Starting from commitpostgres/postgres@cdbc0ca48ca shared preload libraries are become to initialize beforeMaxBackends global variable setup. This might lead to invalid calculation of requested shared memory size if this calculation is based on max number of possible backend processes in postgres instance. Forpg_wait_sampling module the size calculation of queryId shared memoryfollows this rule, i.e. it usesMaxBackends value. As result, for this shared memory fragment we request zero size (UPD. not zero butNUM_AUXILIARY_PROCS + max_prepared_xacts size) but later we assign non-zero memory (with already initializedMaxBackends value) for this purpose. Such behavior might incur future occurrences ofout of shared memory errors.

The hackersthread have to help in finding of possible solution.