Header files are now self-contained, which makes the relationships
between the files clearer, and crucially allows LSP tools like `clangd`
to function correctly in all of our header files. I have verified that
the headers are self-contained (aside from the various Windows shims) by
compiling them as if they were C files – in a follow-up commit I plan to
add this to CI to ensure we don't regress on this front.
Now that all merging go through try_acquire_edata_neighbor, the mergeablility
checks (including head state checking) are done before reaching the merge hook.
In other words, merge hook will never be called if the head state doesn't agree.
Instead of passing down the new_addr, pass down the active edata which allows us
to always use a neighbor-acquiring semantic. In other words, this tells us both
the original edata and neighbor address. With this change, only neighbors of a
"known" edata can be acquired, i.e. acquiring an edata based on an arbitrary
address isn't possible anymore.
This avoids the addr-based mutexes (i.e. the mutex_pool), and instead relies on
the metadata tracked in rtree leaf: the head state and extent_state. Before
trying to access the neighbor edata (e.g. for coalescing), the states will be
verified first -- only neighbor edatas from the same arena and with the same
state will be accessed.
In practice, many rtree_leaf_elm accesses are cache misses. By restructuring,
we can make it more likely that these misses occur without blocking us from
starting later lookups, taking more of those misses in parallel.
The only time sharing an rtree context saves across extent operations isn't a
no-op is when tsd is unavailable. But this happens only in situations like
thread death or initialization, and we don't care about shaving off every
possible cycle in such scenarios.
