Purge old jemalloc files from cache on cache miss (eg on stack or jemalloc version change)

[edmorley]

This buildpack compiles jemalloc during the build and caches the result under a stack and jemalloc-specific path:

heroku-buildpack-jemalloc/bin/compile

Lines 37 to 38 in a7fe927

	# bundle is the full path to the cached jemalloc binaries for this version.
	bundle=$CACHE_DIR/jemalloc/$STACK/$version

This correctly ensures that a recompile happens if the stack or requested jemalloc version changes.

However, currently the buildpack does not remove these cached items on cache miss, meaning the cache will continue to grow in size each time the stack or requested jemalloc version changes (unless users know to manually clear their build cache using a special CLI plugin).

To resolve this, the buildpack could do an rm -rf "${CACHE_DIR}/jemalloc/" on cache miss before the download/recompile, here:

heroku-buildpack-jemalloc/bin/compile

Lines 116 to 121 in a7fe927

	# doesn't exist.
	if [[ ! -d $bundle ]]; then
	echo "-----> jemalloc: Installing $version from source"
	download
	compile

[gaffneyc]

My original intent with not clearing the cached builds was to make it possible to revert to a previously built version of jemalloc without having to deploy. Realistically though that's confusing since you have to build a new slug in order to use a version you've never deployed before.

Purging the builds would probably work but I'd like to noodle on it. Fortunately each build is ~6MB and the cadence of Heroku stack and jemalloc releases is both rather slow.

Another option would be to only clear the cache for stacks other than the current one. Compressing the build artifacts would also reduce them down to ~2MB.

[edmorley]

Ah if it's only 6MB that's not so bad - I had imagined it would be larger than that.