osbuild: introduce mount host service

Allows stages to access file systems provided by devices.
This makes mount handling transparent to the stages, i.e.
the individual stages do not need any code for different
file system types and the underlying devices.

osbuild/formats/v2.py

@@ -12,6 +12,7 @@ from ..sources import Source
 VERSION = "2"
 
 
+# pylint: disable=too-many-statements
 def describe(manifest: Manifest, *, with_id=False) -> Dict:
 
     # Undo the build, runner pairing introduce by the loading
@@ -72,6 +73,24 @@ def describe(manifest: Manifest, *, with_id=False) -> Dict:
         }
         return desc
 
+    def describe_mount(mnt):
+        desc = {
+            "type": mnt.info.name,
+            "device": mnt.device.name,
+            "target": mnt.target
+        }
+
+        if mnt.options:
+            desc["options"] = mnt.options
+        return desc
+
+    def describe_mounts(mounts: Dict):
+        desc = {
+            name: describe_mount(mnt)
+            for name, mnt in mounts.items()
+        }
+        return desc
+
     def describe_stage(s: Stage):
         desc = {
             "type": s.info.name
@@ -87,6 +106,10 @@ def describe(manifest: Manifest, *, with_id=False) -> Dict:
         if devs:
             desc["devices"] = devs
 
+        mounts = describe_mounts(s.mounts)
+        if mounts:
+            desc["mounts"] = mounts
+
         ips = describe_inputs(s.inputs)
         if ips:
             desc["inputs"] = ips
@@ -187,6 +210,22 @@ def load_input(name: str, description: Dict, index: Index, stage: Stage, manifes
         ip.add_reference(r, desc)
 
 
+def load_mount(name: str, description: Dict, index: Index, stage: Stage):
+    mount_type = description["type"]
+    info = index.get_module_info("Mount", mount_type)
+
+    source = description["source"]
+    target = description["target"]
+
+    options = description.get("options", {})
+
+    device = stage.devices.get(source)
+    if not device:
+        raise ValueError(f"Unknown device '{source}' for mount '{name}'")
+
+    stage.add_mount(name, info, device, target, options)
+
+
 def load_stage(description: Dict, index: Index, pipeline: Pipeline, manifest: Manifest):
     stage_type = description["type"]
     opts = description.get("options", {})
@@ -202,6 +241,10 @@ def load_stage(description: Dict, index: Index, pipeline: Pipeline, manifest: Ma
     for name, desc in ips.items():
         load_input(name, desc, index, stage, manifest)
 
+    mounts = description.get("mounts", {})
+    for name, desc in mounts.items():
+        load_mount(name, desc, index, stage)
+
     return stage
 
 
@@ -354,8 +397,8 @@ def validate(manifest: Dict, index: Index) -> ValidationResult:
         res = schema.validate(stage)
         result.merge(res, path=path)
 
-        validate_stage_modules("Device", stage, path)
-        validate_stage_modules("Input", stage, path)
+        for mod in ("Device", "Input", "Mount"):
+            validate_stage_modules(mod, stage, path)
 
     def validate_pipeline(pipeline, path):
         stages = pipeline.get("stages", [])