Flakes: computeLocks: pass correct LockParent when reusing oldLock

Previously, when we were attempting to reuse the old lockfile
information in the computeLocks function, we have passed the parent of
the current input to the next computeLocks call. This was incorrect,
since the follows are resolved relative to the parent. This caused
issues when we tried to reuse oldLock but couldn't for some
reason (read: mustRefetch is true), in that case the follows were
resolved incorrectly.

Fix this by passing the correct parent, and adding some tests to
prevent this particular regression from happening again.

Closes https://github.com/NixOS/nix/issues/5697

src/libexpr/flake/flake.cc

@@ -480,11 +480,16 @@ LockedFlake lockFlake(
                             }
                         }
 
+                        LockParent newParent {
+                            .path = inputPath,
+                            .absolute = false
+                        };
+
                         computeLocks(
                             mustRefetch
                             ? getFlake(state, oldLock->lockedRef, false, flakeCache).inputs
                             : fakeInputs,
-                            childNode, inputPath, oldLock, parent, parentPath);
+                            childNode, inputPath, oldLock, newParent, parentPath);
 
                     } else {
                         /* We need to create a new lock file entry. So fetch

tests/flakes.sh

@@ -722,6 +722,7 @@ cat > $flakeFollowsB/flake.nix <<EOF
     inputs = {
         foobar.url = "path:$flakeFollowsA/flakeE";
         nonFlake.url = "path:$nonFlakeDir";
+        goodoo.follows = "C/goodoo";
         C = {
             url = "path:./flakeC";
             inputs.foobar.follows = "foobar";
@@ -736,6 +737,7 @@ cat > $flakeFollowsC/flake.nix <<EOF
     description = "Flake C";
     inputs = {
         foobar.url = "path:$flakeFollowsA/flakeE";
+        goodoo.follows = "foobar";
     };
     outputs = { ... }: {};
 }
@@ -760,8 +762,18 @@ EOF
 git -C $flakeFollowsA add flake.nix flakeB/flake.nix \
   flakeB/flakeC/flake.nix flakeD/flake.nix flakeE/flake.nix
 
+nix flake update $flakeFollowsA
+
+oldLock="$(cat "$flakeFollowsA/flake.lock")"
+
+# Ensure that locking twice doesn't change anything
+
 nix flake lock $flakeFollowsA
 
+newLock="$(cat "$flakeFollowsA/flake.lock")"
+
+diff <(echo "$newLock") <(echo "$oldLock")
+
 [[ $(jq -c .nodes.B.inputs.C $flakeFollowsA/flake.lock) = '"C"' ]]
 [[ $(jq -c .nodes.B.inputs.foobar $flakeFollowsA/flake.lock) = '["D"]' ]]
 [[ $(jq -c .nodes.C.inputs.foobar $flakeFollowsA/flake.lock) = '["B","foobar"]' ]]