createFileMap

fn createFileMap(
    file: File,
    protection: std.process.MemoryProtection,
    offset: u64,
    populate: bool,
    len: usize,
) CreateFileMapError!File.MemoryMap {
    if (is_windows) {
        try Thread.checkCancel();

        var section = windows.INVALID_HANDLE_VALUE;
        const section_size: windows.LARGE_INTEGER = @intCast(len);
        const page = windows.PAGE.fromProtection(protection) orelse return error.AccessDenied;
        switch (windows.ntdll.NtCreateSection(
            &section,
            .{
                .SPECIFIC = .{ .SECTION = .{
                    .QUERY = true,
                    .MAP_WRITE = protection.write,
                    .MAP_READ = protection.read,
                    .MAP_EXECUTE = protection.execute,
                    .EXTEND_SIZE = true,
                } },
                .STANDARD = .{ .RIGHTS = .REQUIRED },
            },
            null,
            &section_size,
            page,
            .{ .COMMIT = populate },
            file.handle,
        )) {
            .SUCCESS => {},
            .FILE_LOCK_CONFLICT => return error.LockViolation,
            .INVALID_FILE_FOR_SECTION => return error.OperationUnsupported,
            .ACCESS_DENIED => return error.AccessDenied,
            .SECTION_TOO_BIG => return error.SectionOversize,
            else => |status| return windows.unexpectedStatus(status),
        }
        var contents_ptr: ?[*]align(std.heap.page_size_min) u8 = null;
        var contents_len = len;
        switch (windows.ntdll.NtMapViewOfSection(
            section,
            windows.current_process,
            @ptrCast(&contents_ptr),
            null,
            0,
            null,
            &contents_len,
            .Unmap,
            .{},
            page,
        )) {
            .SUCCESS => {},
            .CONFLICTING_ADDRESSES => return error.MappingAlreadyExists,
            .SECTION_PROTECTION => return error.PermissionDenied,
            .ACCESS_DENIED => return error.AccessDenied,
            .INVALID_VIEW_SIZE => |status| return windows.statusBug(status),
            else => |status| return windows.unexpectedStatus(status),
        }
        if (builtin.mode == .debug) {
            const page_size = std.heap.pageSize();
            const alignment: Alignment = .fromByteUnits(page_size);
            assert(contents_len == alignment.forward(len));
        }
        return .{
            .file = file,
            .offset = offset,
            .memory = contents_ptr.?[0..len],
            .section = section,
        };
    } else if (have_mmap) {
        const prot: posix.PROT = .{
            .READ = protection.read,
            .WRITE = protection.write,
            .EXEC = protection.execute,
        };
        const flags: posix.MAP = switch (native_os) {
            .linux => .{
                .TYPE = .SHARED_VALIDATE,
                .POPULATE = populate,
            },
            else => .{
                .TYPE = .SHARED,
            },
        };

        const page_align = std.heap.page_size_min;

        const contents = while (true) {
            const syscall: Syscall = try .start();
            const casted_offset = std.math.cast(i64, offset) orelse return error.Unseekable;
            const rc = mmap_sym(null, len, prot, flags, file.handle, casted_offset);
            syscall.finish();
            const err: posix.E = if (builtin.link_libc) e: {
                if (rc != std.c.MAP_FAILED) {
                    break @as([*]align(page_align) u8, @ptrCast(@alignCast(rc)))[0..len];
                }
                break :e @fromBackingInt(@intCast(posix.system._errno().*));
            } else e: {
                const err = posix.errno(rc);
                if (err == .SUCCESS) {
                    break @as([*]align(page_align) u8, @ptrFromInt(rc))[0..len];
                }
                break :e err;
            };
            switch (err) {
                .SUCCESS => unreachable,
                .INTR => continue,
                .ACCES => return error.AccessDenied,
                .AGAIN => return error.LockedMemoryLimitExceeded,
                .EXIST => return error.MappingAlreadyExists,
                .MFILE => return error.ProcessFdQuotaExceeded,
                .NFILE => return error.SystemFdQuotaExceeded,
                .NODEV => return error.OperationUnsupported,
                .NOMEM => return error.OutOfMemory,
                .PERM => return error.PermissionDenied,
                .TXTBSY => return error.FileBusy,
                .OVERFLOW => return error.Unseekable,
                .BADF => return errnoBug(err), // Always a race condition.
                .INVAL => return errnoBug(err), // Invalid parameters to mmap()
                .OPNOTSUPP => return errnoBug(err), // Bad flags with MAP.SHARED_VALIDATE on Linux.
                else => return posix.unexpectedErrno(err),
            }
        };
        return .{
            .file = file,
            .offset = offset,
            .memory = contents,
            .section = {},
        };
    }

    return error.OperationUnsupported;
}