runners: add blank runner

This adds a new runner called `org.osbuild.blank`, which assumes /usr
is pre-populated and ready to go. It does not perform any OS setup. It
only initializes the environment and executes the stage.

This runner allows easy bootstrapping of new systems. It assumes our
ideal setup where `/usr` describes a host system in its entirety,
without any local policy applied. Thus, this runner is also what we
ultimately want to work towards as a default. This might not happen
anytime soon, though, given how `passwd`, `ldconfig`, `nss`, etc. still
depend on prepopulated caches in `/etc`.

runners/org.osbuild.blank

@@ -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)