[HACKERS] Error: dsa_area could not attach to a segment that has been freed

Use _PG_init and the shmem hook to reserve a little bit of

traditional shared memory and initialise it to zero.  This will be

used just to share the DSA handle, but you can't actually create the

DSA area in postmaster.  In other words, this little bit of shared

memory is for "discovery", since it can be looked up by name from any

backend.

In each backend that wants to use your new in-memory index system,

you need to be able to attach or create the DSA area on-demand.

Perhaps you could have a get_my_shared_state() function (insert better

name) that uses a static local variable to hold a pointer to some

state.  If it's NULL, you know you need to create the state.  That

should happen only once in each backend, the first time through the

function.  In that case you need to create or attach to the DSA area

as appropriate, which you should wrap in

LWLockAcquire(AddinShmemInitLock,

LW_EXCLUSIVE)/LWLockRelease(AddinShmemInitLock) to serialise the code

block.  First, look up the bit of traditional shared memory to see if

there is a DSA handle published in it already.  If there is you can

attach.  If there isn't, you are the first so you need to create, and

publish the handle for others to attach to.  Remember whatever state

you need to remember, such as the dsa_area, in static local variables

so that all future calls to get_my_shared_state() in that backend will

be fast.