fix(linstorvolumemanager): robustify "usable size" getters

Impacted functions: `_get_volumes_info` and `_get_volume_node_names_and_size`.

Before this change "usable_size" validity was
checked too early and which could lead to an exception for
no good reason while the size could be known on at
least one host despite an issue on other machines.

Signed-off-by: Ronan Abhamon <ronan.abhamon@vates.tech>

Author: Wescoeur

drivers/linstorvolumemanager.py

@@ -2111,7 +2111,7 @@ def _get_volumes_info(self, volume_name=None):
         if not self._volume_info_cache_dirty:
             return self._volume_info_cache
 
-        for resource in self._get_resource_cache().resources:
+        def process_resource(resource):
             if resource.name not in all_volume_info:
                 current = all_volume_info[resource.name] = self.VolumeInfo(
                     resource.name
@@ -2143,15 +2143,21 @@ def _get_volumes_info(self, volume_name=None):
                     ):
                         current.virtual_size = usable_size
 
-        if current.virtual_size <= 0:
-            raise LinstorVolumeManagerError(
-               'Failed to get usable size of `{}` on `{}`'
-               .format(resource.name, volume.storage_pool_name)
-            )
+        try:
+            for resource in self._get_resource_cache().resources:
+                process_resource(resource)
+            for volume in all_volume_info.values():
+                if volume.virtual_size <= 0:
+                    raise LinstorVolumeManagerError(
+                        'Failed to get usable size of `{}`'
+                        .format(volume.name)
+                    )
 
-        for current in all_volume_info.values():
-            current.allocated_size *= 1024
-            current.virtual_size *= 1024
+                volume.allocated_size *= 1024
+                volume.virtual_size *= 1024
+        except LinstorVolumeManagerError:
+            self._mark_resource_cache_as_dirty()
+            raise
 
         self._volume_info_cache_dirty = False
         self._volume_info_cache = all_volume_info
@@ -2170,16 +2176,14 @@ def _get_volume_node_names_and_size(self, volume_name):
                     node_names.add(resource.node_name)
 
                     current_size = volume.usable_size
-                    if current_size < 0:
-                        raise LinstorVolumeManagerError(
-                           'Failed to get usable size of `{}` on `{}`'
-                           .format(resource.name, volume.storage_pool_name)
-                        )
-
-                    if size < 0:
+                    if current_size > 0 and (current_size < size or size < 0):
                         size = current_size
-                    else:
-                        size = min(size, current_size)
+
+        if size < 0:
+            raise LinstorVolumeManagerError(
+                'Failed to get usable size of `{}` on `{}`'
+                .format(resource.name, volume.storage_pool_name)
+            )
 
         return (node_names, size * 1024)