Fix import following issue in daemon (#10032)

A special case for dealing with blocking errors caused false positives
when following imports. Narrowed down the special casing to only
trigger when there is blocking errors.

The logic is still quite convoluted, but at least this fixes some
issues with following imports to stub packages.

Fixes #10022.

Author: JukkaL

mypy/server/update.py

@@ -232,15 +232,17 @@ def update(self,
             self.manager.log_fine_grained('previous targets with errors: %s' %
                              sorted(self.previous_targets_with_errors))
 
+        blocking_error = None
         if self.blocking_error:
             # Handle blocking errors first. We'll exit as soon as we find a
             # module that still has blocking errors.
             self.manager.log_fine_grained('existing blocker: %s' % self.blocking_error[0])
             changed_modules = dedupe_modules([self.blocking_error] + changed_modules)
+            blocking_error = self.blocking_error[0]
             self.blocking_error = None
 
         while True:
-            result = self.update_one(changed_modules, initial_set, removed_set)
+            result = self.update_one(changed_modules, initial_set, removed_set, blocking_error)
             changed_modules, (next_id, next_path), blocker_messages = result
 
             if blocker_messages is not None:
@@ -289,9 +291,10 @@ def trigger(self, target: str) -> List[str]:
     def update_one(self,
                    changed_modules: List[Tuple[str, str]],
                    initial_set: Set[str],
-                   removed_set: Set[str]) -> Tuple[List[Tuple[str, str]],
-                                                   Tuple[str, str],
-                                                   Optional[List[str]]]:
+                   removed_set: Set[str],
+                   blocking_error: Optional[str]) -> Tuple[List[Tuple[str, str]],
+                                                           Tuple[str, str],
+                                                           Optional[List[str]]]:
         """Process a module from the list of changed modules.
 
         Returns:
@@ -303,9 +306,17 @@ def update_one(self,
         """
         t0 = time.time()
         next_id, next_path = changed_modules.pop(0)
-        if next_id not in self.previous_modules and next_id not in initial_set:
-            self.manager.log_fine_grained('skip %r (module not in import graph)' % next_id)
+
+        # If we have a module with a blocking error that is no longer
+        # in the import graph, we must skip it as otherwise we'll be
+        # stuck with the blocking error.
+        if (next_id == blocking_error
+                and next_id not in self.previous_modules
+                and next_id not in initial_set):
+            self.manager.log_fine_grained(
+                'skip %r (module with blocking error not in import graph)' % next_id)
             return changed_modules, (next_id, next_path), None
+
         result = self.update_module(next_id, next_path, next_id in removed_set)
         remaining, (next_id, next_path), blocker_messages = result
         changed_modules = [(id, path) for id, path in changed_modules

test-data/unit/fine-grained-follow-imports.test

@@ -719,3 +719,23 @@ def f() -> None: pass
 [out]
 ==
 main.py:2: error: Too many arguments for "f"
+
+[case testFollowImportsNormalMultipleImportedModulesSpecialCase]
+# flags: --follow-imports=normal
+# cmd: mypy main.py
+
+[file main.py]
+import pkg
+
+[file pkg/__init__.py.2]
+from . import mod1
+
+[file pkg/mod1.py.2]
+from . import mod2
+
+[file pkg/mod2.py.2]
+
+[out]
+main.py:1: error: Cannot find implementation or library stub for module named "pkg"
+main.py:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
+==