# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1559509826 -7200
# Node ID 938887b82033d33ceaf9e9a462f1aabf5b7c8df2
# Parent  dc210430ce4c10b10bffa48cab52c1d01f2a005d
Prevent "null" usage of assigned names from escaping via the exception handler.

diff -r dc210430ce4c -r 938887b82033 common.py
--- a/common.py	Mon May 27 11:54:56 2019 +0200
+++ b/common.py	Sun Jun 02 23:10:26 2019 +0200
@@ -563,6 +563,8 @@
         self.next_temporary()
         t2 = self.get_temporary_name()
         self.next_temporary()
+        t3 = self.get_temporary_name()
+        self.next_temporary()
 
         node = compiler.ast.Stmt([
 
@@ -583,9 +585,10 @@
             # try:
             #     while True:
             #         try:
-            #             <var>... = <t2>()
+            #             <t3> = <t2>()
             #         except StopIteration:
             #             raise LoopExit
+            #         <var>... = <t3>
             #         {n.body}
             # except LoopExit:
             #     {n.else_}
@@ -601,13 +604,16 @@
                     compiler.ast.Stmt([
                         compiler.ast.TryExcept(
                             compiler.ast.Assign(
-                                [n.assign],
+                                [compiler.ast.AssName(t3, "OP_ASSIGN")],
                                 compiler.ast.CallFunc(
                                     compiler.ast.Name(t2),
                                     [])),
                             [(compiler.ast.Name("StopIteration"), None,
                               compiler.ast.Raise(compiler.ast.Name("LoopExit")))],
                             None),
+                        compiler.ast.Assign(
+                            [n.assign],
+                            compiler.ast.Name(t3)),
                         n.body]),
                     None),
                 [(compiler.ast.Name("LoopExit"), None, n.else_ or compiler.ast.Pass())],
diff -r dc210430ce4c -r 938887b82033 internal_tests/branches.py
--- a/internal_tests/branches.py	Mon May 27 11:54:56 2019 +0200
+++ b/internal_tests/branches.py	Sun Jun 02 23:10:26 2019 +0200
@@ -687,4 +687,77 @@
     bt.get_assignment_positions_for_branches("a", ar)
 names.append(bt.assignments["a"])
 
+# This demonstrates why the assignment in a "for" loop construct must appear
+# outside the inner "try" body: null usage escapes the loop via the exception
+# handler and the raise statement, even though the assignment would only be
+# valid otherwise.
+
+# Equivalent to...
+#
+# try:
+#   while ...:
+#     try:
+#       a = ...
+#     except:
+#       raise ...
+#     a.p
+# except:
+#   pass
+
+bt = branching.BranchTracker()
+bt.new_branchpoint()                # begin (try)
+bt.new_branchpoint(True)            # begin (while)
+bt.new_branch(True)                 # while ...
+bt.new_branchpoint()                # begin (try)
+a = bt.assign_names(["a"])
+bt.resume_abandoned_branches()      # except
+bt.abandon_branch()                 # raise
+bt.shelve_branch()
+bt.new_branch()                     # (null)
+bt.shelve_branch()
+bt.merge_branches()                 # end (try)
+ap = bt.use_attribute("a", "p")
+bt.resume_continuing_branches()
+bt.shelve_branch(True)
+bt.new_branch()                     # (null)
+bt.shelve_branch()
+bt.merge_branches()                 # end (while)
+bt.resume_broken_branches()
+bt.resume_abandoned_branches()      # except
+bt.shelve_branch()
+bt.new_branch()                     # (null)
+bt.shelve_branch()
+bt.merge_branches()                 # end (try)
+
+print simple_usage(a) == \
+    {'a' : set([('p',), ()])}, simple_usage(a)
+print bt.get_assignment_positions_for_branches("a", ap) == [0], \
+    bt.get_assignment_positions_for_branches("a", ap)
+names.append(bt.assignments["a"])
+
+# Equivalent to...
+#
+# b = ...
+# while ...:
+#   a = ...
+#   a.p
+
+bt = branching.BranchTracker()
+bt.new_branchpoint(True)            # begin
+b = bt.assign_names(["b"])
+bt.new_branch(True)                 # while ...
+a = bt.assign_names(["a"])
+ap = bt.use_attribute("a", "p")
+bt.resume_continuing_branches()
+bt.shelve_branch(True)
+bt.new_branch()                     # (null)
+bt.shelve_branch()
+bt.merge_branches()                 # end
+
+print simple_usage(a) == \
+    {'a' : set([('p',)])}, simple_usage(a)
+print bt.get_assignment_positions_for_branches("a", ap) == [0], \
+    bt.get_assignment_positions_for_branches("a", ap)
+names.append(bt.assignments["a"])
+
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