Postgres memory allocators, by default, ERROR out on memory allocation failures. An ERROR leads to a longjmp, which means that the caller of the allocator will never see a NULL return value. We can explicitly let the compiler know about this behavior by adding the returns_nonnull attribute to the allocators that follow this behavior. Postgres does support _extended versions of some of the allocators that take a flaks argument. The caller can provide the MCXT_ALLOC_NO_OOM flag to these allocators to request that they return NULL on allocation failure instead of ERROR-ing out. The _extended allocators cannot be marked as returns_nonnull because of that.
By using returns_nonnull, we can help the compiler to optimize call sites. -- Tristan Partin PostgreSQL Contributors Team AWS (https://aws.amazon.com)
