static int write_entry(struct mailbox *mailbox,
                       unsigned int uid,
                       const char *entry,
                       const char *userid,
                       const struct buf *value,
                       int ignorequota,
                       int silent,
                       const struct annotate_metadata *mdata,
                       int maywrite)

{
    char key[MAX_MAILBOX_PATH+1];
    int keylen, r;
    annotate_db_t *d = NULL;
    struct buf oldval = BUF_INITIALIZER;
    const char *mboxname = mailbox ? mailbox->name : "";
    modseq_t modseq = mdata ? mdata->modseq : 0;

    r = _annotate_getdb(mboxname, uid, CYRUSDB_CREATE, &d);
    if (r)
        return r;

    /* must be in a transaction to modify the db */
    annotate_begin(d);

    keylen = make_key(mboxname, uid, entry, userid, key, sizeof(key));

    struct annotate_metadata oldmdata;
    r = read_old_value(d, key, keylen, &oldval, &oldmdata);
    if (r) goto out;

    /* if the value is identical, don't touch the mailbox */
    if (oldval.len == value->len && (!value->len || !memcmp(oldval.s, value->s, value->len)))
        goto out;

    if (!maywrite) {
        r = IMAP_PERMISSION_DENIED;
        if (r) goto out;
    }

    if (mailbox) {
        if (!ignorequota) {
            quota_t qdiffs[QUOTA_NUMRESOURCES] = QUOTA_DIFFS_DONTCARE_INITIALIZER;
            qdiffs[QUOTA_ANNOTSTORAGE] = value->len - (quota_t)oldval.len;
            r = mailbox_quota_check(mailbox, qdiffs);
            if (r) goto out;
        }

        /* do the annot-changed here before altering the DB */
        mailbox_annot_changed(mailbox, uid, entry, userid, &oldval, value, silent);

        /* grab the message annotation modseq, if not overridden */
        if (uid && !mdata) {
            modseq = mailbox->i.highestmodseq;
        }
    }

    /* zero length annotation is deletion.
     * keep tombstones for message annotations */
    if (!value->len && !uid) {

#if DEBUG
        syslog(LOG_ERR, "write_entry: deleting key %s from %s",
                key_as_string(d, key, keylen), d->filename);
#endif

        do {
            r = cyrusdb_delete(d->db, key, keylen, tid(d), /*force*/1);
        } while (r == CYRUSDB_AGAIN);
    }
    else {
        struct buf data = BUF_INITIALIZER;
        unsigned char flags = 0;
        if (!value->len || value->s == NULL) {
            flags |= ANNOTATE_FLAG_DELETED;
        }
        else {
            // this is only here to allow cleanup of invalid values in the past...
            // the calling of this API with a NULL "userid" is bogus, because that's
            // supposed to be reserved for the make_key of prefixes - but there has
            // been API abuse in the past, so some of these are in the wild.  *sigh*.
            // Don't allow new ones to be written
            if (!userid) goto out;
        }
        make_entry(&data, value, modseq, flags);

#if DEBUG
        syslog(LOG_ERR, "write_entry: storing key %s (value: %s) to %s (modseq=" MODSEQ_FMT ")",
                key_as_string(d, key, keylen), value->s, d->filename, modseq);
#endif

        do {
            r = cyrusdb_store(d->db, key, keylen, data.s, data.len, tid(d));
        } while (r == CYRUSDB_AGAIN);
        buf_free(&data);
    }

    if (!mailbox)
        sync_log_annotation("");

out:
    annotate_putdb(&d);
    buf_free(&oldval);

    return r;
}