cmd/sharefs: opaque setup state

This allows unrestricted use of the type system and prepares setup code for cross-process initialisation.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/sharefs/test/test.py

@@ -8,8 +8,8 @@ print(machine.succeed("/etc/sharefs -V"))
 machine.wait_for_unit("sharefs.service")
 
 machine.succeed("mkdir /mnt")
-def check_bad_opts_output(opts, want, privileged=False):
-    output = machine.fail(("" if privileged else "sudo -u alice -i ") + f"/etc/sharefs -f -o source=/proc/nonexistent,{opts} /mnt 2>&1")
+def check_bad_opts_output(opts, want, source="/etc", privileged=False):
+    output = machine.fail(("" if privileged else "sudo -u alice -i ") + f"/etc/sharefs -f -o source={source},{opts} /mnt 2>&1")
     if output != want:
         raise Exception(f"unexpected output: {output}")
 
@@ -33,6 +33,11 @@ check_bad_opts_output("allow_other", "sharefs: setuid and setgid must not be 0\n
 check_bad_opts_output("setuid=1023", "sharefs: setuid and setgid must not be 0\n", privileged=True)
 check_bad_opts_output("setgid=1023", "sharefs: setuid and setgid must not be 0\n", privileged=True)
 
+# Bad backing directory:
+check_bad_opts_output("clone_fd", "sharefs: cannot open source: no such file or directory\n", source="/proc/nonexistent")
+check_bad_opts_output("clone_fd", "sharefs: cannot open source: not a directory\n", source="/proc/self/exe")
+check_bad_opts_output("clone_fd", "sharefs: cannot open source: permission denied\n", source="/root")
+
 # Make sure nothing actually got mounted:
 machine.fail("umount /mnt")
 machine.succeed("rmdir /mnt")