WirePlumber script to automatically attach a filter to a device

auto_attach_filter.lua

#!/usr/bin/wpexec

local mic_name = "alsa_input.usb-C-Media_Electronics_Inc._USB_Audio_Device-00.mono-fallback"
local filter_name = "rnnoise_source"

local link_props  =  {
    ["link.output.port"] = nil,
    ["link.input.port"] = nil,
    ["link.output.node"] = nil,
    ["link.input.node"] = nil
}
local link = nil 

mic_om = ObjectManager {
    Interest {
        type = "node",
        Constraint {
            "node.name", "=", mic_name
        }
    }
}
mic_port_om = ObjectManager {
    Interest {
        type = "port",
        Constraint {
            "port.direction", "=", "out"
        }
    }
}

filter_om = ObjectManager {
    Interest {
        type = "node",
        Constraint {
            "node.name", "=", filter_name
        }
    }
}

filter_port_om = ObjectManager {
    Interest {
        type = "port",
        Constraint {
            "port.direction", "=", "in"
        }
    }
}

link_om = ObjectManager {
    Interest {
        type = "link"
    }
}

function find_port(node_id, port_om)
    for port in port_om:iterate() do
        if port.properties["node.id"] == node_id then
            return port.properties["object.id"]
        end
    end
end

function add_node(node, direction, port_om)
    local props = node.properties
    local object_id = props["object.id"]
    link_props["link." .. direction .. ".node"] = object_id
    if link_props["link." .. direction .. ".port"] == nil then
        link_props["link." .. direction .. ".port"] = find_port(object_id, port_om)
    end
end

function try_activate_link()
    if link ~= nil then
        print("Link is active")
        return
    end
    local count = 0
    for k,v in pairs(link_props) do
        count = count + 1
    end
    if count ~= 4 then
        print("Not all properties are set")
        return
    end
    print("Setup link")
    for l in link_om:iterate {Constraint {"link.input.node","=", link_props["link.input.node"]}} do
        print("Removed link that were attached to filter")
        l:request_destroy()
    end
    link = Link("link-factory", link_props)
    link:activate(1)
end

function deactivate_link()
        if link == nil then
            return 
        end
        link:request_destroy()
        link = nil
        link_props["link.output.port"] = nil
        link_props["link.output.node"] = nil
end

function add_port(port, direction)
    if link_props["link." .. direction .. ".node"] == port.properties["node.id"] then
        link_props["link." .. direction .. ".port"] = port.properties["object.id"]
    end
end

mic_port_om:connect("object-added", function(om, port)
    add_port(port, "output")
    try_activate_link()
end)

filter_port_om:connect("object-added", function(om, port)
    add_port(port, "input")
    try_activate_link()
end)

filter_om:connect("object-added", function(om, node)
    add_node(node, "input", filter_port_om)
    try_activate_link()
end)


mic_om:connect("object-added", function(om, node)
    add_node(node, "output", mic_port_om)
    try_activate_link()
end)

mic_om:connect("object-removed", function (om, node)
    deactivate_link()
end)

link_om:connect("object-removed", function (om, l)
    if l == link then
        print("Link was removed")
        link = nil
        print("Try to add the link again")
        try_activate_link()
    end
end)

mic_om:activate()
filter_om:activate()
link_om:activate()
mic_port_om:activate()
filter_port_om:activate()

An obvious alternative is to use the target.object property for the capture.node argument when the filter (chain) is initialised.