[Andrey Lepikhov <a.lepikhov@postgrespro.ru>]

Hi, Hackers!

In the AQO project (Adaptive Query Optimization) [1] the nodeToString() 
function is used by the planner to convert an query parse tree into a 
string to generate a hash value [2].
In PostgreSQL v.11 call nodeToString(parse) segfaulted.
The reason is: parse tree node for XMLNAMESPACES clause has null pointer 
in the case of DEFAULT namespace (the pointer will be initialized at 
executor on the first call). Function _outValue() uses value->val.str[0] 
[3] without checking of value->val.str.

I want to know, which of next options is correct:
1. Converting a parse tree into string with nodeToString() is illegal 
operation. We need to add a comment to the description of nodeToString().
2. We can use nodeToString() for parse tree convertation. In this case 
we need to check node variable 'value->val.str' to NULL pointer (Now I 
use this approach, see attachment).

[1] https://github.com/postgrespro/aqo
[2] hash.c, line 55.
[3] outfuncs.c, line 3312.

-- 
Andrey Lepikhov
Postgres Professional
https://postgrespro.com
The Russian Postgres Company

Attachments:

0001-XML-Bug-fix.patch (text/x-patch, 824B) download
From 26bfe91a4901b3b342e1b3ed58252ac750773207 Mon Sep 17 00:00:00 2001
From: "Andrey V. Lepikhov" <a.lepikhov@postgrespro.ru>
Date: Sun, 16 Sep 2018 08:30:19 +0500
Subject: [PATCH] XML Bug fix

---
 src/backend/nodes/outfuncs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 744a8b91b8..20eb033eac 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -3310,7 +3310,7 @@ _outValue(StringInfo str, const Value *value)
 			 * but we don't want it to do anything with an empty string.
 			 */
 			appendStringInfoChar(str, '"');
-			if (value->val.str[0] != '\0')
+			if ((value->val.str) && (value->val.str[0] != '\0'))
 				outToken(str, value->val.str);
 			appendStringInfoChar(str, '"');
 			break;
-- 
2.17.1