Finally jumps back to exiting lines

In Python 3.8, when a finally clause is run because a line in the try
block is exiting the block, the exiting line is visited again after the
finally block.

4 files changed, 151 insertions, 45 deletions

diff --git a/coverage/env.py b/coverage/env.py
index e35d026b..aa8bb8f6 100644
--- a/coverage/env.py
+++ b/coverage/env.py
@@ -24,6 +24,16 @@ PYVERSION = sys.version_info
 PY2 = PYVERSION < (3, 0)
 PY3 = PYVERSION >= (3, 0)
 
+# Python behavior
+class PYBEHAVIOR(object):
+    """Flags indicating this Python's behavior."""
+
+    # When a break/continue/return statement in a try block jumps to a finally
+    # block, does the finally block do the break/continue/return (pre-3.8), or
+    # does the finally jump back to the break/continue/return (3.8) to do the
+    # work?
+    finally_jumps_back = (PYVERSION >= (3, 8))
+
 # Coverage.py specifics.
 
 # Are we using the C-implemented trace function?
diff --git a/coverage/parser.py b/coverage/parser.py
index 2eae4eb5..5ffcad8c 100644
--- a/coverage/parser.py
+++ b/coverage/parser.py
@@ -505,6 +505,10 @@ class NodeList(object):
         self.lineno = body[0].lineno
 
 
+# TODO: some add_arcs methods here don't add arcs, they return them. Rename them.
+# TODO: the cause messages have too many commas.
+# TODO: Shouldn't the cause messages join with "and" instead of "or"?
+
 class AstArcAnalyzer(object):
     """Analyze source text with an AST to find executable code paths."""
 
@@ -546,6 +550,7 @@ class AstArcAnalyzer(object):
             if code_object_handler is not None:
                 code_object_handler(node)
 
+    @contract(start=int, end=int)
     def add_arc(self, start, end, smsg=None, emsg=None):
         """Add an arc, including message fragments to use if it is missing."""
         if self.debug:                      # pragma: debugging
@@ -970,21 +975,45 @@ class AstArcAnalyzer(object):
             final_exits = self.add_body_arcs(node.finalbody, prev_starts=final_from)
 
             if try_block.break_from:
-                self.process_break_exits(
-                    self._combine_finally_starts(try_block.break_from, final_exits)
-                )
+                if env.PYBEHAVIOR.finally_jumps_back:
+                    for break_line in try_block.break_from:
+                        lineno = break_line.lineno
+                        cause = break_line.cause.format(lineno=lineno)
+                        for final_exit in final_exits:
+                            self.add_arc(final_exit.lineno, lineno, cause)
+                    breaks = try_block.break_from
+                else:
+                    breaks = self._combine_finally_starts(try_block.break_from, final_exits)
+                self.process_break_exits(breaks)
+
             if try_block.continue_from:
-                self.process_continue_exits(
-                    self._combine_finally_starts(try_block.continue_from, final_exits)
-                )
+                if env.PYBEHAVIOR.finally_jumps_back:
+                    for continue_line in try_block.continue_from:
+                        lineno = continue_line.lineno
+                        cause = continue_line.cause.format(lineno=lineno)
+                        for final_exit in final_exits:
+                            self.add_arc(final_exit.lineno, lineno, cause)
+                    continues = try_block.continue_from
+                else:
+                    continues = self._combine_finally_starts(try_block.continue_from, final_exits)
+                self.process_continue_exits(continues)
+
             if try_block.raise_from:
                 self.process_raise_exits(
                     self._combine_finally_starts(try_block.raise_from, final_exits)
                 )
+
             if try_block.return_from:
-                self.process_return_exits(
-                    self._combine_finally_starts(try_block.return_from, final_exits)
-                )
+                if env.PYBEHAVIOR.finally_jumps_back:
+                    for return_line in try_block.return_from:
+                        lineno = return_line.lineno
+                        cause = return_line.cause.format(lineno=lineno)
+                        for final_exit in final_exits:
+                            self.add_arc(final_exit.lineno, lineno, cause)
+                    returns = try_block.return_from
+                else:
+                    returns = self._combine_finally_starts(try_block.return_from, final_exits)
+                self.process_return_exits(returns)
 
             if exits:
                 # The finally clause's exits are only exits for the try block
diff --git a/tests/test_arcs.py b/tests/test_arcs.py
index c86147b7..f20c8cad 100644
--- a/tests/test_arcs.py
+++ b/tests/test_arcs.py
@@ -623,6 +623,10 @@ class ExceptionArcTest(CoverageTest):
 
