Add the CHATHISTORY extension specification - refactored

extensions/chathistory.md

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+---
+title: IRCv3 chathistory extension
+layout: spec
+work-in-progress: true
+copyrights:
+  -
+    name: "Evan Magaliff"
+    period: "2017"
+    email: "evan@muffinmedic.net"
+  -
+    name: "Darren Whitlen"
+    period: "2018-2019"
+    email: "darren@kiwiirc.com"
+---
+## Description
+This document describes the format of the `chathistory` extension. This enables clients to request messages that were previously sent if they are still available on the server.
+
+The server as mentioned in this document may refer to either an IRC server or an IRC bouncer.
+
+## Implementation
+The `chathistory` extension uses the [chathistory][batch/chathistory] batch type and introduces a client command, `chathistory`.
+
+To fully support this extension, clients MUST support the [`batch`][batch], [`server-time`][server-time] and [`draft/message-tags`][draft/message-tags] capabilities.
+
+The `chathistory` capability MUST be negotiated. This allows the server and client to act differently when delivering message history on connection.
+
+An ISUPPORT token MUST be sent to the client to state the maximum number of messages a client can request in a single command, represented by an integer. `CHATHISTORY=50`. If `0`, the client SHOULD assume that there is no maximum number of messages.
+
+### `CHATHISTORY` Command
+`CHATHISTORY` content can be requested by the client by sending the `CHATHISTORY` command to the server. A `batch` MUST be returned by the server. If no content exists to return, an empty batch SHOULD be returned to avoid the client waiting for a reply and to indicate that no content is available.
+
+The `chathistory` command uses the following general syntax structure:
+
+    CHATHISTORY <subcommand> <target> <timestamp | msgid> <limit>
+
+The `target` parameter specifies a single buffer (channel or query name) from which history SHOULD be retrieved. Any `timestamp` values or parameters MUST be in the format of `YYYY-MM-DDThh:mm:ss.sssZ`.
+
+#### Subcommands
+
+The following subcommands are used to describe how the server should return messages relative to the `timestamp` or `msgid` given.
+
+#### `BEFORE`
+
+    CHATHISTORY BEFORE <target> <timestamp=YYYY-MM-DDThh:mm:ss.sssZ | msgid=1234> <limit>
+
+Request up to `limit` number of messages before and including the given `timestamp` or `msgid`. Only one timestamp or msgid MUST be given, not both.
+
+#### `AFTER`
+    CHATHISTORY AFTER <target> <timestamp=YYYY-MM-DDThh:mm:ss.sssZ | msgid=1234> <limit>
+Request up to `limit` number of messages after and excluding the given `timestamp` or `msgid`. Only one timestamp or msgid MUST be given, not both.
+
+#### `LATEST`
+    CHATHISTORY LATEST <target> <* | timestamp=YYYY-MM-DDThh:mm:ss.sssZ | msgid=1234> <limit>
+Request the most recent messages that have been sent since the given `timestamp` or `msgid`. If a `*` is given instead of a timestamp or msgid, the server MUST use the current time as a timestamp. The number of messages returned MUST be equal to or less than `limit`. Only one timestamp or msgid MUST be given, not both.
+
+#### `AROUND`
+    CHATHISTORY AROUND <target> <timestamp=YYYY-MM-DDThh:mm:ss.sssZ | msgid=1234> <limit>
+Request a number of messages before and after the `timestamp` or `msgid` with the total number of returned messages not exceeding `limit`. The implementation may decide how many messages to include before and after the selected message. Only one timestamp or msgid MUST be given, not both.
+
+This is useful for retrieving conversation context around a single message.
+
+#### `BETWEEN`
+    CHATHISTORY BETWEEN <target> <timestamp=YYYY-MM-DDThh:mm:ss.sssZ | msgid=1234> <timestamp=YYYY-MM-DDThh:mm:ss.sssZ | msgid=1234> <limit>
+Request up to `limit` number of messages between the given `timestamp` or `msgid` values. The returned messages MUST start from the inclusive first message selector, while excluding and finishing on the second - this may be forwards or backwards in time.
+
+#### Returned message notes
+The returned messages MUST be in ascending time order and the `server-time` SHOULD be the time at which the message was received from the IRC server. The `msgid` that identifies each individual message in a response MUST be the `msgid` as when  received from the IRC server.
+
+#### Errors and Warnings
+Errors are returned using the standard replies syntax.
+
+If the server receives a `CHATHISTORY` command with an unknown subcommand, the `UNKNOWN_COMMAND` error code MUST be returned.
+> FAIL CHATHISTORY UNKNOWN_COMMAND the_given_command :Unknown command
+
+If the server receives a `CHATHISTORY` command with missing parameters, the `NEED_MORE_PARAMS` error code MUST be returned.
+> FAIL CHATHISTORY NEED_MORE_PARAMS the_given_command :Missing parameters
+
+If no message history can be returned due to an error, the `MESSAGE_ERROR` error code SHOULD be returned.
+> FAIL CHATHISTORY MESSAGE_ERROR the_given_command [extra_context] :Messages could not be retrieved
+
+### Examples
+
+Requesting the latest conversation upon joining a channel
+~~~~
+[c] CHATHISTORY LATEST #channel * 50
+[s] :irc.host BATCH +ID chathistory #channel
+[s] @batch=ID;draft/msgid=1234;time=2019-01-04T14:33:26.123Z :nick!ident@host PRIVMSG #channel :message
+[s] @batch=ID;draft/msgid=1235;time=2019-01-04T14:33:38.123Z :nick!ident@host NOTICE #channel :message
+[s] @batch=ID;draft/msgid=1238;time=2019-01-04T14:34:17.123Z;+client_tag=val :nick!ident@host PRIVMSG #channel :
ACTION message
+[s] :irc.host BATCH -ID
+~~~~
+
+Requesting further message history than our client currently has
+~~~~
+[c] CHATHISTORY BEFORE bob timestamp=2019-01-04T14:34:17.123Z 50
+[s] :irc.host BATCH +ID chathistory bob
+[s] @batch=ID;draft/msgid=1234;time=2019-01-04T14:34:09.123Z :bob!ident@host PRIVMSG alice :hello
+[s] @batch=ID;draft/msgid=1235;time=2019-01-04T14:34:10.123Z :alice!ident@host PRIVMSG bob :hi! how are you?
+[s] @batch=ID;draft/msgid=1238;time=2019-01-04T14:34:16.123Z; :bob!ident@host PRIVMSG alice :I'm good, thank you!
+[s] :irc.host BATCH -ID
+~~~~
+
+## Use Cases
+The batch type and supporting command are useful for allowing an "infinite scroll" type capability within the client. A client will, upon scrolling to the top of the active window or a manual trigger, may request `chathistory` from the server and, after receiving returned content, append it to the top of the window. Users can repeat this historic scrolling to retrieve prior history until limitations are met (see below).
+
+Upon joining a channel, a client may request the latest messages for the buffer so that the active conversation context may be retrieved.
+
+## Security Considerations
+Secure identification of users and clients MUST exist in order to ensure that users cannot obtain history they are not authorised to view. Use of account names, internal account identifiers, or certificate fingerprints SHOULD be strongly considered when matching content to users. If a client requests content for a target that they do not have permission for, eg. a channel they are banned from, an empty batch SHOULD be returned as if no content exists.
+
+While an ISUPPORT token value of `0` may be used to indicate no message limit exists, servers SHOULD set and enforce a reasonable maximum and properly throttle `CHATHISTORY` commands to prevent abuse.