pkl.project: check symlink targets

The pkl module extension uses symlinks to refer to the PklProject and PklProject.deps.json files.
The rctx.symlink function does not guarantee that the symlink points at a real file on the filesystem and it doesn't watch the file for changes.

With this change, we ensure the file exists before symlinking to it and watch it for changes (in case it's deleted by the user).

Author: malt3

pkl/private/pkl_project.bzl

@@ -89,17 +89,35 @@ def convert_dict_to_options(option_name, items_dict):
         list.append("{key}={value}".format(key = key, value = value))
     return list
 
+def _watch_symlink_target(rctx, label):
+    """Validates that a label points to an existing file and watches it for changes.
+
+    Args:
+        rctx: The repository context.
+        label: The label to validate.
+    Returns:
+        The resolved path.
+    """
+    path = rctx.path(label)
+    if not path.exists:
+        fail("Cannot symlink '{}': file does not exist.".format(label))
+    rctx.watch(path)
+    return path
+
 def _pkl_project_impl(rctx):
-    packages = parse_pkl_project_deps_json(rctx.read(rctx.attr.pkl_project_deps))
+    pkl_project_path = _watch_symlink_target(rctx, rctx.attr.pkl_project)
+    pkl_project_deps_path = _watch_symlink_target(rctx, rctx.attr.pkl_project_deps)
+
+    packages = parse_pkl_project_deps_json(rctx.read(pkl_project_deps_path))
     targets_for_all = []
     for package in packages:
         targets_for_all.append("@%s//:item" % package.workspace_name)
 
-    rctx.symlink(rctx.attr.pkl_project, "PklProject")
-    rctx.symlink(rctx.path(rctx.attr.pkl_project_deps), "PklProject.deps.json")
+    rctx.symlink(pkl_project_path, "PklProject")
+    rctx.symlink(pkl_project_deps_path, "PklProject.deps.json")
 
     env_vars = convert_dict_to_options("--env-var", rctx.attr.environment)
-    pkl_project_metadata = _eval_pkl_project(rctx, "PklProject", extra_args = env_vars + rctx.attr.extra_flags)
+    pkl_project_metadata = _eval_pkl_project(rctx, pkl_project_path, extra_args = env_vars + rctx.attr.extra_flags)
     has_package = "package" in pkl_project_metadata
 
     build_bazel_content = ""
@@ -138,7 +156,8 @@ pkl_cache(
     rctx.file("BUILD.bazel", content = build_bazel_content, executable = False)
 
 def _pkl_project_http_rewrites_impl(rctx):
-    rules = _eval_pkl_project(rctx, rctx.path(rctx.attr.pkl_project), extra_args = [
+    pkl_project_path = _watch_symlink_target(rctx, rctx.attr.pkl_project)
+    rules = _eval_pkl_project(rctx, pkl_project_path, extra_args = [
         "--expression",
         # Render the rewrite rules as list, to make sure the order is preserved.
         """