fix: a machine with no ifupdown configuration is not an error

The release validator caught it: in a container there is no
/etc/network/interfaces at all, and a missing file was being treated as
a read failure, so --check printed nothing and said nothing was wrong
rather than saying there was nothing to do.

A machine with no ifupdown configuration is the ordinary case for a
fresh install, and the answer is simply that this command has no work.

Co-Authored-By: Claude Opus 5 <[email protected]>

VERSION

@@ -1 +1 @@
-0.2.4
+0.2.5

scripts/classroom-wifi

@@ -208,6 +208,11 @@ def carrying(name, device):
 
 def find_stanza():
     """The one wireless stanza ifupdown holds, or None, plus the file."""
+    if not os.path.exists(INTERFACES):
+        # A machine with no ifupdown configuration at all. Nothing is
+        # wrong with that and nothing here has anything to do.
+        return None, ""
+
     try:
         with open(INTERFACES, "r", encoding="utf-8") as handle:
             text = handle.read()

tests/test-classroom-network-adopt.sh

@@ -166,7 +166,15 @@ check("it refuses to start when it could not undo itself",
       "shutil.which" in wifi
       and "leaving the network exactly as it is" in wifi)
 
-print("9. the upgrade does not do any of this on its own")
+print("9. a machine with no ifupdown configuration at all")
+# A container, and any machine installed without ifupdown. Found by the
+# release validator: --check said nothing at all, because a missing file
+# was being treated as a read error.
+check("a missing interfaces file is not an error, it is just nothing to do",
+      "not os.path.exists(INTERFACES)" in wifi
+      and 'return None, ""' in wifi)
+
+print("10. the upgrade does not do any of this on its own")
 # Moving a machine's only link during an unattended upgrade is fine until
 # the once it is not, and the machine it would strand is a laptop in a
 # locked room.