check for patches not referenced in conandata.yml

reimplement part of https://github.com/ericLemanissier/conan-center-lint-patches/blob/main/lint_patches.py
results are tracked in conan-io/conan-center-index#21146

hooks/conan-center.py

@@ -922,10 +922,12 @@ def test(out):
         conandata_yml = load_yml(conandata_path)
 
         if not conandata_yml:
-            return
+            conandata_yml = {}
 
-        if 'sources' not in conandata_yml or 'patches' not in conandata_yml:
-            return
+        if 'sources' not in conandata_yml:
+            conandata_yml['sources'] = []
+        if 'patches' not in conandata_yml:
+            conandata_yml['patches'] = []
 
         versions_conandata = conandata_yml['sources'].keys()
         versions_patches = conandata_yml['patches'].keys()
@@ -943,6 +945,24 @@ def test(out):
                     continue
                 if not os.path.isfile(os.path.join(export_folder_path, patch['patch_file'])):
                     out.error("The patch file '{}' does not exist.".format(patch['patch_file']))
+
+        patches_path = os.path.join(export_folder_path, "patches")
+        unused_patches: list[str] = []
+        if os.path.isdir(patches_path):
+            unused_patches.extend(
+                os.path.join(root[len(patches_path) + 1:], f)
+                for root, _, files in os.walk(patches_path) for f in files)
+        unused_patches.sort()
+        for patches in parsed["patches"]:
+            for patch in patches:
+                patch_file_name = str(patch["patch_file"])
+                patch_file_name = os.path.relpath(patch_file_name)  # fixes the path (double slashes for example)
+                patch_file_name = patch_file_name[8:]
+                if patch_file_name in unused_patches:
+                    unused_patches.remove(patch_file_name)
+        for patch in unused_patches:
+            print(f"Following patch files is not referenced in conandata.yml: patches/{patch}")
+
 
 
 @raise_if_error_output

tests/test_hooks/conan-center/test_conan-center.py

@@ -1248,3 +1248,22 @@ def test_dangling_patches(self):
         output = self.conan(['export', 'all', 'name/version@user/test'])
         self.assertIn("The patch file 'patches/patch.diff' does not exist.",
                       output)
+
+        conandata = textwrap.dedent("""
+                                    sources:
+                                        1.0:
+                                            url: fakeurl
+                                            md5: 12323423423
+                        """)
+
+        tools.save(os.path.join("all", "conandata.yml"), content=conandata)
+        output = self.conan(['export', 'all', 'name/version@user/test'])
+        self.assertNotIn("Following patch files is not referenced in conandata.yml",
+                         output)
+
+        tools.save(os.path.join("all", "patches", "patch.diff"), content="")
+        output = self.conan(['export', 'all', 'name/version@user/test'])
+        self.assertIn("Following patch files is not referenced in conandata.yml: patches/patch.diff",
+                      output)
+
+