fix(memoize): release cache with file, reset on re-parse

[seflue]

In long-running Neovim sessions with org files open, memory use keeps growing as I edit them and never comes back down. I have seen this for a while; one instance was at over 10 GB.

The memory is tree-sitter trees, pinned through the memoize cache, which never lets go of them. It is meant to shrink on its own. It is a module-level table declared __mode = 'k', which reads as "entries go away when their key is collected". They do not, because the keys are filenames and string keys are never weak in Lua (iscleared() in Lua 5.1's lgc.c, 5.4 manual §2.5.4). The obvious repair, keying by the file object, does not work either. A cached value references the file it was built from, and a weak-key table still marks its values (gc_traverse_tab skips marking only when keys and values are weak), so every entry keeps its own key alive. Lua 5.2 added ephemeron tables for exactly this case; LuaJIT has none. This is why the leak was easy to miss. The code looks like it handles the lifetime, and the reasons it does not are two GC details that hold only for strings and only for LuaJIT.

On top of that, an entry grows while its file is edited. Results are bucketed by node id, lookups after a re-parse use the ids of the new tree, and the entry is only reset when the mtime changes, so a file collects one set of dead buckets, and with it one tree, per re-parse between saves.

Two commits:

• The cache moves from the module-level Memoize.cache to a field on the OrgFile instance, file.memoize_cache, so it lives exactly as long as the instance, with no invalidation call to remember.
• The entry carries __generation, the root node id, next to __version. When either changes the entry is replaced as a whole, so a re-parse drops the old buckets and with them the tree they pinned.

Measured with a headless harness: load a set, render the week agenda, then rounds of edit a buffer, reload, redraw. Lua heap from collectgarbage('count'), C heap from mallinfo2(), RSS from /proc.

About 1500 files, 4.5 MB of org text, agenda open: 52 MB Lua heap, 209 MB C heap, 360 MB RSS on both versions. After unloading all files:

	Lua heap	C heap	RSS
master	49 MB	196 MB	358 MB
this branch	17 MB	9 MB	164 MB

Ten rounds of edit, reload, redraw on 350 files with 100 of them open in buffers, live Lua + C heap:

	this branch	master
after the first render	32 MB	32 MB
after 10 rounds	35 MB	52 MB

master gains about 2 MB per round, one more tree of each edited file.

[kristijanhusak]

Great find! Thanks!