fix(instances): a snapshot needs free space in the VM's storage pools

A disk-only snapshot was taken without any space check, though its
overlays grow as the guest writes and a later delete merges them down;
a full pool pauses the guest and can leave the snapshot undeletable.
Only a memory snapshot was checked, for the memory alone.

Before a snapshot, each storage pool with a disk of the VM must have
free a share of those disks' virtual size (10%, at least 1 GiB), and
the pool of the memory file the VM's memory on top. Both values are on
the Settings page (Snapshot Free Percent, Snapshot Free Min GiB; 0
turns a rule off, an invalid value keeps the default). The pools are
refreshed first, and a failed refresh or an unreadable volume refuses
the snapshot instead of skipping the check; a disk in no pool is not
checked. Nothing is reserved: later writes can still fill a pool.

appsettings/migrations/0017_snapshot_free_space_settings.py

@@ -0,0 +1,35 @@
+from django.core.management.color import no_style
+from django.db import migrations
+
+
+SETTINGS = [
+    ("Snapshot Free Percent", "SNAPSHOT_FREE_SPACE_PERCENT", "10",
+     "Free space a storage pool keeps for a snapshot, in percent of the VM's disks in it (0: none)"),
+    ("Snapshot Free Min GiB", "SNAPSHOT_FREE_SPACE_MIN_GIB", "1",
+     "Free space a storage pool keeps for a snapshot at least, in GiB (0: none)"),
+]
+
+
+def add_settings(apps, schema_editor):
+    setting = apps.get_model("appsettings", "AppSettings")
+    db_alias = schema_editor.connection.alias
+    # see 0014: move a PostgreSQL id sequence past the fixed ids first
+    with schema_editor.connection.cursor() as cursor:
+        for sql in schema_editor.connection.ops.sequence_reset_sql(no_style(), [setting]):
+            cursor.execute(sql)
+    for name, key, value, description in SETTINGS:
+        setting.objects.using(db_alias).get_or_create(
+            key=key, defaults={"name": name, "value": value, "choices": "", "description": description}
+        )
+
+
+def del_settings(apps, schema_editor):
+    setting = apps.get_model("appsettings", "AppSettings")
+    db_alias = schema_editor.connection.alias
+    setting.objects.using(db_alias).filter(key__in=[key for _, key, _, _ in SETTINGS]).delete()
+
+
+class Migration(migrations.Migration):
+    dependencies = [("appsettings", "0016_nic_type_rtl8139")]
+
+    operations = [migrations.RunPython(add_settings, del_settings)]

appsettings/tests/test_migrations.py

@@ -40,3 +40,14 @@ class NicTypeTestCase(TestCase):
         nic_type.fix_rtl8139(apps, SimpleNamespace(connection=connection))
         setting.refresh_from_db()
         self.assertEqual((setting.value, setting.choices), ("rtl8139", "default,e1000,e1000e,rtl8139,virtio"))
+
+
+class SnapshotFreeSpaceSettingsTestCase(TestCase):
+    def test_the_rows_fit_their_columns(self):
+        # PostgreSQL refuses a value longer than the column, sqlite does not
+        for key in ("SNAPSHOT_FREE_SPACE_PERCENT", "SNAPSHOT_FREE_SPACE_MIN_GIB"):
+            setting = AppSettings.objects.get(key=key)
+            for field in ("name", "key", "value", "description"):
+                limit = AppSettings._meta.get_field(field).max_length
+                if limit:
+                    self.assertLessEqual(len(getattr(setting, field)), limit, f"{key} {field}")