rbd: avoid accidental root disk downsizes

PL-133166

CHANGES.txt

@@ -12,6 +12,9 @@ Release notes
   impatient clients and we ended up not thawing correctly when
   creating the snapshot resulted in an ImageExists error. (PL-133149)
 
+- rbd: fix disk downsizing that causes potential data loss
+  when inventory data and real data mismatch and may shrink root images
+  accidentally. (PL-133166)
 
 1.6 (2024-10-23)
 ----------------

src/fc/qemu/hazmat/volume.py

@@ -234,7 +234,10 @@ def size(self):
         return self.rbdimage.size()
 
     def ensure_size(self, size):
-        if self.size == size:
+        # The existing size must be considered as a minimum, because we can't
+        # just reduce images that already exist and are bigger and expect them
+        # to work properly.
+        if self.size >= size:
             return
         self.rbdimage.resize(size)
 

tests/hazmat/test_volume.py

@@ -94,6 +94,9 @@ def test_volume_size(tmp_spec):
     # Call ensure multiple times to help triggering caching code paths.
     volume.ensure_size(2048)
     assert volume.size == 2048
+    # Trying to reduce the size doesn't make a change
+    volume.ensure_size(1024)
+    assert volume.size == 2048
 
 
 def test_volume_shared_lock_protection(tmp_spec):

tests/test_vm.py

@@ -638,6 +638,17 @@ def test_vm_resize_disk(vm, patterns):
 
     assert get_log() == resize_noop
 
+    # Decreasing the desired disk size does not trigger a change.
+    vm.cfg["root_size"] = 6442450944 - 1
+    vm.ensure_online_disk_size()
+    resize_noop2 = patterns.resize_noop2
+    resize_noop2.in_order(
+        """
+check-disk-size action=none found=6442450944 machine=simplevm wanted=6442450943
+"""
+    )
+    assert get_log() == resize_noop2
+
 
 def test_swap_size():
     assert swap_size(512) == 1024 * MiB