Re: How gvdb_table_is_valid works on glib/dconf

[Ryan Lortie <desrt desrt ca>]

hi Joshua

On 11-02-16 04:09 AM, Joshua Lee wrote:
> I can not understand while gvdb_table_is_valid can check the on-disk
> gvdb table is valid.

The very first thing that comes in any GVDB file is the string 
"GVariant".  That's normally why *data should be non-zero, so !!*data 
will be 1.

When dconf system databases are replaced, the old file has its header 
overwritten with zeros to indicate that applications need to reopen it 
on their next access.  This cases *data to be 0, so !!*data is also zero.

See this code in dconf-update, for example:


        var fd = Posix.open (filename, Posix.O_WRONLY);

        if (fd < 0 && errno != Posix.ENOENT) {
            var saved_error = errno;
            throw new FileError.FAILED ("Can not open '%s' for 
replacement: %s", filename, strerror (saved_error));
        }

        try {
            table.write_contents (filename);

            if (fd >= 0) {
                Posix.write (fd, "\0\0\0\0\0\0\0\0", 8);
            }
        } finally {
            if (fd >= 0) {
                Posix.close (fd);
            }
        }


Basically, it opens the old file and holds on to the fd while replacing 
it with the new file.  It then zeros-out the header of the old file to 
indicate that users should reopen (thus getting the new file).

Hope that helps.

Cheers