allow system definition to be in live layer

Rather than always assuming it has to be mounted in from the layer to be
installed (snapd will now populate the seed in the target system when
its install API is called if it is empty).

examples/snaps/v2-systems.json

@@ -0,0 +1,43 @@
+{
+  "type": "sync",
+  "status-code": 200,
+  "status": "OK",
+  "result": {
+    "systems": [
+      {
+        "current": true,
+        "label": "20240314",
+        "model": {
+          "model": "ubuntu-core-24-amd64-dangerous",
+          "brand-id": "canonical",
+          "display-name": "ubuntu-core-24-amd64-dangerous"
+        },
+        "brand": {
+          "id": "canonical",
+          "username": "canonical",
+          "display-name": "Canonical",
+          "validation": "verified"
+        },
+        "actions": [
+          {
+            "title": "Reinstall",
+            "mode": "install"
+          },
+          {
+            "title": "Recover",
+            "mode": "recover"
+          },
+          {
+            "title": "Factory reset",
+            "mode": "factory-reset"
+          },
+          {
+            "title": "Run normally",
+            "mode": "run"
+          }
+        ],
+        "default-recovery-system": true
+      }
+    ]
+  }
+}

subiquity/server/controllers/filesystem.py

@@ -337,6 +337,15 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
             self._source_handler = None
 
     async def _get_system(self, variation_name, label):
+        systems = await self.app.snapdapi.v2.systems.GET()
+        labels = {system.label for system in systems.systems}
+        if label in labels:
+            try:
+                system = await self.app.snapdapi.v2.systems[label].GET()
+            except requests.exceptions.HTTPError as http_err:
+                log.warning("v2/systems/%s returned %s", label, http_err.response.text)
+                raise
+        else:
             try:
                 await self._mount_systems_dir(variation_name)
             except NoSnapdSystemsOnSource:
@@ -858,6 +867,9 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
     async def guided_core_boot(self, disk: Disk):
         # Formatting for a core boot classic system relies on some curtin
         # features that are only available with v2 partitioning.
+        systems = await self.app.snapdapi.v2.systems.GET()
+        labels = {system.label for system in systems.systems}
+        if self._info.label not in labels:
             await self._mount_systems_dir(self._info.name)
         self.model.storage_version = 2
         [volume] = self._info.system.volumes.values()

subiquity/server/controllers/tests/test_filesystem.py

@@ -432,6 +432,18 @@ class TestSubiquityControllerFilesystem(IsolatedAsyncioTestCase):
             },
         }
         requests_mocker = requests_mock.Mocker()
+        requests_mocker.get(
+            "http+unix://snapd/v2/systems",
+            json={
+                "type": "sync",
+                "status-code": 200,
+                "status": "OK",
+                "result": {
+                    "systems": [],
+                },
+            },
+            status_code=200,
+        )
         requests_mocker.get(
             "http+unix://snapd/v2/systems/enhanced-secureboot-desktop",
             json=json_body,
@@ -1397,9 +1409,10 @@ class TestCoreBootInstallMethods(IsolatedAsyncioTestCase):
             name="foo",
             label="system",
             system=snapdapi.SystemDetails(
+                label="system",
                 volumes={
                     "pc": snapdapi.Volume(schema="gpt", structure=structures),
-                }
+                },
             ),
         )
 

subiquity/server/snapdapi.py

@@ -203,6 +203,7 @@ class StorageEncryption:
 
 @attr.s(auto_attribs=True)
 class SystemDetails:
+    label: str
     current: bool = False
     volumes: Dict[str, Volume] = attr.Factory(dict)
     storage_encryption: Optional[StorageEncryption] = named_field(
@@ -210,6 +211,11 @@ class SystemDetails:
     )
 
 
+@attr.s(auto_attribs=True)
+class SystemsResponse:
+    systems: List[SystemDetails] = attr.Factory(list)
+
+
 class SystemAction(enum.Enum):
     INSTALL = "install"
 
@@ -258,6 +264,9 @@ class SnapdAPI:
                 ...
 
         class systems:
+            def GET() -> SystemsResponse:
+                ...
+
             @path_parameter
             class label:
                 def GET() -> SystemDetails: