This comment was _originally_ for the `default` memcached cache, back
when it was added all of the way back in 0a84d7ac62. 9e64750083
made it a lie, and edc718951c made it even more confusing when it
removed the `default` cache configuration block, leaving the wrong
comment next to the wrong cache configuration block.
Banish the comment.
The only purpose of this seems to be to not have to reset the cache;
fae59502ab added it without any explanation for why it is necessary.
Remove it, and explicitly flush the cache in the one place where it is
necessary.
This cache was added in da33b72848 to serve as a replacement for the
durable database cache, in development; the previous commit has
switched that to be the non-durable memcached backend.
The special-case for "in-memory" in development is mostly-unnecessary
in contrast to memcached -- `./tools/run-dev.py` flushes memcached on
every startup. This differs in behaviour slightly, in that if the
codepath is changed and `run-dev` restarts Django, the cache is not
cleared. This seems an unlikely occurrence, however, and the code
cleanup from its removal is worth it.
The choice to cache these in the database dates back to c93f1d4eda,
with the comment added in da33b72848 while working around the
durability of the "database" cache in local development.
The values were stored in a durable cache, as they needed to be
ensured to persist between when they were inserted in
`get_link_embed_data` and when they were used in
`render_incoming_message` via `link_embed_data_from_cache`.
However, database accesses are not fast compared to memcached, and we
wish to avoid the overhead of the database connection from the
`embed_links` worker. Specifically, making the connection may not be
thread-safe -- and in low-memory (and Docker) configurations, all
workers run as separate threads in a single process. This can lead to
stalled database connections in `embed_links` workers, and failed
previews.
Since the previous commit made the durability of the cache no longer
necessary, this will have minimal effect; at worst, posting the same
URL twice, on either side of an upgrade, will result in two preview
fetches of it.
The `get_link_embed_data` / `link_embed_data_from_cache` pair as
introduced in c93f1d4eda uses the cache
as a temporary store inside of the `embed_links` worker; this means
that it must be durable storage, or the worker will stall and re-fetch
the same links to preview them.
Switch to plumbing through the fetched URL embed data as an parameter
to the Markdown evaluation which uses them, rather than using the
cache as an intermediary. This frees up the cache to be merely a
non-durable cache.
As a side-effect, this removes get_cache_with_key, and
link_embed_data_from_cache which was its only callsite.
76deb30312 changed this to not just be the URL, but rather a
prefixed hash of the URL, but failed to update this location which
wrote to it. This meant that this pre-population step was writing to
the wrong keys in the durable cache, and thus ineffective.
Then, da33b72848 switched the cache to be in-memory, making this
write to the wrong keys in an in-process memory store. There is no
way to pre-fill this sort of cache, except at server start-up.
Finally, and most fundamentally, 8c0c9ca7a4 then disabled
`inline_url_embed_preview` by default, making the code entirely moot.
Remove the triply-unnecessary code.
Alex Vandiver
Mateusz Mandera
Anders Kaseorg