MemoryContext slingshot

There are cases when we'd like to have a callback, but Postgres doesn't provide one. Fortunately, in some cases, there is still a way to do this.

When MemoryContext is deleted or reset, you can use this as a "slingshot" to inject your callback there.

Examples of such use:

Upon PostmasterContext deletion, initialize the backend
When transaction is committed and visible to other backends, TopTransactionContext is reset. Transaction callbacks don't get us that far.

MemoryContextCallback *cb = MemoryContextAlloc(/* TargetMemoryContext */, sizeof(*cb));
cb->func = /* callback function */;
cb->arg = /* function argument */
MemoryContextRegisterResetCallback(/* TargetMemoryContext */, cb);

Caution

Newly registered callbacks will be called before those registered earlier, effectively reversing the order in which the callbacks are called.