inputs: introduce new input concept

A pipeline input provides data in various forms to a `Stage`, like
files, OSTree commits or trees. The content can either be obtained
via a `Source` or have been built by a `Pipeline`. Thus an `Input`
is the bridge between various types of content that originate from
different types of sources.

The acceptable origin of the data is determined by the `Input`
itself. What types of input are allowed and required is determined
by the `Stage`.

To osbuild itself this is all transparent. The only data visible to
osbuild is the path. The input options are just passed to the
`Input` as is and the result is forwarded to the `Stage`.

osbuild/inputs.py

@@ -0,0 +1,82 @@
+"""
+Pipeline inputs
+
+A pipeline input provides data in various forms to a `Stage`, like
+files, OSTree commits or trees. The content can either be obtained
+via a `Source` or have been built by a `Pipeline`. Thus an `Input`
+is the bridge between various types of content that originate from
+different types of sources.
+
+The acceptable origin of the data is determined by the `Input`
+itself. What types of input are allowed and required is determined
+by the `Stage`.
+
+To osbuild itself this is all transparent. The only data visible to
+osbuild is the path. The input options are just passed to the
+`Input` as is and the result is forwarded to the `Stage`.
+"""
+
+
+import importlib
+import json
+import os
+import subprocess
+
+from typing import Dict, Tuple
+
+from .meta import ModuleInfo
+from .objectstore import StoreServer
+
+
+class Input:
+    """
+    A single input with its corresponding options.
+    """
+
+    def __init__(self, info: ModuleInfo, options: Dict):
+        self.info = info
+        self.options = options or {}
+
+    def name(self) -> str:
+        return self.info.name
+
+    def run(self, storeapi: StoreServer) -> Tuple[str, Dict]:
+        name = self.info.name
+        msg = {
+            "options": self.options,
+            "origin": name,
+            "api": {
+                "store": storeapi.socket_address
+            }
+        }
+
+        # 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"{name}: error: {r.stderr}") from None
+
+        if "error" in reply:
+            raise RuntimeError(f"{name}: " + reply["error"])
+
+        if r.returncode != 0:
+            raise RuntimeError(f"{name}: error {r.returncode}")
+
+        path, data = reply["path"], reply.get("data", {})
+
+        return path, data