populate: replace symlinks before syncing with rsync

rsync follows a symlink at the target path, so a source switching from the
flake or flake-input type to file, pass, passage, or pipe got written into
the store path the link points to, and one switching from the symlink
type into what that link points to, e.g. another source.  Populate
removes a symlink at the target path before syncing with rsync.

The roots in .populate/flake-inputs/<name> of a source that is neither a
flake nor a flake-input source anymore get removed as well.

An edge case switches a flake source to a file source.

1 files changed, 19 insertions, 0 deletions

diff --git a/tests/scenarios.nix b/tests/scenarios.nix
index a738eca..38f8736 100644
--- a/tests/scenarios.nix
+++ b/tests/scenarios.nix
@@ -808,6 +808,25 @@ assert !(krops.isLocalTarget (targets.remote "/"));
         check = checkFlake { attr = "msg"; expected = "rel:lib"; };
       };
     })
+    (s "flake-to-file" {
+      phases = [
+        {
+          sources.x.flake = { flake = "/fixtures/top"; inherit fetchOnTarget; };
+          check = roots 3;
+          mutate = ''
+            readlink "$T"/x > "$T"/old
+          '';
+        }
+        {
+          sources.x.file = "/fixtures/alt";
+          check = ''
+            test ! -L "$T"/x || fail "x is still a link to $(readlink "$T"/x)"
+            test -e "$T"/x/flake.nix || fail "flake.nix missing"
+            nix-store --verify-path "$(cat "$T"/old)" || fail "store path changed"
+          '' + roots 0;
+        }
+      ];
+    })
   ] ++ lib.optional (targetKind == "local")
     # The target has the path the input is locked to.
     (s "uselocal" {