Re: Column Filtering in Logical Replication

From: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
To: Rahila Syed <rahilasyed90(at)gmail(dot)com>
Cc: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Peter Smith <smithpb2250(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Column Filtering in Logical Replication
Date: 2021-09-06 19:16:03
Message-ID: 202109061916.xxrml7wrmuoe@alvherre.pgsql
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The code in get_rel_sync_entry() changes current memory context to
CacheMemoryContext, then does a bunch of memory-leaking things. This is
not good, because that memory context has to be very carefully managed
to avoid permanent memory leaks. I suppose you added that because you
need something -- probably entry->att_map -- to survive memory context
resets, but if so then you need to change to CacheMemoryContext only
when that memory is allocated, not other chunks of memory. I suspect
you can fix this by moving the MemoryContextSwitchTo() to just before
calling get_table_columnset; then all the leaky thinkgs are done in
whatever the original memory context is, which is fine.

(However, you also need to make sure that ->att_map is carefully freed
at the right time. It looks like this already happens in
rel_sync_cache_relation_cb, but is rel_sync_cache_publication_cb
correct? And in get_rel_sync_entry() itself, what if the entry already
has att_map -- should it be freed prior to allocating another one?)

By the way, I notice that your patch doesn't add documentation changes,
which are of course necessary.

/me is left wondering about PGOutputData->publication_names memory
handling ...

Álvaro Herrera Valdivia, Chile —

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