fix(installer): resolve nested loop ancestry to a fixed point

hydra/installer.py

@@ -74,6 +74,7 @@ class Topology:
     def __init__(self):
         self.nodes = {}
         self.edges = {}
+        self.parents = {}
         self.protected = set()
 
     @classmethod
@@ -112,6 +113,7 @@ class Topology:
                 raise InstallError('Inconsistent duplicate lsblk node')
             result.nodes[path] = node
             result.edges.setdefault(path, set())
+            result.parents.setdefault(path, set())
             if any(m for m in mounts):
                 result.protected.add(path)
             pkname = node.get('pkname')
@@ -168,6 +170,17 @@ class Topology:
     def link(self, child, parent):
         if child == parent or child not in self.nodes or parent not in self.nodes:
             raise InstallError('Unresolved or cyclic block-device parent')
+        # Backing links discovered after lsblk must obey the same directed
+        # ancestry rule. An undirected shared disk is not itself a cycle.
+        seen, pending = set(), [parent]
+        while pending:
+            current = pending.pop()
+            if current == child:
+                raise InstallError('Cyclic block-device backing ancestry')
+            if current not in seen:
+                seen.add(current)
+                pending.extend(self.parents[current] - seen)
+        self.parents[child].add(parent)
         self.edges[child].add(parent)
         self.edges[parent].add(child)
 
@@ -240,16 +253,6 @@ class SystemProbe:
             source = self._block_source(mount, topo)
             if source:
                 topo.protect(source)
-        # Mounted loops may hide the physical filesystem that stores their file.
-        for path, node in topo.nodes.items():
-            if node['type'] == 'loop' and path in topo.protected:
-                identity = self._loop_backing_identity(path, node['maj:min'])
-                sources = [p for p, n in topo.nodes.items() if n['maj:min'] == identity]
-                if len(sources) != 1:
-                    raise InstallError(f'Unknown physical backing for mounted loop: {path} ({identity})')
-                source = sources[0]
-                topo.protect(source)
-                topo.link(path, source)
         if not Path(payload).is_file():
             raise InstallError(f'Prepared squashfs payload is not a regular file: {payload}')
         source = self._source_for_path(self.resolve(payload), topo)
@@ -257,6 +260,27 @@ class SystemProbe:
             # tmpfs/overlay payload location cannot establish live-medium ancestry.
             raise InstallError('Cannot identify the payload backing block device; use --payload on the mounted live medium')
         topo.protect(source)
+        # Loops can hide further loops, including ancestors of a mounted
+        # partition or device mapper. Resolve protected components to a fixed
+        # point, independent of lsblk ordering and payload discovery order.
+        inspected = set()
+        while True:
+            protected_devices = set()
+            for path in topo.protected:
+                protected_devices.update(topo.component(path))
+            pending = [path for path in topo.nodes if path in protected_devices
+                       and topo.nodes[path]['type'] == 'loop' and path not in inspected]
+            if not pending:
+                break
+            for path in pending:
+                identity = self._loop_backing_identity(path, topo.nodes[path]['maj:min'])
+                sources = [p for p, n in topo.nodes.items() if n['maj:min'] == identity]
+                if len(sources) != 1:
+                    raise InstallError(f'Unknown physical backing for mounted loop: {path} ({identity})')
+                source = sources[0]
+                topo.link(path, source)
+                topo.protect(source)
+                inspected.add(path)
         return topo
 
     @staticmethod