Handle global disposable preload feature

For: QubesOS/qubes-issues#1512
For: QubesOS/qubes-desktop-linux-manager#262

Author: ben-grande

qubes/vm/adminvm.py

@@ -347,3 +347,71 @@ async def run_service_for_stdio(self, *args, input=None, **kwargs):
             )
 
         return stdouterr
+
+    @qubes.events.handler("property-set:default_dispvm")
+    def on_property_set_default_dispvm(
+        self, event, name, newvalue, oldvalue=None
+    ):  # pylint: disable=unused-argument
+        if newvalue == oldvalue:
+            return
+        old_appvm = self.app.domains[oldvalue]
+        appvm = self.app.domains[newvalue]
+        appvm.preload_max_ignore_global = False
+        for qube in [old_appvm, appvm]:
+            asyncio.ensure_future(
+                qube.fire_event_async("domain-preload-dispvm-start")
+            )
+
+    @qubes.events.handler("property-del:default_dispvm")
+    @qubes.events.handler("property-reset:default_dispvm")
+    def on_property_del_default_dispvm(
+        self, event, name, oldvalue=None
+    ):  # pylint: disable=unused-argument
+        if oldvalue:
+            appvm = self.app.domains[oldvalue]
+            appvm.preload_max_ignore_global = True
+            asyncio.ensure_future(
+                appvm.fire_event_async("domain-preload-dispvm-start")
+            )
+
+    @qubes.events.handler("domain-feature-delete:preload-dispvm-max")
+    def on_feature_delete_preload_dispvm_max(
+        self, event, feature
+    ):  # pylint: disable=unused-argument
+        appvm = getattr(self, "default_dispvm", None)
+        if not appvm:
+            return
+        appvm.preload_max_ignore_global = True
+        asyncio.ensure_future(
+            appvm.fire_event_async("domain-preload-dispvm-start")
+        )
+
+    @qubes.events.handler("domain-feature-pre-set:preload-dispvm-max")
+    def on_feature_pre_set_preload_dispvm_max(
+        self, event, feature, value, oldvalue=None
+    ):  # pylint: disable=unused-argument
+        appvm = getattr(self, "default_dispvm", None)
+        if not appvm:
+            return
+        appvm.fire_event(
+            "domain-feature-pre-set:preload-dispvm-max",
+            pre_event=True,
+            feature="preload-dispvm-max",
+            value=value,
+            oldvalue=oldvalue,
+        )
+
+    @qubes.events.handler("domain-feature-set:preload-dispvm-max")
+    def on_feature_set_preload_dispvm_max(
+        self, event, feature, value, oldvalue=None
+    ):  # pylint: disable=unused-argument
+        appvm = getattr(self, "default_dispvm", None)
+        if not appvm:
+            return
+        appvm.preload_max_ignore_global = False
+        appvm.fire_event(
+            "domain-feature-set:preload-dispvm-max",
+            feature="preload-dispvm-max",
+            value=value,
+            oldvalue=oldvalue,
+        )

qubes/vm/mix/dvmtemplate.py

@@ -91,6 +91,11 @@ def on_domain_loaded(self, event):  # pylint: disable=unused-argument
     def on_feature_delete_preload_dispvm_max(
         self, event, feature
     ):  # pylint: disable=unused-argument
+        if (
+            self == getattr(self, "default_dispvm", None)
+            and "preload-dispvm-max" in self.app.domains["dom0"].features
+        ):
+            return
         self.remove_preload_excess(0)
 
     @qubes.events.handler("domain-feature-pre-set:preload-dispvm-max")
@@ -294,11 +299,22 @@ def get_feat_preload(self) -> list[str]:
         return value.split(" ") if value else []
 
     def get_feat_preload_max(self) -> int:
-        """Get the ``preload-dispvm-max`` feature as an integer."""
+        """Get the ``preload-dispvm-max`` feature as an integer.
+
+        :param global_feat: ignore global setting.
+        """
         feature = "preload-dispvm-max"
         assert isinstance(self, qubes.vm.BaseVM)
-        value = self.features.get(feature, 0)
-        return int(value) if value else 0
+        default_dispvm = getattr(self.app, "default_dispvm", None)
+        value = None
+        if not getattr(self, "preload_max_ignore_global", False):
+            if self == default_dispvm:
+                global_features = self.app.domains["dom0"].features
+                if feature in global_features:
+                    value = global_features.get(feature) or 0
+        if value is None:
+            value = self.features.get(feature) or 0
+        return int(value)
 
     def can_preload(self) -> bool:
         """Returns ``True`` if there is preload vacancy."""