Unlogged relations and WAL-logging

Unlogged relations are not WAL-logged, but creating the init-fork is. 
There are a few things around that seem sloppy:

1. In index_build(), we do this:

> 
>     /*
>      * If this is an unlogged index, we may need to write out an init fork for
>      * it -- but we must first check whether one already exists.  If, for
>      * example, an unlogged relation is truncated in the transaction that
>      * created it, or truncated twice in a subsequent transaction, the
>      * relfilenode won't change, and nothing needs to be done here.
>      */
>     if (indexRelation->rd_rel->relpersistence == RELPERSISTENCE_UNLOGGED &&
>         !smgrexists(RelationGetSmgr(indexRelation), INIT_FORKNUM))
>     {
>         smgrcreate(RelationGetSmgr(indexRelation), INIT_FORKNUM, false);
>         indexRelation->rd_indam->ambuildempty(indexRelation);
>     }

Shouldn't we call log_smgrcreate() here? Creating the init fork is 
otherwise not WAL-logged at all. In practice, all the ambuildempty() 
implementations create and WAL-log a metapage and replay of that will 
implicitly create the underlying relation. But if you created an index 
access method whose ambuildempty() function does nothing, i.e. the init 
fork is just an empty file, there would be no trace in the WAL about 
creating the init fork, which is bad. In fact, 
src/test/modules/dummy_index_am is an example of that, but because it 
does nothing at all with the file, you cannot see any ill effect from 
the missing init fork.

2. Some implementations of ambuildempty() use the buffer cache (hash, 
gist, gin, brin), while others bypass it and call smgrimmedsync() 
instead (btree, spgist, bloom). I don't see any particular reason for 
those decisions, it seems to be based purely on which example the author 
happened to copy-paste.

Using the buffer cache seems better to me, because it avoids the 
smgrimmedsync() call. That makes creating unlogged indexes faster. 
That's not significant for any real world application, but still.

3. Those ambuildempty implementations that bypass the buffer cache use 
smgrwrite() to write the pages. That doesn't make any difference in 
practice, but in principle it's wrong: You are supposed to use 
smgrextend() when extending a relation. Using smgrwrite() skips updating 
the relation size cache, which is harmless in this case because it 
wasn't initialized previously either, and I'm not sure if we ever call 
smgrnblocks() on the init-fork. But if you build with 
CHECK_WRITE_VS_EXTEND, you get an assertion failure.

4. Also, the smgrwrite() calls are performed before WAL-logging the 
pages, so the page that's written to disk has 0/0 as the LSN, not the 
LSN of the WAL record. That's harmless too, but seems a bit sloppy.

(I remember we had a discussion once whether we should always extend the 
relation first, and WAL-log only after that, but I can't find the thread 
right now. The point is to avoid the situation that the operation fails 
because you run out of disk space, and then you crash and WAL replay 
also fails because you are still out of disk space. But most places 
currently call log_newpage() first, and smgrextend() after that.)

5. In heapam_relation_set_new_filenode(), we do this:

> 
>     /*
>      * If required, set up an init fork for an unlogged table so that it can
>      * be correctly reinitialized on restart.  An immediate sync is required
>      * even if the page has been logged, because the write did not go through
>      * shared_buffers and therefore a concurrent checkpoint may have moved the
>      * redo pointer past our xlog record.  Recovery may as well remove it
>      * while replaying, for example, XLOG_DBASE_CREATE or XLOG_TBLSPC_CREATE
>      * record. Therefore, logging is necessary even if wal_level=minimal.
>      */
>     if (persistence == RELPERSISTENCE_UNLOGGED)
>     {
>         Assert(rel->rd_rel->relkind == RELKIND_RELATION ||
>                rel->rd_rel->relkind == RELKIND_MATVIEW ||
>                rel->rd_rel->relkind == RELKIND_TOASTVALUE);
>         smgrcreate(srel, INIT_FORKNUM, false);
>         log_smgrcreate(newrnode, INIT_FORKNUM);
>         smgrimmedsync(srel, INIT_FORKNUM);
>     }

The comment doesn't make much sense, we haven't written nor WAL-logged 
any page here, with nor without the buffer cache. It made more sense 
before commit fa0f466d53.

This is what actually led me to discover the bug I just reported at [1], 
with regular tables. If we fix that bug I like I proposed there, then 
smgrcreate() will register and fsync request, and we won't need 
smgrimmedsync here anymore.

Attached is a patch to tighten those up. The third one should arguably 
be part of [1], not this thread, but here it goes too.

[1] 
https://www.postgresql.org/message-id/d47d8122-415e-425c-d0a2-e0160829702d%40iki.fi

- Heikki