 
     def test_break_through_finally(self):
+        if env.PYBEHAVIOR.finally_jumps_back:
+            arcz = ".1 12 23 34 3D 45 56 67 68 7A 7D 8A A3 A7 BC CD D."
+        else:
+            arcz = ".1 12 23 34 3D 45 56 67 68 7A 8A A3 AD BC CD D."
         self.check_coverage("""\
             a, c, d, i = 1, 1, 1, 99
             try:
@@ -638,11 +642,15 @@ class ExceptionArcTest(CoverageTest):
                 d = 12                              # C
             assert a == 5 and c == 10 and d == 1    # D
             """,
-            arcz=".1 12 23 34 3D 45 56 67 68 7A 8A A3 AD BC CD D.",
+            arcz=arcz,
             arcz_missing="3D BC CD",
         )
 
     def test_continue_through_finally(self):
+        if env.PYBEHAVIOR.finally_jumps_back:
+            arcz = ".1 12 23 34 3D 45 56 67 68 73 7A 8A A3 A7 BC CD D."
+        else:
+            arcz = ".1 12 23 34 3D 45 56 67 68 7A 8A A3 BC CD D."
         self.check_coverage("""\
             a, b, c, d, i = 1, 1, 1, 1, 99
             try:
@@ -658,7 +666,7 @@ class ExceptionArcTest(CoverageTest):
                 d = 12                              # C
             assert (a, b, c, d) == (5, 8, 10, 1)    # D
             """,
-            arcz=".1 12 23 34 3D 45 56 67 68 7A 8A A3 BC CD D.",
+            arcz=arcz,
             arcz_missing="BC CD",
         )
 
@@ -794,6 +802,10 @@ class ExceptionArcTest(CoverageTest):
         )
 
     def test_return_finally(self):
+        if env.PYBEHAVIOR.finally_jumps_back:
+            arcz = ".1 12 29 9A AB BC C-1   -23 34 45 5-2 57 75 38 8-2"
+        else:
+            arcz = ".1 12 29 9A AB BC C-1   -23 34 45 57 7-2 38 8-2"
         self.check_coverage("""\
             a = [1]
             def check_token(data):
@@ -808,10 +820,26 @@ class ExceptionArcTest(CoverageTest):
             assert check_token(True) == 5
             assert a == [1, 7]
             """,
-            arcz=".1 12 29 9A AB BC C-1   -23 34 45 57 7-2 38 8-2",
+            arcz=arcz,
         )
 
     def test_except_jump_finally(self):
+        if env.PYBEHAVIOR.finally_jumps_back:
+            arcz = (
+                ".1 1Q QR RS ST TU U. "
+                ".2 23 34 45 56 4O 6L "
+                "78 89 9A AL LA AO  8B BC CD DL LD D4  BE EF FG GL LG G.  EH HI IJ JL  HL "
+                "L4 LM "
+                "MN NO O."
+            )
+        else:
+            arcz = (
+                ".1 1Q QR RS ST TU U. "
+                ".2 23 34 45 56 4O 6L "
+                "78 89 9A AL  8B BC CD DL  BE EF FG GL  EH HI IJ JL  HL "
+                "LO L4 L. LM "
+                "MN NO O."
+            )
         self.check_coverage("""\
             def func(x):
                 a = f = g = 2
@@ -842,18 +870,30 @@ class ExceptionArcTest(CoverageTest):
             assert func('continue') == (12, 21, 2, 3)       # R
             assert func('return') == (15, 2, 2, 0)          # S
             assert func('raise') == (18, 21, 23, 0)         # T
+            assert func('other') == (2, 21, 2, 3)           # U 30
             """,
-            arcz=
-                ".1 1Q QR RS ST T. "
-                ".2 23 34 45 56 4O 6L "
-                "78 89 9A AL  8B BC CD DL  BE EF FG GL  EH HI IJ JL  HL "
-                "LO L4 L. LM "
-                "MN NO O.",
-            arcz_missing="6L HL",
+            arcz=arcz,
+            arcz_missing="6L",
             arcz_unpredicted="67",
         )
 
