sources: introduce the Source class

Very much like, stages and inputs, a new `Source` calls represents
a source type with its options. A `download` method on the `Source`
class can be used to only donwload content to the cache.

osbuild/sources.py

@@ -1,7 +1,59 @@
+import os
+import importlib
 import json
 import subprocess
+
 from . import api
+from .objectstore import ObjectStore
 from .util import jsoncomm
+from .util.types import PathLike
+
+
+class Source:
+    """
+    A single source with is corresponding options.
+    """
+    def __init__(self, info, options) -> None:
+        self.info = info
+        self.options = options
+
+    def download(self, store: ObjectStore, libdir: PathLike):
+        source = self.info.name
+        cache = os.path.join(store.store, "sources", source)
+        msg = {
+            "options": self.options,
+            "cache": cache,
+            "output": None,
+            "checksums": [],
+            "libdir": os.fspath(libdir)
+        }
+
+        # We want the `osbuild` python package that contains this
+        # very module, which might be different from the system wide
+        # installed one, to be accessible to the Input programs so
+        # we detect our origin and set the `PYTHONPATH` accordingly
+        modorigin = importlib.util.find_spec("osbuild").origin
+        modpath = os.path.dirname(modorigin)
+        env = os.environ.copy()
+        env["PYTHONPATH"] = os.path.dirname(modpath)
+
+        r = subprocess.run([self.info.path],
+                           env=env,
+                           input=json.dumps(msg),
+                           stdout=subprocess.PIPE,
+                           encoding="utf-8",
+                           check=False)
+
+        try:
+            reply = json.loads(r.stdout)
+        except ValueError:
+            raise RuntimeError(f"{source}: error: {r.stderr}") from None
+
+        if "error" in reply:
+            raise RuntimeError(f"{source}: " + reply["error"])
+
+        if r.returncode != 0:
+            raise RuntimeError(f"{source}: error {r.returncode}")
 
 
 class SourcesServer(api.BaseAPI):