runners: org.osbuild.blank -> org.osbuild.linux

Now with `os-release` using `linux` as default ID+VERSION string, we
have a proper fallback name for our blank runner. Rename the blank
runner to `org.osbuild.linux`. It now serves as default fallback for
anything not further specified.

runners/org.osbuild.linux

@@ -0,0 +1,38 @@
+#!/usr/bin/python3
+
+import array
+import json
+import os
+import socket
+import subprocess
+import sys
+
+
+def load_fds(sock, msglen):
+    fds = array.array("i")   # Array of ints
+    msg, ancdata, _, addr = sock.recvmsg(msglen, socket.CMSG_LEN(253 * fds.itemsize))
+    for cmsg_level, cmsg_type, cmsg_data in ancdata:
+        if (cmsg_level == socket.SOL_SOCKET and cmsg_type == socket.SCM_RIGHTS):
+            # Append data, ignoring any truncated integers at the end.
+            fds.frombytes(cmsg_data[:len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
+    return json.loads(msg), list(fds), addr
+
+def setup_stdio():
+    with socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM) as sock:
+        sock.setsockopt(socket.SOL_SOCKET, socket.SO_PASSCRED, 1)
+        sock.connect("/run/osbuild/api/osbuild")
+        req = {'method': 'setup-stdio'}
+        sock.send(json.dumps(req).encode('utf-8'))
+        msg, fds, _ = load_fds(sock, 1024)
+        for io in ['stdin', 'stdout', 'stderr']:
+            target = getattr(sys, io)
+            source = fds[msg[io]]
+            os.dup2(source, target.fileno())
+            os.close(source)
+
+
+if __name__ == "__main__":
+    setup_stdio()
+
+    r = subprocess.run(sys.argv[1:], check=False)
+    sys.exit(r.returncode)