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.

2 files changed, 12 insertions, 2 deletions

diff --git a/tests/scenarios.nix b/tests/scenarios.nix
index 43b63b4..e01c5a9 100644
--- a/tests/scenarios.nix
+++ b/tests/scenarios.nix
@@ -109,10 +109,12 @@ let
     # evaluation fail.  env sets environment variables for populate, as
     # arguments of env.  interrupt ends populate after that many seconds,
     # which it's expected not to finish before.  output lists what a
-    # succeeding populate is expected to print.
+    # succeeding populate is expected to print, and unexpected what populate
+    # is expected not to print, succeeding or not.
     phase = {
       sources, backup ? false, prune ? false, deploy ? null,
       targetPath ? path, env ? "", interrupt ? null, output ? [],
+      unexpected ? [],
       expect ? "success", control ? null, controlTargetPath ? null,
       check ? "", mutate ? null, checkDeployer ? null, mutateDeployer ? null,
     }: let
@@ -147,7 +149,7 @@ let
           null
       else {
         populate = toString (command sources targetPath);
-        inherit env interrupt output;
+        inherit env interrupt output unexpected;
         expect = if expect == "success" then null else expect.fail;
         # No phase gets around checking the canaries.
         check = script (check + "\ncanary_check\n");
@@ -1258,6 +1260,9 @@ assert !(krops.isLocalTarget (targets.remote "/"));
           p.pipe = "/fixtures/pipe";
         };
         expect.fail = "error: source p is not a directory";
+        # Locking the flake with an empty URL would lock the current
+        # directory, the login directory on a remote target.
+        unexpected = [ "while fetching the input" ];
       }];
     })
     (s "noinput-nested" {
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}")