[Maria-developers] custom storage backend rnd_next() implementation and caching questions

30 Jan 2015

      Hi Everyone,

 I am developing a custom mariadb storage backend which is intended to give filesystem related information via mariadb tables.

 For example one could tell:

CREATE TEMPORARY TABLE IF NOT EXISTS `/var/log` ENGINE=fsview;

which would create a table (using the assisted discovery method) with the following schema:

MariaDB [test]> describe `/var`;
+-----------+--------------+------+-----+---------+-------+
| Field     | Type         | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+-------+
| file_name | varchar(255) | YES  |     | NULL    |       |
| contents  | longblob     | YES  |     | NULL    |       |
+-----------+--------------+------+-----+---------+———+

However i have some implementation issues:

1. I am having trouble implementing rnd_next() in the backend:

int ha_fsview::rnd_next(uchar *buf) {
    DBUG_ENTER(__PRETTY_FUNCTION__);

    if (!dirp) {
        DBUG_RETURN(HA_ERR_INTERNAL_ERROR);
    }
    if (struct dirent* entry = readdir(dirp)) {
        uchar *pos = buf;
        *pos = 0;
        ++pos;

        // file name (1bytes size + str)
        *pos = strlen(entry->d_name);
        ++pos;

        size_t bytes_to_copy = strlen(entry->d_name);
        memcpy(pos, entry->d_name, bytes_to_copy);
        pos+=bytes_to_copy;
        DBUG_PRINT("info", ("fsview filename: %s", entry->d_name));

        // content (4 bytes size + content ptr)

        static const std::string fake_content = "hello world";
        int s = fake_content.size();
        memcpy(pos, &s, sizeof(s));
        pos+=sizeof(s);

        const char* c = fake_content.c_str();
        memcpy(pos, &c, sizeof(c));
        pos+=sizeof(c);

        DBUG_RETURN(0);
    } else {
        DBUG_RETURN(HA_ERR_END_OF_FILE);
    }
}

My problem is that filenames (first field) is ok, but i cannot really see the expected content “hello world” in the second field. I am assuming that i am doing something wrong, but i cannot really find the reason. Could somebody help me, please?

2. If i am issuing:

 SELECT * FROM `/var`;

twice, then the server is not initiating a table scan again on my backend (most likely results from the first run are stored in some cache). So if filesystem contents are changed between the two statements, then it won’t be reflected in the resultset.

 Of course if i do a:

 SELECT SQL_NO_CACHE * FROM `/var`;

then everything is fine, however i would not like to put the burden on the user to specify the option to not cache the results.

My question is whether it’s possible to specify in the storage backend that the results from the tables should not be cached.

Thank for the help in advance

-- 
Andras Szabo
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