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, 11 insertions, 4 deletions

diff --git a/pkgs/populate/default.nix b/pkgs/populate/default.nix
index 7b22662..163f1de 100644
--- a/pkgs/populate/default.nix
+++ b/pkgs/populate/default.nix
@@ -383,18 +383,23 @@ let
         ++ inputChecks from (drop (length parent) path);
     # Sets the positional parameters to nix flake lock's arguments for
     # drawing the inputs of the flake in source name from other sources.
+    # URLs get assigned first, so failing to find one stops the script,
+    # which a command substitution in an argument wouldn't: Nix would take
+    # the empty URL for the current directory.
     overrideArgs = name: /* sh */ ''
       set --
       ${concatMapStrings (x: /* sh */ ''
-        set -- "$@" --override-input ${quote (concatStringsSep "/" (drawnPath name x.path))} ${url (check name x.source)}
+        drawn_url=${url (check name x.source)}
+        set -- "$@" --override-input ${quote (concatStringsSep "/" (drawnPath name x.path))} "$drawn_url"
       '') (flakeFollows sources.${name})}
     '';
     # Rewrites the lock file of the flake in a file or git source in place.
     rewrite = name: /* sh */ ''
       (
         ${overrideArgs name}
+        own_url=$(flake_url ${quote name})
         own_lock "$(readlink -f ${quote "${target.path}/${name}"})"
-        nix flake lock "$@" "$(flake_url ${quote name})" >&2
+        nix flake lock "$@" "$own_url" >&2
       )
     '';
     # Derives what the target path of a flake or flake-input source gets from
@@ -489,8 +494,9 @@ let
           fi
           ${optionalString rewriteLock /* sh */ ''
             ${overrideArgs name}
+            own_url=$(flake_url ${quote name})
             own_lock ${quote targetPath}
-            nix flake lock "$@" "$(flake_url ${quote name})" >&2
+            nix flake lock "$@" "$own_url" >&2
           ''}
         '' else /* sh */ ''
           if test -e ${quote "${dir}/key"} &&
@@ -674,8 +680,9 @@ let
           ${concatMapStrings (x: /* sh */ ''
             set -- "$@" --override-input ${quote (concatStringsSep "/" x.path)} "path:$locks_dir"
           '') follows}
+          own_url=$(own_flake_url ${quote name})
           if ! nix flake metadata --json --no-write-lock-file "$@" \
-              "$(own_flake_url ${quote name})" > "$locks_dir"/${quote name}.json; then
+              "$own_url" > "$locks_dir"/${quote name}.json; then
             >&2 printf 'error: source %s draws inputs from other sources, but its flake can'"'"'t be locked\n' \
                 ${quote name}
             exit 1