populate: stop if a flake's URL can't be found

The URLs of flakes went into nix flake lock's and nix flake metadata's
arguments as command substitutions, whose failure doesn't stop a script
under set -e.  After "error: source p is not a directory", nix flake
lock ran with an empty URL, which Nix takes for the current directory:
on a remote target, the login directory, which ended up in the
world-readable store, .ssh included.  URLs get assigned first now, so
failing to find one stops populate.

A phase's unexpected lists what populate is expected not to print,
whether it succeeds or not; notdir expects populate not to go on
locking.

1 files changed, 5 insertions, 0 deletions

diff --git a/tests/vm.nix b/tests/vm.nix
index d7b2f60..7c362f5 100644
--- a/tests/vm.nix
+++ b/tests/vm.nix
@@ -158,6 +158,11 @@ in pkgs.testers.runNixOSTest {
                         f"env HOME=/root {phase['env']} timeout {limit} {phase['populate']} </dev/null 2>&1",
                         timeout=900,
                     )
+                    for unexpected in phase["unexpected"]:
+                        if unexpected in out:
+                            raise Exception(
+                                f"populate {i} prints {unexpected!r}:\n{out}"
+                            )
                     if phase["interrupt"] is not None:
                         if status != 124:
                             raise Exception(f"populate {i} ends on its own:\n{out}")