On Fri, 7 Oct 2022 at 09:05, Tom Lane <t...@sss.pgh.pa.us> wrote: > > Here's a v2 incorporating discussed changes. > > In reordering enum MemoryContextMethodID, I arranged to avoid using > 000 and 111 as valid IDs, since those bit patterns will appear in > zeroed and wipe_mem'd memory respectively. Those should probably be > more-or-less-permanent exclusions, so I added comments about it.
I'm just considering some future developer here that is writing a new MemoryContext type and there's no more space left and she or he needs to either use 000 or 111. I think if that was me, I might be unsure if I should be looking to expand the bit-space to make room. I might think that based on the word "avoid" in: > + MCTX_UNUSED1_ID, /* avoid: 000 occurs in never-used memory */ > + MCTX_UNUSED5_ID /* avoid: 111 occurs in wipe_mem'd memory */ but the final sentence in: > + * dummy entries for unused IDs in the mcxt_methods[] array. We also try > + * to avoid using bit-patterns as valid IDs if they are likely to occur in > + * garbage data. leads me to believe we're just *trying* to avoid using these bit-patterns. Also, the comment in mcxt_methods[] might make me believe that it's ok for me to use them if I really need to. > + * Unused (as yet) IDs should have dummy entries here. This allows us to Based on these comments, I'm not quite sure if I should be completely avoiding using 000 and 111 or I should just use those last when there are no other free slots in the array. It might be quite a long time before someone is in this situation, so should we be more clear? However, maybe you've left it this way as you feel it's a decision that must be made in the future, perhaps based on how difficult it would be to free up another bit? David
