Fix the exportjson tool (.ff cache to .json conversion) (#21628)

It was broken by lazy cache deserialization, and the tests didn't catch
it since the fixup state was persisted within the test. Both fix the
exportjson tool and make the test more realistic.

Author: JukkaL

mypy/exportjson.py

@@ -128,8 +128,12 @@ def convert_symbol_table_node(self: SymbolTableNode, cfg: Config) -> Json:
         data["plugin_generated"] = True
     if self.cross_ref:
         data["cross_ref"] = self.cross_ref
-    elif self.node is not None:
-        data["node"] = convert_symbol_node(self.node, cfg)
+    else:
+        # Read the raw node without cross-reference fixup, since exportjson reads
+        # cache files in isolation and no node fixer is available.
+        node = self.read_node_no_fixup()
+        if node is not None:
+            data["node"] = convert_symbol_node(node, cfg)
     return data
 
 

mypy/nodes.py

@@ -4952,6 +4952,17 @@ def node(self) -> SymbolNode | None:
                 self.unfixed = False
         return self._node
 
+    def read_node_no_fixup(self) -> SymbolNode | None:
+        """Return the deserialized node without performing cross-reference fixup.
+
+        This is intended for introspection tools (such as mypy.exportjson) that read
+        cache files in isolation, where no node fixer is available.
+        """
+        if self._node is None and self._node_bytes:
+            self._node = read_symbol(ReadBuffer(self._node_bytes), self._node_tag)
+            self._node_bytes = b""
+        return self._node
+
     def copy(self) -> SymbolTableNode:
         new = SymbolTableNode(
             self.kind, self._node, self.module_public, self.implicit, self.module_hidden

mypy/test/testexportjson.py

@@ -11,6 +11,7 @@
 from mypy.errors import CompileError
 from mypy.exportjson import convert_binary_cache_meta_to_json, convert_binary_cache_to_json
 from mypy.modulefinder import BuildSource
+from mypy.modules_state import modules_state
 from mypy.options import Options
 from mypy.test.config import test_temp_dir
 from mypy.test.data import DataDrivenTestCase, DataSuite
@@ -44,6 +45,11 @@ def run_case(self, testcase: DataDrivenTestCase) -> None:
             major, minor = sys.version_info[:2]
             cache_dir = os.path.join(".mypy_cache", f"{major}.{minor}")
 
+            # Reset the global fixup state, since the exportjson tool
+            # reads cache files in isolation (no node fixer available).
+            modules_state.node_fixer = None
+            modules_state.modules = {}
+
             for module in result.files:
                 if module in (
                     "builtins",