Firstly, we need to understand that ActiveRecord can be
connected to a role which prevent writes and this happens in Discourse when a
replica database has been setup for failover purposes. When a role
prevent writes from happening, ActiveRecord will raise the
`ActiveRecord::ReadOnlyError` if a write query is attempted.
Secondly, theme fields are baked at runtime within GET requests. The
baking process involves writing the baked value to the
`ThemeField#baked_value` column in the database.
If we combine the two points above, we can see how the writing of the
baked value to the database will trigger a `ActiveRecord::ReadOnlyError`
in a GET requests when the database is connected to a role preventing
writes. However, failing to bake a theme is not the end of the world and
should not cause GET requests to fail. Therefore, this commit adds a rescue
for `ActiveRecord::ReadOnlyError` in the `ThemeField#ensure_baked!`
method.
Constants should always be only assigned once. The logical OR assignment
of a constant is a relic of the past before we used zeitwerk for
autoloading and had bugs where a file could be loaded twice resulting in
constant redefinition warnings.
When the thread is aborted, an exception is raised before the `start` of a job is set, and therefore raises an exception in the `ensure` block. This commit checks that `start` exists, and also adds `abort_on_exception=true` so that this issue would have caused test failures.
This will give us some aggregate stats on the defer queue performance.
It is limited to 100 entries (for safety) which is stored in an LRU cache.
Scheduler::Defer.stats can then be used to get an array that denotes:
- number of runs and completions (queued, finished)
- error count (errors)
- total duration (duration)
We can look later at exposing these metrics to gain visibility on the reason
the defer queue is clogged.
Use a dedicated thread to run Scheduler::Defer
This avoids blocking of a worker during operations that require waiting.
In particular uploads risked blocking a unicorn.
This also add a queue "length" that discourse prometheus consumes.
