Issue 3116: fix quadratic behavior in marshal.dumps().

Misc/NEWS

@@ -50,6 +50,8 @@ Core and Builtins
 Extension Modules
 -----------------
 
+- Issue #3116:  marshal.dumps() had quadratic behavior for strings > 32Mb.
+
 - Issue #2138: Add factorial() the math module.
 
 - The heapq module does comparisons using LT instead of LE.  This

Python/marshal.c

@@ -67,7 +67,7 @@ w_more(int c, WFILE *p)
 	size = PyString_Size(p->str);
 	newsize = size + size + 1024;
 	if (newsize > 32*1024*1024) {
-		newsize = size + 1024*1024;
+		newsize = size + (size >> 3);	/* 12.5% overallocation */
 	}
 	if (_PyString_Resize(&p->str, newsize) != 0) {
 		p->ptr = p->end = NULL;