If backfilling is slow then the client may time out and retry, causing
Synapse to start a new `/backfill` before the existing backfill has
finished, duplicating work.
This adds quite a lot of OpenTracing decoration for database activity. Specifically it adds tracing at four different levels:
* emit a span for each "interaction" - ie, the top level database function that we tend to call "transaction", but isn't really, because it can end up as multiple transactions.
* emit a span while we hold a database connection open
* emit a span for each database transaction - actual actual transaction.
* emit a span for each database query.
I'm aware this might be quite a lot of overhead, but even just running it on a local Synapse it looks really interesting, and I hope the overhead can be offset just by turning down the sampling frequency and finding other ways of tracing requests of interest (eg, the `force_tracing_for_users` setting).
The existing tracing reports an error each time there is a timeout, which isn't
really representative.
Additionally, we log things about the way `wait_for_events` works
(eg, the result of the callback) to the *parent* span, which is confusing.
So that they render nicely in mdbook (see #10086), and so that we no longer have a mix of structured text languages in our documentation (excluding files outside of `docs/`).
Empirically, this helped my server considerably when handling gaps in Matrix HQ. The problem was that we would repeatedly call have_seen_events for the same set of (50K or so) auth_events, each of which would take many minutes to complete, even though it's only an index scan.
==============================
Bugfixes
--------
- Fix a bug introduced in v1.35.0rc1 when calling the spaces summary API via a GET request. ([\#10079](https://github.com/matrix-org/synapse/issues/10079))
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Merge tag 'v1.35.0rc2' into develop
Synapse 1.35.0rc2 (2021-05-27)
==============================
Bugfixes
--------
- Fix a bug introduced in v1.35.0rc1 when calling the spaces summary API via a GET request. ([\#10079](https://github.com/matrix-org/synapse/issues/10079))
* Make `invalidate` and `invalidate_many` do the same thing
... so that we can do either over the invalidation replication stream, and also
because they always confused me a bit.
* Kill off `invalidate_many`
* changelog
