Wez Furlong f92df0fca0 lruttl: introduce allow_stale_reads option
This is paired with a corresponding field in the memoize options
struct, which is in turn exposed to the shaping helper shaping_data
cache via the `allow_stale_shaping_data` field.

The effect of setting this to true is:

* Undoes the effect of invalidate_with_epoch, as stale reads cannot
  be epoch based
* If a semaphore wait times out, and a stale value was present in
  the cache, that stale value will be returned to the caller

To facilitate this behavior, the lruttl cache introduces a new
state for a cached value: `Refreshing`, which is a combination
of both `Present` and `Pending`.

When stale reads are allowed, we take care to avoid unilaterally
replacing `Present` with `Pending` when an item has expired.

Whenever we satisfy a lookup with a stale value, we will bump a new
lruttl_stale_count counter for that cache.  Previously, we would
classify those as errors.

Something else that changed as part of this commit, is that we now set
an overall deadline for the semaphore wait operation; previously, each
wait would allow for up to the specified semaphore timeout, but now the
total wait time for that lookup will be bounded to the specified
timeout.
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KumoMTA

KumoMTA is an open-source Message Transfer Agent (MTA) designed for high-performance outbound email functionality, similar to commercial enterprise MTAs such as Momentum, PowerMTA, and Halon.

The KumoMTA project was founded by a group of email industry veterans with decades of experience building and managing high-performance On-Prem MTAs and is supported by a community of some of the largest senders in the world.

Because it is designed for high-performance sending environments, KumoMTA is for experienced email operations professionals who are accustomed to high-performance sending environments and familiar with DevOps practices.

Learn more in our FAQ and at https://kumomta.com/.

Documentation

You can learn more about KumoMTA from the Documentation.

Community

Real-time discussion is available on Our Discord.

Developers

If you are interested in contributing/extending KumoMTA, take a look at DEVELOPERS.md. The #devel channel on Our Discord is for contributors to discuss KumoMTA development.

Reporting Bugs

See How to Report Bugs.

Getting Help

See How to Get Help. For paid support see https://kumomta.com/support.

Getting Updates

You can subscribe to updates at https://kumomta.com/subscribe, we send updates periodically and will never sell nor share your information.

Talk to Us

We're available to talk about the project, book us at https://cal.com/team/kumomta/talk-with-kumomta.

S
Description
The first Open-Source high-performance MTA developed from the ground-up for high-volume email sending environments.
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