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This requires some cooperation between the plugin and the host, and will only work for specific messages.
The way it works is as follows:
* the plugin sends a message containing the key "blocking_request" (with any value)
* this will cause the "send message" function to block (continuing to pull incoming messages off the network socket and queue them) until it receives a message from the host containing the key "blocking_response"
** NOTE: if the plugin sends a blocking_request that isn't set up to cause the host to send back a blocking_response, it will block forever
* the blocking_response message will be handed to the plugin's "receive message" function _immediately_ (before the "send message" function returns)
** this means that the plugin can extract response data for use by the the code that sent the message (but is still blocked inside the "send message" call)
** NOTE: this BREAKS the invariant stating that the plugin's "receive message" function will never be called recursively, and the plugin MUST be prepared to deal with this
* after the plugin finishes processing the blocking_response message, the "send message" function that was called with the blocking_request message will return to the plugin
* any queued messages will be delivered after the outer invocation of the plugin's "receive message" function returns (as normal)
Inside the viewer, the code can tell when a plugin is in this blocked state by calling LLPluginProcessParent::isBlocked(). LLPluginClassMedia uses this to avoid sending mouse-move and size-change messages to blocked plugins.
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