With hash and btree there is no guaranteed way to read a table after
it is updated, even when using locks. LMDB tables are better in
this respect, as long as you use the right locking procedure (man 5
lmdb_table) but you'd need to make the table group readable so that
your tool can lock and read it.
That leaves two problems.
The table format is an internal interface. I do make incompatible
changes internally, and that is OK because no-one is supported when
they rely on an internal interface. The format is described with a
comment in the verify.c source code.
Instead of hacking file permissions you could try to set up a
proxymap service over TCP on 127.0.0.1. But again the protocol is
an internal interface and breaking changes happen. I just noticed
that this protocol still needs to be changed into "server speaks
first" like the rest of Postfix. I have added that on the TODO list
for Postfix 3.7.
Reading the verify map (and other tables) would need a proxymap
like service with a stable protool. It should probably output JSON
format, like postqueue -j does.
Wietse
