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Problem: missing eventual group consistency |
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If group members are concurrently adding new members, |
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the new members will miss each other's additions, example: |
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1. Alice and Bob are in a two-member group |
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2. Then Alice adds Carol, while concurrently Bob adds Doris |
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Right now, the group has inconsistent memberships: |
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- Alice and Carol see a (Alice, Carol, Bob) group |
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- Bob and Doris see a (Bob, Doris, Alice) |
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This then leads to "sender is unknown" messages in the chat, |
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for example when Alice receives a message from Doris, |
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or when Bob receives a message from Carol. |
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There are also other sources for group membership inconsistency: |
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- leaving/deleting/adding in larger groups, while being offline, |
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increases chances for inconsistent group membership |
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- dropped group-membership messages |
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- group-membership messages landing in "Spam" |
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Note that all these problems (can) also happen with verified groups, |
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then raising "false alarms" which could lure people to ignore such issues. |
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IOW, it's clear we need to do something about it to improve overall |
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reliability in group-settings. |
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Solution: replay group modification messages on inconsistencies |
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For brevity let's abbreviate "group membership modification" as **GMM**. |
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Delta chat has explicit GMM messages, typically encrypted to the group members |
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as seen by the device that sends the GMM. The `Spec <https://github.com/deltachat/deltachat-core-rust/blob/master/spec.md#add-and-remove-members>`_ details the Mime headers and format. |
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If we detect membership inconsistencies we can resend relevant GMM messages |
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to the respective chat. The receiving devices can process those GMM messages |
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as if it would be an incoming message. If for example they have already seen |
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the Message-ID of the GMM message, they will ignore the message. It's |
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probably useful to record GMM message in their original MIME-format and |
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not invent a new recording format. Few notes on three aspects: |
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- **group-membership-tracking**: All valid GMM messages are persisted in |
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their full raw (signed/encrypted?) MIME-format in the DB. Note that GMM messages |
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already are in the msgs table, and there is a mime_header column which we could |
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extend to contain the raw Mime GMM message. |
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- **consistency_checking**: If an incoming GMM has a member list which is |
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not consistent with our own view, broadcast a "Group-Member-Correction" |
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message to all members containing a multipart list of GMMs. |
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- **correcting_memberships**: Upon receiving a Group-Member-Correction |
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message we pass the contained GMMs to the "incoming mail pipeline" |
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(without **consistency_checking** them, to avoid recursion issues) |
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Alice/Carol and Bob/Doris getting on the same page |
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++++++++++++++++++++++++++++++++++++++++++++++++++ |
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Recall that Alice/Carol and Bob/Doris had a differening view of |
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group membership. With the proposed solution, when Bob receives |
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Alice's "Carol added" message, he will notice that Alice (and thus |
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also carol) did not know about Doris. Bob's device sends a |
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"Chat-Group-Member-Correction" message containing his own GMM |
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when adding Doris. Therefore, the group's membership is healed |
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for everyone in a single broadcast message. |
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Alice might also send a Group-member-Correction message, |
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so there is a second chance that the group gets to know all GMMs. |
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Note, for example, that if for some reason Bobs and Carols provider |
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drop GMM messages between them (spam) that Alice and Doris can heal |
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it by resending GMM messages whenever they detect them to be out of sync. |
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Discussions of variants |
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++++++++++++++++++++++++ |
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- instead of acting on GMM messages we could send corrections |
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for any received message that addresses inconsistent group members but |
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a) this could delay group-membership healing |
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b) could lead to a lot of members sending corrections |
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c) means we might rely on "To-Addresses" which we also like to strike |
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at least for protected chats. |
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- instead of broadcasting correction messages we could only send it to |
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the sender of the inconsistent member-added/removed message. |
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A receiver of such a correction message would then need to forward |
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the message to the members it thinks also have an inconsistent view. |
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This sounds complicated and error-prone. Concretely, if Alice |
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receives Bob's "Member-added: Doris" message, then Alice |
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broadcasting the correction message with "Member-added: Carol" |
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would reach all four members, healing group consistency in one step. |
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If Bob meanwhile receives Alice's "Member-Added: Carol" message, |
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Bob would broadcast a correction message to all four members as well. |
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(Imagine a situation where Alice/Bob added Carol/Doris |
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while both being in an offline or bad-connection situation). |
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