Global statements from one function leaked into parallel functions.

Re http://bugs.python.org/issue4315

The symbol table used the same name dictionaries to recursively
analyze each of its child blocks, even though the dictionaries are
modified during analysis.  The fix is to create new temporary
dictionaries via the analyze_child_block().  The only information that
needs to propagate back up is the names of the free variables.

Add more comments and break out a helper function.  This code doesn't
get any easier to understand when you only look at it once a year.

Lib/test/test_scope.py

@@ -632,6 +632,30 @@ self.assert_(X.passed)
 
         f() # used to crash the interpreter...
 
+    def testGlobalInParallelNestedFunctions(self):
+        # A symbol table bug leaked the global statement from one
+        # function to other nested functions in the same block.
+        # This test verifies that a global statement in the first
+        # function does not affect the second function.
+        CODE = """def f():
+    y = 1
+    def g():
+        global y
+        return y
+    def h():
+        return y + 1
+    return g, h
+
+y = 9
+g, h = f()
+result9 = g()
+result2 = h()
+"""
+        local_ns = {}
+        global_ns = {}
+        exec CODE in local_ns, global_ns
+        self.assertEqual(2, global_ns["result2"])
+        self.assertEqual(9, global_ns["result9"])
 
 
 def test_main():

Python/symtable.c

@@ -361,8 +361,9 @@ PyST_GetScope(PySTEntryObject *ste, PyObject *name)
 
 /* Decide on scope of name, given flags.
 
-   The dicts passed in as arguments are modified as necessary.
+   The namespace dictionaries may be modified to record information
-   ste is passed so that flags can be updated.
+   about the new name.  For example, a new global will add an entry to
+   global.  A name that was global can be changed to local.
 */
 
 static int 
@@ -415,7 +416,7 @@ analyze_name(PySTEntryObject *ste, PyObject *dict, PyObject *name, long flags,
 	   explicit?  It could also be global implicit.
 	 */
 	else if (global && PyDict_GetItem(global, name)) {
-		SET_SCOPE(dict, name, GLOBAL_EXPLICIT);
+		SET_SCOPE(dict, name, GLOBAL_IMPLICIT);
 		return 1;
 	}
 	else {
@@ -424,7 +425,10 @@ analyze_name(PySTEntryObject *ste, PyObject *dict, PyObject *name, long flags,
 		SET_SCOPE(dict, name, GLOBAL_IMPLICIT);
 		return 1;
 	}
-	return 0; /* Can't get here */
+	/* Should never get here. */
+	PyErr_Format(PyExc_SystemError, "failed to set scope for %s",
+		     PyString_AS_STRING(name));
+	return 0;
 }
 
 #undef SET_SCOPE
@@ -588,43 +592,68 @@ update_symbols(PyObject *symbols, PyObject *scope,
 }   
 
 /* Make final symbol table decisions for block of ste.
+
    Arguments:
    ste -- current symtable entry (input/output)
-   bound -- set of variables bound in enclosing scopes (input)
+   bound -- set of variables bound in enclosing scopes (input).  bound
+       is NULL for module blocks.
    free -- set of free variables in enclosed scopes (output)
    globals -- set of declared global variables in enclosing scopes (input)
+
+   The implementation uses two mutually recursive functions,
+   analyze_block() and analyze_child_block().  analyze_block() is
+   responsible for analyzing the individual names defined in a block.
+   analyze_child_block() prepares temporary namespace dictionaries
+   used to evaluated nested blocks.
+
+   The two functions exist because a child block should see the name
+   bindings of its enclosing blocks, but those bindings should not
+   propagate back to a parent block.
 */
 
+static int
+analyze_child_block(PySTEntryObject *entry, PyObject *bound, PyObject *free, 
+		    PyObject *global, PyObject* child_free);
+
 static int
 analyze_block(PySTEntryObject *ste, PyObject *bound, PyObject *free, 
 	      PyObject *global)
 {
-	PyObject *name, *v, *local = NULL, *scope = NULL, *newbound = NULL;
+	PyObject *name, *v, *local = NULL, *scope = NULL;
-	PyObject *newglobal = NULL, *newfree = NULL;
+	PyObject *newbound = NULL, *newglobal = NULL;
+	PyObject *newfree = NULL, *allfree = NULL;
 	int i, success = 0;
 	Py_ssize_t pos = 0;
 