     def test_else_jump_finally(self):
+        if env.PYBEHAVIOR.finally_jumps_back:
+            arcz = (
+                ".1 1S ST TU UV VW W. "
+                ".2 23 34 45 56 6A 78 8N 4Q "
+                "AB BC CN NC CQ  AD DE EF FN NF F4  DG GH HI IN NI I.  GJ JK KL LN  JN "
+                "N4 NO "
+                "OP PQ Q."
+            )
+        else:
+            arcz = (
+                ".1 1S ST TU UV VW W. "
+                ".2 23 34 45 56 6A 78 8N 4Q "
+                "AB BC CN  AD DE EF FN  DG GH HI IN  GJ JK KL LN  JN "
+                "N4 NQ N. NO "
+                "OP PQ Q."
+            )
         self.check_coverage("""\
             def func(x):
                 a = f = g = 2
@@ -886,14 +926,10 @@ class ExceptionArcTest(CoverageTest):
             assert func('continue') == (14, 23, 2, 3)       # T
             assert func('return') == (17, 2, 2, 0)          # U
             assert func('raise') == (20, 23, 25, 0)         # V
+            assert func('other') == (2, 23, 2, 3)           # W 32
             """,
-            arcz=
-                ".1 1S ST TU UV V. "
-                ".2 23 34 45 56 6A 78 8N 4Q "
-                "AB BC CN  AD DE EF FN  DG GH HI IN  GJ JK KL LN  JN "
-                "NQ N4 N. NO "
-                "OP PQ Q.",
-            arcz_missing="78 8N JN",
+            arcz=arcz,
+            arcz_missing="78 8N",
             arcz_unpredicted="",
         )
 
diff --git a/tests/test_parser.py b/tests/test_parser.py
index c2d70ee5..6340a44b 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -326,26 +326,57 @@ class ParserMissingArcDescriptionTest(CoverageTest):
                         this_thing(16)
                 that_thing(17)
             """)
-        self.assertEqual(
-            parser.missing_arc_description(16, 17),
-            "line 16 didn't jump to line 17, because the break on line 5 wasn't executed"
-        )
-        self.assertEqual(
-            parser.missing_arc_description(16, 2),
-            "line 16 didn't jump to line 2, "
-                "because the continue on line 8 wasn't executed"
+        if env.PYBEHAVIOR.finally_jumps_back:
+            self.assertEqual(
+                parser.missing_arc_description(16, 5),
+                "line 16 didn't jump to line 5, because the break on line 5 wasn't executed"
+            )
+            self.assertEqual(
+                parser.missing_arc_description(5, 17),
+                "line 5 didn't jump to line 17, because the break on line 5 wasn't executed"
+            )
+            self.assertEqual(
+                parser.missing_arc_description(16, 8),
+                "line 16 didn't jump to line 8, because the continue on line 8 wasn't executed"
+            )
+            self.assertEqual(
+                parser.missing_arc_description(8, 2),
+                "line 8 didn't jump to line 2, because the continue on line 8 wasn't executed"
+            )
+            self.assertEqual(
+                parser.missing_arc_description(16, 12),
+                "line 16 didn't jump to line 12, because the return on line 12 wasn't executed"
+            )
+            self.assertEqual(
+                parser.missing_arc_description(12, -1),
+                "line 12 didn't return from function 'function', "
+                    "because the return on line 12 wasn't executed"
+            )
+            self.assertEqual(
+                parser.missing_arc_description(16, -1),
+                "line 16 didn't except from function 'function', "
+                    "because the raise on line 14 wasn't executed"
+            )
+        else:
+            self.assertEqual(
+                parser.missing_arc_description(16, 17),
+                "line 16 didn't jump to line 17, because the break on line 5 wasn't executed"
+            )
+            self.assertEqual(
+                parser.missing_arc_description(16, 2),
+                "line 16 didn't jump to line 2, "
+                    "because the continue on line 8 wasn't executed"
+                    " or "
+                    "the continue on line 10 wasn't executed"
+            )
+            self.assertEqual(
+                parser.missing_arc_description(16, -1),
+                "line 16 didn't except from function 'function', "
+                    "because the raise on line 14 wasn't executed"
                 " or "
-                "the continue on line 10 wasn't executed"
-        )
-        self.assertEqual(
-            parser.missing_arc_description(16, -1),
-            "line 16 didn't except from function 'function', "
-                "because the raise on line 14 wasn't executed"
-            " or "
-            "line 16 didn't return from function 'function', "
-                "because the return on line 12 wasn't executed"
-        )
-
+                "line 16 didn't return from function 'function', "
+                    "because the return on line 12 wasn't executed"
+            )
     def test_missing_arc_descriptions_bug460(self):
         parser = self.parse_text(u"""\
             x = 1