inputs: add files input

Create a new input for resources of the `org.osbuild.files` type,
i.e. individual files, addressed in the store via their content
hash.

inputs/org.osbuild.files

@@ -0,0 +1,105 @@
+#!/usr/bin/python3
+"""
+Inputs for individual files
+
+Provides all the files, named via their content hash, specified
+via `references` in a new directory.
+
+The returned data in `refs` is a dictionary where the keys are
+the content hash of a file and the values dictionaries with
+metadata for it. The input itself currently does not set any
+metadata itself, but will forward any metadata set via the
+`metadata` property. Keys in that must start with a prefix,
+like `rpm.` to avoid namespace clashes. This is enforced via
+schema validation.
+"""
+
+
+import json
+import sys
+import subprocess
+
+from osbuild.objectstore import StoreClient
+
+
+SCHEMA = r"""
+"additionalProperties": false,
+"required": ["origin", "references"],
+"properties": {
+  "origin": {
+    "description": "The origin of the input (must be 'org.osbuild.source')",
+    "type": "string",
+    "enum": ["org.osbuild.source"]
+  },
+  "references": {
+    "description": "Checksums of files to use as files input",
+    "oneOf": [{
+      "type": "array",
+      "items": {
+        "type": "string"
+      }
+    }, {
+      "type": "object",
+      "additionalProperties": false,
+      "minProperties": 1,
+      "patternProperties": {
+        ".*": {
+          "type": "object",
+          "additionalProperties": false,
+          "properties": {
+            "metadata": {
+              "description": "Additinal metadata to forward to the stage",
+              "type": "object",
+              "additionalProperties": false,
+              "patternProperties": {
+                "^\\w+[.]{1}\\w+$": {
+                  "additionalProperties": false
+                }
+              }
+            }
+          }
+        }
+      }
+    }]
+  }
+}
+"""
+
+
+def main():
+    args = json.load(sys.stdin)
+    refs = args["refs"]
+
+    store = StoreClient(connect_to=args["api"]["store"])
+    source = store.source("org.osbuild.files")
+    output = store.mkdtemp(prefix="files-input-")
+
+    for checksum in refs:
+        try:
+            subprocess.run(
+                [
+                    "ln",
+                    f"{source}/{checksum}",
+                    f"{output}/{checksum}",
+                ],
+                check=True,
+            )
+        except subprocess.CalledProcessError as e:
+            json.dump({"error": e.output}, sys.stdout)
+            return 1
+
+
+    reply = {
+        "path": output,
+        "data": {
+          "refs": refs
+        }
+    }
+
+    json.dump(reply, sys.stdout)
+    return 0
+
+
+if __name__ == '__main__':
+    r = main()
+    sys.exit(r)