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diff --git a/doc/info/mds.texinfo b/doc/info/mds.texinfo index fbe2993..498b187 100644 --- a/doc/info/mds.texinfo +++ b/doc/info/mds.texinfo @@ -295,6 +295,7 @@ and wait for a reply from all of them. * Signals:: Signalling individual servers * Filesystem:: The display server's footprint on the filesystem * Message Passing:: Sending messages between servers and clients +* Interception:: Implementing protocols and writing unanticipated clients @end menu @node Environment Variables @@ -516,6 +517,247 @@ included in the header. +@node Interception +@section Interception + +As discussed in @ref{Interprocess Communication}, +interception in the primary feature of +@command{mds}'s message passing system. +Not only does it enable servers to select +which message it wants to receive in order +to provide it's service. It also enables +clients to do anything, things that was +never anticipated. As an exaple of its +power, @command{mds} does not provide any +protocol for taking screenshots or recording +a session. Instead, a screenshot application +signs up for messages pass between the +compositor and presentation servers, and +simply requests that the compositor resends +the screen, a feature intended for the +presentation servers. A screen recoding +application would do the same and just +hang on and record all message passed +between the servers. + +If you want your server or client to +receive all messages passed around in +the display server, simply sign up for +all messages: + +@example +Command: intercept\n +Message ID: 0\n +\n +@end example + +But if you only want messages contain +the header @code{Command}, include +that header in the payload of the message: + +@example +Command: intercept\n +Message ID: 0\n +Length: 8\n +\n +Command\n +@end example + +It is allowed to include multiple headers. +You can also be more strict, and require +a specific value for a header, for example: + +@example +Command: intercept\n +Message ID: 0\n +Length: 16\n +\n +Command: get-vt\n +@end example + +You may mix these two types of requirements +freely. Your client will receive any message +that satisfies at least one of the requirements, +these requirements may be split into multiple +message or coalesced into one message; but +you cannot request to include receive a message +if multiple requirements are satisfied. + +Alternatively you can choose to stop receiving +message that satisfies requirements. For example: + +@example +Command: intercept\n +Stop: yes\n +Message ID: 1\n +Length: 16\n +\n +Command: get-vt\n +@end example + +Or stop receiving all messages: + +@example +Command: intercept\n +Stop: yes\n +Message ID: 1\n +\n +@end example + +Note that this will stop you from receiving +messages contain the @code{To} header addressed +to you until you request to receiving such +messages again. + +When you sign up for message you may request +to be able to modify them before that are +send to the next client in the list of client +that should receive them. To do this include +the header--value pair @code{Modifying: yes}: + +@example +Command: intercept\n +Modifying: yes\n +Message ID: 0\n +Length: 30\n +\n +Command: keyboard-enumeration\n +@end example + +It is up to the client to keep track of +which message that it may modify. When +you receive a message that you can modify +you must respond when you are done with +the message. + +For example, if you have signed up +for @code{Command: keyboard-enumeration} +with the ability to modify such messages +and the message + +@example +Command: keyboard-enumeration\n +To: 0:1\n +In response to: 2\n +Message ID: 1\n +Length: 7\n +\n +kernel\n +@end example + +is send from a server, you may receive +it as + +@example +Command: keyboard-enumeration\n +To: 0:1\n +In response to: 2\n +Message ID: 1\n +Length: 7\n +Modify ID: 4\n +\n +kernel\n +@end example + +Be aware that the @code{Modify ID} may +be included even if you have not signed +up to be able to modify the message, +it is enough that one client before you +has or it was originally included +@footnote{You may however not include +this header when you send out an +orginal message}. + +If you receive the message as such +and want to add the line +@code{on-screen-keyboard-20376} to +the payload should send out: +@footnote{The first line containing +starting with @code{Message ID} is an +example, it should be whatever is +appropriate for your client.} + +@example +Modify ID: 4\n +Message ID: 2\n +Modify: yes\n +Length: 127\n +\n +Command: keyboard-enumeration\n +To: 0:1\n +In response to: 2\n +Message ID: 1\n +Length: 32\n +Modify ID: 4\n +\n +kernel\n +on-screen-keyboard-20376\n +@end example + +If you however decide not to modify +the message send out + +@example +Modify ID: 4\n +Message ID: 2\n +Modify: no\n +\n +@end example + +There is also a third option: +to consume to the message. This +stops any further clients from +receiving the message. This is +done by modifying the message +into an empty message: + +@example +Modify ID: 4\n +Message ID: 2\n +Modify: yes\n +\n +@end example + +You may choose to include the +header--value pair @code{Length: 0}, +it is however redundant and +discouraged. + +This mechanism of being able to +modify message does not make much +sense unless you can control in +the order the clients receive +messages. This is done with what +is called priority. The higher +priority you have, the earlier +you will receive the message. The +default priority is zero, and the +priority is bound to a signed +64-bit integer. If you want to +be able to list yourself in +@code{Command: keyboard-enumeration} +message, you should sign up +with a positive priority since +the final recipient or requested +the enumeration will receive it +with priority zero. Therefore +you should sign up for such message +with a message like: +@footnote{4611686018427387904, is +halfway to the maximium value.} + +@example +Command: intercept\n +Modifying: yes\n +Priority: 4611686018427387904\n +Message ID: 0\n +Length: 30\n +\n +Command: keyboard-enumeration\n +@end example + + + @node GNU Free Documentation License @appendix GNU Free Documentation License @include fdl.texinfo |