Fix a few more memory leaks

Document more info about the benefits of configuring without
pymalloc when running valgrind

Author: nnorwitz

Misc/README.valgrind

@@ -12,6 +12,19 @@ Misc/valgrind-python.supp.  Second, you must do one of the following:
   * Uncomment the lines in Misc/valgrind-python.supp that
     suppress the warnings for PyObject_Free and PyObject_Realloc
 
+If you want to use Valgrind more effectively and catch even more
+memory leaks, you will need to configure python --without-pymalloc.
+PyMalloc allocates a few blocks in big chunks and most object
+allocations don't call malloc, they use chunks doled about by PyMalloc
+from the big blocks.  This means Valgrind can't detect
+many allocations (and frees), except for those that are forwarded
+to the system malloc.  Note: configuring python --without-pymalloc
+makes Python run much slower, especially when running under Valgrind.
+You may need to run the tests in batches under Valgrind to keep
+the memory usage down to allow the tests to complete.  It seems to take
+about 5 times longer to run --without-pymalloc.
+
+
 Details:
 --------
 Python uses its own small-object allocation scheme on top of malloc,
@@ -21,7 +34,8 @@ Valgrind may show some unexpected results when PyMalloc is used.
 Starting with Python 2.3, PyMalloc is used by default.  You can disable
 PyMalloc when configuring python by adding the --without-pymalloc option.
 If you disable PyMalloc, most of the information in this document and
-the supplied suppressions file will not be useful.
+the supplied suppressions file will not be useful.  As discussed above,
+disabling PyMalloc can catch more problems.
 
 If you use valgrind on a default build of Python,  you will see
 many errors like:

Python/ast.c

@@ -1054,8 +1054,12 @@ ast_for_listcomp(struct compiling *c, const node *n)
             return NULL;
         }
 
-	if (asdl_seq_LEN(t) == 1)
+	if (asdl_seq_LEN(t) == 1) {
 	    lc = comprehension(asdl_seq_GET(t, 0), expression, NULL);
+	    /* only free the sequence since we grabbed element 0 above */
+	    if (lc)
+	        asdl_seq_free(t); /* ok */
+	}
 	else
 	    lc = comprehension(Tuple(t, Store, LINENO(ch)), expression, NULL);
 
@@ -1222,9 +1226,13 @@ ast_for_genexp(struct compiling *c, const node *n)
             return NULL;
         }
 
-        if (asdl_seq_LEN(t) == 1)
+        if (asdl_seq_LEN(t) == 1) {
             ge = comprehension(asdl_seq_GET(t, 0), expression,
                                NULL);
+	    /* only free the sequence since we grabbed element 0 above */
+	    if (ge)
+	        asdl_seq_free(t); /* ok */
+	}
         else
             ge = comprehension(Tuple(t, Store, LINENO(ch)),
                                expression, NULL);

Python/compile.c

@@ -2384,7 +2384,10 @@ compiler_import_as(struct compiler *c, identifier name, identifier asname)
 			dot = strchr(src, '.');
 			attr = PyString_FromStringAndSize(src, 
 					    dot ? dot - src : strlen(src));
+			if (!attr)
+				return -1;
 			ADDOP_O(c, LOAD_ATTR, attr, names);
+			Py_DECREF(attr);
 			src = dot + 1;
 		}
 	}

Python/modsupport.c

@@ -82,6 +82,7 @@ Py_InitModule4(char *name, PyMethodDef *methods, char *doc,
 			}
 			Py_DECREF(v);
 		}
+		Py_DECREF(n);
 	}
 	if (doc != NULL) {
 		v = PyString_FromString(doc);