It turns out that TablespaceCreateDbspace fails badly if a relcache flush
occurs when it tries to heap_open pg_tablespace. When control returns to smgrcreate, that routine will be holding a dangling pointer to a closed SMgrRelation, resulting in mayhem. This is of course a consequence of the violation of proper module layering inherent in having smgr.c call a tablespace command routine, but the simplest fix seems to be to change the locking mechanism. There's no real need for TablespaceCreateDbspace to touch pg_tablespace at all --- it's only opening it as a way of locking against a parallel DROP TABLESPACE command. A much better answer is to create a special-purpose LWLock to interlock these two operations. This drops TablespaceCreateDbspace quite a few layers down the food chain and makes it something reasonably safe for smgr to call.
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