Do not conflict with patches

internal/go_repository.bzl

@@ -295,13 +295,11 @@ def _go_repository_impl(ctx):
     if generate:
         # Build file generation is needed. Populate Gazelle directive at root build file
         build_file_name = existing_build_file or build_file_names[0]
-        ctx.file(
-            build_file_name,
-            "\n".join(["# " + d for d in ctx.attr.build_directives] + [_generate_package_info(
-                importpath = ctx.attr.importpath,
-                version = ctx.attr.version,
-            )]),
-        )
+        if len(ctx.attr.build_directives) > 0:
+            ctx.file(
+                build_file_name,
+                "\n".join(["# " + d for d in ctx.attr.build_directives]),
+            )
 
         # Run Gazelle
         if gazelle_path == None:
@@ -363,15 +361,32 @@ def _go_repository_impl(ctx):
     patch(ctx)
 
     if generate:
-        ctx.file("REPO.bazel", """\
+        # Do not override a REPO.bazel patched in by users. This also provides a
+        # way for users to opt out of Gazelle-generated package_info.
+        repo_file = ctx.path("REPO.bazel")
+        if not repo_file.exists:
+            ctx.file("REPO.bazel", """\
 repo(default_package_metadata = ["//:gazelle_generated_package_info"])
 """)
 
+            # Modify the top-level build file after patches have been applied as the
+            # patches may otherwise conflict with our generated content.
+            build_file = ctx.path(build_file_name)
+            if build_file.exists:
+                build_file_content = ctx.read(build_file)
+            else:
+                build_file_content = ""
+            build_file_content += _generate_package_info(
+                importpath = ctx.attr.importpath,
+                version = ctx.attr.version,
+            )
+            ctx.file(build_file_name, build_file_content)
+
 def _generate_package_info(*, importpath, version):
     package_name = importpath
     package_url = "https://" + importpath
     package_version = version.removeprefix("v")
-    return """\
+    return """
 load("@rules_license//rules:package_info.bzl", "package_info")
 
 package_info(

tests/bcr/go_mod/tests.bzl

@@ -25,7 +25,7 @@ def _package_info_aspect_impl(_, ctx):
         attr = ctx.rule.attr.applicable_licenses
     elif hasattr(ctx.rule.attr, "package_metadata"):
         attr = ctx.rule.attr.package_metadata
-    if PackageInfo in attr[0]:
+    if attr and PackageInfo in attr[0]:
         return [attr[0][PackageInfo]]
     return []