Fix compatibility.py migration overwrite when no changes were necessary

conan/internal/graph/compatibility.py

@@ -89,17 +89,20 @@ def migrate_compatibility_files(cache_folder):
     compatibility_file = os.path.join(compatible_folder, "compatibility.py")
     cppstd_compat_file = os.path.join(compatible_folder, "cppstd_compat.py")
 
-    def _should_migrate_file(file_path):
+    def _is_migratable(file_path):
         if not os.path.exists(file_path):
             return True
         content = load(file_path)
         first_line = content.lstrip().split("\n", 1)[0]
         return CONAN_GENERATED_COMMENT in first_line
 
-    if _should_migrate_file(compatibility_file) and _should_migrate_file(cppstd_compat_file):
-        if os.path.exists(compatibility_file) and load(compatibility_file) != _default_compat:
-            ConanOutput().success("Migration: Successfully updated compatibility.py")
-        save(compatibility_file, _default_compat)
+    if _is_migratable(compatibility_file) and _is_migratable(cppstd_compat_file):
+        compatibility_exists = os.path.exists(compatibility_file)
+        needs_update = not compatibility_exists or load(compatibility_file) != _default_compat
+        if needs_update:
+            save(compatibility_file, _default_compat)
+            if compatibility_exists:
+                ConanOutput().success("Migration: Successfully updated compatibility.py")
         if os.path.exists(cppstd_compat_file):
             os.remove(cppstd_compat_file)
 

test/integration/test_migrations.py

@@ -15,7 +15,7 @@
                          [("profile.py", "msvc", "EME_ESE_VC"),
                           ("compatibility/compatibility.py", "conanfile", "conian_file")])
 def test_migration_profile_checker_plugin(plugin_path, string_replace, new_string):
-    t = TestClient()
+    t = TestClient(light=True)
     # Any command that checks the package cache generates the DB
     t.run("list")
     assert os.path.exists(os.path.join(t.cache_folder, "p", "cache.sqlite3"))
@@ -41,7 +41,7 @@ def test_migration_profile_checker_plugin(plugin_path, string_replace, new_strin
     assert new_string not in contents
 
     # New client, everything new
-    t2 = TestClient()
+    t2 = TestClient(light=True)
     # This generates the new plugin file
     t2.run("list")
 
@@ -68,7 +68,7 @@ def test_migration_profile_checker_plugin(plugin_path, string_replace, new_strin
 
 
 def test_back_migrations():
-    t = TestClient()
+    t = TestClient(light=True)
 
     # add 3 migrations
     for number in (1, 2, 3):
@@ -97,7 +97,7 @@ def migrate(cache_folder):
 
 
 def test_back_default_compatibility_migration():
-    t = TestClient()
+    t = TestClient(light=True)
     t.run("-v")  # Fire the backward migration
     migration_file = os.path.join(t.cache_folder, "migrations", "2.4_1-migrate.py")
     assert os.path.exists(migration_file)
@@ -113,7 +113,7 @@ def test_back_default_compatibility_migration():
 
 class TestMigrationCppstdCompat:
     def test_migration(self):
-        t = TestClient()
+        t = TestClient(light=True)
         t.run("-v")
         cppstd_compat_path = "extensions/plugins/compatibility/cppstd_compat.py"
         compatibility_path = "extensions/plugins/compatibility/compatibility.py"
@@ -125,7 +125,7 @@ def test_migration(self):
         assert not os.path.exists(os.path.join(t.cache_folder, cppstd_compat_path))
 
     def test_cppstd_modified(self):
-        t = TestClient()
+        t = TestClient(light=True)
         t.run("-v")
         cppstd_compat_path = "extensions/plugins/compatibility/cppstd_compat.py"
         compatibility_path = "extensions/plugins/compatibility/compatibility.py"
@@ -139,7 +139,7 @@ def test_cppstd_modified(self):
         assert "def cppstd_compat(conanfile)" not in t.load_home(compatibility_path)
 
     def test_compatibility_modified(self):
-        t = TestClient()
+        t = TestClient(light=True)
         t.run("-v")
         cppstd_compat_path = "extensions/plugins/compatibility/cppstd_compat.py"
         compatibility_path = "extensions/plugins/compatibility/compatibility.py"
@@ -150,3 +150,26 @@ def test_compatibility_modified(self):
         assert t.load_home(compatibility_path) == "Modified file"
         # not Removed because compatibility was modified
         assert "POTATO" in t.load_home(cppstd_compat_path)
+
+    def test_compatibility_modified_by_conan(self):
+        t = TestClient(light=True)
+        t.run("-v")
+        compatibility_path = "extensions/plugins/compatibility/compatibility.py"
+        old = t.load_home(compatibility_path)
+        # In old versions of Conan, assume POTATO was present in the file
+        new = old + "\n# POTATO"
+        t.save_home({"version.txt": "2.11",
+                     compatibility_path: new})
+        # But now with the newer version, Conan removed it
+        # so the triggered migration should remove it
+        t.run("-v")
+        assert "Successfully updated compatibility.py" in t.out
+        assert "# POTATO" not in t.load_home(compatibility_path)
+
+    def test_compatibility_no_rewrite(self):
+        t = TestClient(light=True)
+        t.run("-v")
+        # compatibility.py exists and is not modified
+        t.save_home({"version.txt": "2.11"})
+        t.run("-v")
+        assert "Successfully updated compatibility.py" not in t.out