> What a mess.
The mess with db_get_record() and db_open_record() is even deeper than I thought. There are several anomalies.
Records opened by db_open_record() are later accessed via db_get_record() which requires
that the odb structure and the C structure match exactly.
Of course anybody can modify anything in odb at any time, so there are protections against
modifying the odb structures "from under" db_open_record():
a) db_open_record(MODE_WRITE) makes the odb structure immutable by setting the "exclusive" flag. This works well. In the past
there were problems with "exclusive mode" getting stuck behind dead clients, but these days it is efficiently cleaned and recovered
by the odb validation code at the start of all midas programs.
b) db_create_record(), db_reorder_key() and db_delete_key() refuse to function on watched/hotlinked odb structures. One would
think this is good, but there is a side-effect. If I run "odbedit watch /", all odb delete operations fail (including deletion of temporary
items in /system/tmp).
c) db_create_key() and db_set_data()/db_set_value() do not have such protections, and they can (and do) add new odb entries and
change size of existing entries (especially size of strings), and make db_get_record() fail. note that db_get_record() inside
db_open_record() fails silently and odb hotlinks mysteriously stop working.
One could keep fixing this by adding protections against modification of hotlinked odb structures, but unfortunately, one cannot tell
db_watch() hotlinks from db_open_record() hotlinks. Only the latter ones require protection. db_watch() does not require such
protections because it does not use db_get_record() internally, it leaves it to the user to sort out any mismatches.
Also it would be nice if "odbedit watch /" did not have the nasty side effect of making all odb unchangable (presently it only makes
To sort it all out, I am moving in this direction:
1) replace all uses of db_get_record() with db_get_record1() which automatically cures any structure mismatch
2) replace all uses of db_open_record(MODE_READ) with db_watch() in conjunction with db_get_record1(). This is done in mfe.c
and seems to work ok.
2a) automatic repair of structure mismatch is presently defeated by db_create_record() refusing to work on hotlinked odb entries.
3) with db_get_record() and db_open_record(MODE_READ) removed from use, turn off hotlink protection in item (b) above. This will
fix problem (2a).
4) maybe replace db_open_record(MODE_WRITE) with explicit db_set_record(). I personally do not like it's "magical" operation,
when in fact, it is just a short hand for "db_get_key/db_set_record" hidden inside db_send_changed_records().
4a) db_open_record(MODE_WRITE) works well enough right now, no need to touch it.