loop. Requires copy_file_range to support cross-filesystem copying.
    static int impl(int infile, int outfile, uintmax_t size, std::size_t blksize)
    {
        // Although copy_file_range does not document any particular upper limit of one transfer, still use some upper bound to guarantee
        // that size_t is not overflown in case if off_t is larger and the file size does not fit in size_t.
        BOOST_CONSTEXPR_OR_CONST std::size_t max_batch_size = 0x7ffff000u;
        uintmax_t offset = 0u;
        while (offset < size)
        {
            uintmax_t size_left = size - offset;
            std::size_t size_to_copy = max_batch_size;
            if (size_left < static_cast< uintmax_t >(max_batch_size))
                size_to_copy = static_cast< std::size_t >(size_left);
            // Note: Use syscall directly to avoid depending on libc version. copy_file_range is added in glibc 2.27.
            // uClibc-ng does not have copy_file_range as of the time of this writing (the latest uClibc-ng release is 1.0.33).
            loff_t sz = ::syscall(__NR_copy_file_range, infile, (loff_t*)nullptr, outfile, (loff_t*)nullptr, size_to_copy, (unsigned int)0u);
            if (BOOST_LIKELY(sz > 0))
            {
                offset += sz;
            }
            else if (sz < 0)
            {
                int err = errno;
                if (err == EINTR)
                    continue;

                if (offset == 0u)
                {
                    // copy_file_range may fail with EINVAL if the underlying filesystem does not support it.
                    // In some RHEL/CentOS 7.7-7.8 kernel versions, copy_file_range on NFSv4 is also known to return EOPNOTSUPP
                    // if the remote server does not support COPY, despite that it is not a documented error code.
                    // See https://patchwork.kernel.org/project/linux-nfs/patch/20190411183418.4510-1-olga.kornievskaia@gmail.com/
                    // and https://bugzilla.redhat.com/show_bug.cgi?id=1783554.
                    if (err == EINVAL || err == EOPNOTSUPP)
                    {
#if !defined(BOOST_FILESYSTEM_USE_SENDFILE)
                    fallback_to_read_write:
#endif
                        return copy_file_data_read_write(infile, outfile, size, blksize);
                    }

                    if (err == EXDEV)
                    {
#if defined(BOOST_FILESYSTEM_USE_SENDFILE)
                    fallback_to_sendfile:
                        return copy_file_data_sendfile::impl(infile, outfile, size, blksize);
#else
                        goto fallback_to_read_write;
#endif
                    }

                    if (err == ENOSYS)
                    {
#if defined(BOOST_FILESYSTEM_USE_SENDFILE)
                        filesystem::detail::atomic_store_relaxed(copy_file_data, &check_fs_type< copy_file_data_preallocate< copy_file_data_sendfile > >);
                        goto fallback_to_sendfile;
#else
                        filesystem::detail::atomic_store_relaxed(copy_file_data, &copy_file_data_read_write);
                        goto fallback_to_read_write;