Fix module __getattr__ in incremental mode (#5306)

For module `__getattr__` to work correctly in incremental mode, we need to de-serialise potential parent package files during an attempt to find a partial package. Otherwise there will be spurious "Module not found" errors during the second and subsequent runs.

Author: ilevkivskyi

mypy/build.py

@@ -1870,6 +1870,8 @@ def __init__(self,
             self.dep_line_map = {id: line
                                  for id, line in zip(all_deps, self.meta.dep_lines)}
             self.child_modules = set(self.meta.child_modules)
+            if temporary:
+                self.load_tree(temporary=True)
         else:
             # When doing a fine-grained cache load, pretend we only
             # know about modules that have cache information and defer
@@ -1952,16 +1954,16 @@ def load_fine_grained_deps(self) -> None:
         # TODO: Assert deps file wasn't changed.
         self.fine_grained_deps = {k: set(v) for k, v in deps.items()}
 
-    def load_tree(self) -> None:
+    def load_tree(self, temporary: bool = False) -> None:
         assert self.meta is not None, "Internal error: this method must be called only" \
                                       " for cached modules"
         with open(self.meta.data_json) as f:
             data = json.load(f)
         # TODO: Assert data file wasn't changed.
         self.tree = MypyFile.deserialize(data)
-
-        self.manager.modules[self.id] = self.tree
-        self.manager.add_stats(fresh_trees=1)
+        if not temporary:
+            self.manager.modules[self.id] = self.tree
+            self.manager.add_stats(fresh_trees=1)
 
     def fix_cross_refs(self) -> None:
         assert self.tree is not None, "Internal error: method must be called on parsed file only"

test-data/unit/check-incremental.test

@@ -4754,3 +4754,39 @@ def f(): pass
 [file a.py.2]
 [out]
 [out2]
+
+[case testModuleGetattrInitIncremental]
+import c
+[file c.py]
+import a.b
+x = a.b.f()
+[file c.py.2]
+import a.b
+x = a.b.f()
+# touch
+[file a/__init__.pyi]
+from typing import Any
+def __getattr__(attr: str) -> Any: ...
+[builtins fixtures/module.pyi]
+[out]
+[out2]
+
+[case testModuleGetattrInitIncremental2]
+import c
+[file c.py]
+import a.b.c
+[file c.py.2]
+import a.b.c
+# touch
+[file a/__init__.pyi]
+from typing import Any
+def __getattr__(attr: str) -> Any: ...
+[file a/b.pyi]
+# empty
+[builtins fixtures/module.pyi]
+[out]
+tmp/c.py:1: error: Cannot find module named 'a.b.c'
+tmp/c.py:1: note: (Perhaps setting MYPYPATH or using the "--ignore-missing-imports" flag would help)
+[out2]
+tmp/c.py:1: error: Cannot find module named 'a.b.c'
+tmp/c.py:1: note: (Perhaps setting MYPYPATH or using the "--ignore-missing-imports" flag would help)