Resource Management in Lua
Recently while working on
nvim-surround, I ran into a
scenario where I needed to manage resources; in particular
extmarks. The only thing
you really need to know about them is that they represent coordinates in a text
buffer and get moved around as the buffer gets updated, which can be helpful for
capturing "semantic" locations. Here's some pseudocode for how I was using them:
local extmark_id = set_extmark(initial_pos)
edit_buffer()
local extmark_pos = get_extmark(extmark_id)
del_extmark(extmark_id)
-- Do something with the "updated" position
-- ...
However, it began to irk me that the "garbage collection" for the extmarks
wasn't automatic. Moreover, forgetting to clean them up could result in
hundreds, if not thousands of extmarks being re-calculated with every buffer
update. It occurred to me that I wanted bind the lifetime of the extmark to
the lifetime of its ID1, to avoid all the normal "footguns" like
use after free or
invalid dereference. To accomplish
this, I created a wrapper function that would handle instantiating and
destroying the extmark automatically:
local with_extmark = function(initial_pos, callback)
local extmark_id = set_extmark(initial_pos)
callback()
local extmark_pos = get_extmark(extmark_id)
del_extmark(extmark_id)
return extmark_pos
end
Then, whenever I needed to use an extmark, I could simply call with_extmark,
without worrying about cleaning up any references:
local position = with_extmark(initial_pos, function()
edit_buffer()
end)
-- Do something with the "updated" position
-- There are no extmark references visible here!
It's worth mentioning that none of the content here has been specific to Neovim
extmarks; we could have instead discussed any other resource, like memory,
mutexes, sockets, and more! As long as the language supports creating closures,
we can use wrapper functions to ensure that no resources get leaked in our
programs, avoiding a whole host of issues.