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Set an optional max connection lifetime for SMTP connections
Zulip's email_senders worker process opens an SMTP connection on start-up, and re-checks that connection is still live with a NOOP command immediately before attempting to re-use it every time:
https://github.com/zulip/zulip/blob/77fd03e426e75b5914bece7da46a3bbaa8121f17/zerver/worker/queue_processors.py#L735-L751
https://github.com/zulip/zulip/blob/77fd03e426e75b5914bece7da46a3bbaa8121f17/zerver/lib/send_email.py#L294-L331
However, some SMTP servers react poorly to long-lived connections, closing connections not based on idle time, but rather connection time. This can result in connections dropping mid-send, which leaves it unclear to Zulip if the message has been processed or not.
We should add a configuration option which controls the max lifetime (in minutes) which we will reuse an open SMTP connection. If the connection has been open longer than this threshold, we should close the old connection and re-open a new one. If the setting is 0, we should tear open and close SMTP connections as needed, and never keep them open.
We should probably default this setting to 0, because the connection keep-alive is an optimization that most small installs won't need.
Hello @zulip/server-production members, this issue was labeled with the "area: production" label, so you may want to check it out!
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We should probably default this setting to 0, because the connection keep-alive is an optimization that most small installs won't need.
Should we keep the default as None in the beginning? where None defaults to the existing behaviour of not closing the connection. This might help to avoid any unexpected changes for people with large-ish installs. Or would this setting only impact zulip cloud in a meaningful way?
