client access: add support for snap permissions

This patch adds to wireplumber code to manage the Snap audio
permissions.

SNAP containers have two main "audio" rules:

 * audio-playback: the applications inside the container can
   send audio samples into a sink
 * audio-record: the applications inside the container can
   get audio samples from a source

Also, old SNAP containers had the "pulseaudio" rule, which just
exposed the pulseaudio socket directly, without limits. This
is similar to the current Flatpak audio permissions.

In the pulseaudio days, an specific pulseaudio module was used
that checked the permissions given to the application and
allowed or forbide access to the pulseaudio operations.
With the change to pipewire, this functionality must be
implemented in pipewire-pulse and wireplumber to guarantee
the sandbox security.

The current code checks for the presence of the pipewire.snap.id
property in a client, in which case it will read the
pipewire.snap.audio.playback and pipewire.snap.audio.record
properties, and allow or deny access to that client to
the nodes with Audio/Sink or Audio/Source media.class
property.

See !567 and pipewire!1779

src/config/wireplumber.conf

@@ -406,10 +406,15 @@ wireplumber.components = [
     provides = script.client.access-portal
     requires = [ support.portal-permissionstore ]
   }
+  {
+    name = client/access-snap.lua, type = script/lua
+    provides = script.client.access-snap
+  }
   {
     type = virtual, provides = policy.client.access
     wants = [ script.client.access-default,
-              script.client.access-portal ]
+              script.client.access-portal,
+              script.client.access-snap ]
   }
 
   ## Device profile selection hooks

src/scripts/client/access-snap.lua

@@ -0,0 +1,87 @@
+-- Manage snap audio permissions
+--
+-- Copyright © 2023 Canonical Ltd.
+--    @author Sergio Costas Rodriguez <sergio.costas@canonical.com>
+--
+-- SPDX-License-Identifier: MIT
+
+function removeClientPermissionsForOtherClients (client)
+  -- Remove access to any other clients, but allow all the process of the
+  -- same snap to access their elements
+  local client_id = client.properties["pipewire.snap.id"]
+  for snap_client in clients_snap:iterate() do
+    local snap_client_id = snap_client.properties["pipewire.snap.id"]
+    if snap_client_id ~= client_id then
+      client:update_permissions { [snap_client["bound-id"]] = "-" }
+    end
+  end
+  for no_snap_client in clients_no_snap:iterate() do
+    client:update_permissions { [no_snap_client["bound-id"]] = "-" }
+  end
+end
+
+function updateClientPermissions (client)
+  -- Remove access to Audio/Sources and Audio/Sinks based on snap permissions
+  for node in nodes_om:iterate() do
+    local node_id = node["bound-id"]
+    local property = "pipewire.snap.audio.playback"
+
+    if node.properties["media.class"] == "Audio/Source" then
+      property = "pipewire.snap.audio.record"
+    end
+
+    if client.properties[property] ~= "true" then
+      client:update_permissions { [node_id] = "-" }
+    end
+  end
+end
+
+clients_snap = ObjectManager {
+  Interest {
+    type = "client",
+    Constraint { "pipewire.snap.id", "+", type = "pw"},
+  }
+}
+
+clients_no_snap = ObjectManager {
+  Interest {
+    type = "client",
+    Constraint { "pipewire.snap.id", "-", type = "pw"},
+  }
+}
+
+nodes_om = ObjectManager {
+  Interest {
+    type = "node",
+    Constraint { "media.class", "matches", "Audio/*"}
+  }
+}
+
+clients_snap:connect("object-added", function (om, client)
+  -- If a new snap client is added, adjust its permissions
+  updateClientPermissions (client)
+  removeClientPermissionsForOtherClients (client)
+end)
+
+clients_no_snap:connect("object-added", function (om, client)
+  -- If a new, non-snap client is added,
+  -- remove access to it from other snaps
+  client_id = client["bound-id"]
+  for snap_client in clients_snap:iterate() do
+    if client.properties["pipewire.snap.id"] ~= nil then
+      snap_client:update_permissions { [client_id] = "-" }
+    end
+  end
+end)
+
+nodes_om:connect("object-added", function (om, node)
+  -- If a new Audio/Sink or Audio/Source node is added,
+  -- adjust the permissions in the snap clients
+  for client in clients_snap:iterate() do
+    updateClientPermissions (client)
+  end
+end)
+
+clients_snap:activate()
+clients_no_snap:activate()
+nodes_om:activate()