-	local = PyDict_New();
+	local = PyDict_New();  /* collect new names bound in block */
 	if (!local)
 		goto error;
-	scope = PyDict_New();
+	scope = PyDict_New(); /* collect scopes defined for each name */
 	if (!scope)
 		goto error;
+
+	/* Allocate new global and bound variable dictionaries.  These
+	   dictionaries hold the names visible in nested blocks.  For
+	   ClassBlocks, the bound and global names are initialized
+	   before analyzing names, because class bindings aren't
+	   visible in methods.  For other blocks, they are initialized
+	   after names are analyzed.
+	 */
+
+	/* TODO(jhylton): Package these dicts in a struct so that we
+	   can write reasonable helper functions?
+	*/
 	newglobal = PyDict_New();
 	if (!newglobal)
 		goto error;
-	newfree = PyDict_New();
-	if (!newfree)
-		goto error;
 	newbound = PyDict_New();
 	if (!newbound)
 		goto error;
+	newfree = PyDict_New();
+	if (!newfree)
+		goto error;
 
 	if (ste->ste_type == ClassBlock) {
-		/* make a copy of globals before calling analyze_name(),
-		   because global statements in the class have no effect
-		   on nested functions.
-		*/
 		if (PyDict_Update(newglobal, global) < 0)
 			goto error;
 		if (bound)
@@ -632,12 +661,10 @@ analyze_block(PySTEntryObject *ste, PyObject *bound, PyObject *free,
 				goto error;
 	}
 
-	assert(PySTEntry_Check(ste));
-	assert(PyDict_Check(ste->ste_symbols));
 	while (PyDict_Next(ste->ste_symbols, &pos, &name, &v)) {
 		long flags = PyInt_AS_LONG(v);
-		if (!analyze_name(ste, scope, name, flags, bound, local, free,
+		if (!analyze_name(ste, scope, name, flags,
-				  global))
+				  bound, local, free, global))
 			goto error;
 	}
 
@@ -654,18 +681,28 @@ analyze_block(PySTEntryObject *ste, PyObject *bound, PyObject *free,
 			goto error;
 	}
 
-	/* Recursively call analyze_block() on each child block */
+	/* Recursively call analyze_block() on each child block.
+
+	   newbound, newglobal now contain the names visible in
+	   nested blocks.  The free variables in the children will
+	   be collected in allfree.
+	*/
+	allfree = PyDict_New();
+	if (!allfree) 
+		goto error;
 	for (i = 0; i < PyList_GET_SIZE(ste->ste_children); ++i) {
 		PyObject *c = PyList_GET_ITEM(ste->ste_children, i);
 		PySTEntryObject* entry;
 		assert(c && PySTEntry_Check(c));
 		entry = (PySTEntryObject*)c;
-		if (!analyze_block(entry, newbound, newfree, newglobal))
+		if (!analyze_child_block(entry, newbound, newfree, newglobal,
+					 allfree))
 			goto error;
 		if (entry->ste_free || entry->ste_child_free)
 			ste->ste_child_free = 1;
 	}
 
+	PyDict_Update(newfree, allfree);
 	if (ste->ste_type == FunctionBlock && !analyze_cells(scope, newfree))
 		goto error;
 	if (!update_symbols(ste->ste_symbols, scope, bound, newfree,
@@ -683,11 +720,53 @@ analyze_block(PySTEntryObject *ste, PyObject *bound, PyObject *free,
 	Py_XDECREF(newbound);
 	Py_XDECREF(newglobal);
 	Py_XDECREF(newfree);
+	Py_XDECREF(allfree);
 	if (!success)
 		assert(PyErr_Occurred());
 	return success;
 }
 
+static int
+analyze_child_block(PySTEntryObject *entry, PyObject *bound, PyObject *free, 
+		    PyObject *global, PyObject* child_free)
+{
+	PyObject *temp_bound = NULL, *temp_global = NULL, *temp_free = NULL;
+	int success = 0;
+
+	/* Copy the bound and global dictionaries.
+
+	   These dictionary are used by all blocks enclosed by the
+	   current block.  The analyze_block() call modifies these
+	   dictionaries.
+
+	*/
+	temp_bound = PyDict_New();
+	if (!temp_bound)
+		goto error;
+	if (PyDict_Update(temp_bound, bound) < 0)
+		goto error;
+	temp_free = PyDict_New();
+	if (!temp_free)
+		goto error;
+	if (PyDict_Update(temp_free, free) < 0)
+		goto error;
+	temp_global = PyDict_New();
+	if (!temp_global)
+		goto error;
+	if (PyDict_Update(temp_global, global) < 0)
+		goto error;
+
+	if (!analyze_block(entry, temp_bound, temp_free, temp_global))
+		goto error;
+	success = PyDict_Update(child_free, temp_free) >= 0;
+	success = 1;
+ error:
+	Py_XDECREF(temp_bound);
+	Py_XDECREF(temp_free);
+	Py_XDECREF(temp_global);
+	return success;
+}
+
 static int
 symtable_analyze(struct symtable *st)
 {