fix: nested context managers shouldn't cause a phantom missing branch #1876

Author: nedbat

CHANGES.rst

@@ -23,10 +23,15 @@ upgrading your version of coverage.py.
 Unreleased
 ----------
 
+- Fix: nested context managers could incorrectly be analyzed to flag a missing
+  branch on the last context manager, as described in `issue 1876`_.  This is
+  now fixed.
+
 - Fix: the missing branch message about not exiting a module had an extra
   "didn't," as described in `issue 1873`_.  This is now fixed.
 
 .. _issue 1873: https://github.com/nedbat/coveragepy/issues/1873
+.. _issue 1876: https://github.com/nedbat/coveragepy/issues/1876
 
 
 .. start-releases

coverage/parser.py

@@ -315,12 +315,18 @@ def fix_with_jumps(self, arcs: Iterable[TArc]) -> set[TArc]:
         to_add = set()
         for arc in arcs:
             if arc in self._with_jump_fixers:
+                start = arc[0]
+                to_remove.add(arc)
                 start_next, prev_next = self._with_jump_fixers[arc]
-                if start_next in arcs:
-                    to_add.add(prev_next)
-                    to_remove.add(arc)
+                while start_next in self._with_jump_fixers:
                     to_remove.add(start_next)
-        return (set(arcs) | to_add) - to_remove
+                    start_next, prev_next = self._with_jump_fixers[start_next]
+                    to_remove.add(prev_next)
+                to_add.add((start, prev_next[1]))
+                to_remove.add(arc)
+                to_remove.add(start_next)
+        arcs = (set(arcs) | to_add) - to_remove
+        return arcs
 
     @functools.lru_cache
     def exit_counts(self) -> dict[TLineNo, int]:

tests/test_parser.py

@@ -151,6 +151,24 @@ def bar(self):
         assert expected_arcs == parser.arcs()
         assert expected_exits == parser.exit_counts()
 
+    def test_nested_context_managers(self) -> None:
+        # https://github.com/nedbat/coveragepy/issues/1876
+        parser = self.parse_text("""\
+            a = 1
+            with suppress(ValueError):
+                with suppress(ValueError):
+                    x = 4
+                with suppress(ValueError):
+                    x = 6
+                with suppress(ValueError):
+                    x = 8
+            a = 9
+            """)
+
+        one_nine = set(range(1, 10))
+        assert parser.statements == one_nine
+        assert parser.exit_counts() == dict.fromkeys(one_nine, 1)
+
     def test_module_docstrings(self) -> None:
         parser = self.parse_text("""\
             '''The docstring on line 1'''