Re: [PATCH] Fix libxml leaks in contrib/xml2 XPath functions

Hi Michael,

I checked v2, and the two additional error-path cases make sense to me.

While looking at pgxml_xpath(), I noticed one more nearby cleanup gap.
pgxml_xpath() builds an xpath_workspace before returning it to callers.
The callers already wrap pgxml_xpath() with PG_TRY/PG_CATCH, but if an
ERROR is thrown before pgxml_xpath() returns, the assignment of the
workspace pointer in the caller has not completed yet.  The caller-side
PG_CATCH blocks therefore cannot call cleanup_workspace() for the
partially-built workspace.

Attached is an incremental patch on top of your v2-0001..v2-0003.  It
adds local cleanup in pgxml_xpath(), tracks comppath for the error path,
and checks xmlXPathNewContext() before dereferencing the returned
context. I kept it separate for review, but it can of course be folded
if that makes the final series cleaner.

I also used the attached manual repro script, which catches repeated
XPath syntax errors in a single backend and samples VmRSS from /proc.
On my checkout without this pgxml_xpath() cleanup it kept growing after
each round, for example:

postgres=# \i xml2-pgxml-xpath-error-leak-repro.sql
NOTICE:  error i=1, failures=20, total_kb=109748, diff_kb=88008
NOTICE:  error i=2, failures=20, total_kb=193016, diff_kb=83268
NOTICE:  error i=3, failures=20, total_kb=276220, diff_kb=83204
NOTICE:  error i=4, failures=20, total_kb=359428, diff_kb=83208
NOTICE:  error i=5, failures=20, total_kb=442632, diff_kb=83204
NOTICE:  error i=6, failures=20, total_kb=525840, diff_kb=83208
NOTICE:  error i=7, failures=20, total_kb=609044, diff_kb=83204
NOTICE:  error i=8, failures=20, total_kb=692252, diff_kb=83208
NOTICE:  error i=9, failures=20, total_kb=775456, diff_kb=83204
NOTICE:  error i=10, failures=20, total_kb=858664, diff_kb=83208

With this patch applied, it plateaued after the initial warmup:

postgres=# \i xml2-pgxml-xpath-error-leak-repro.sql
NOTICE:  error i=1, failures=20, total_kb=22672, diff_kb=972
NOTICE:  error i=2, failures=20, total_kb=22692, diff_kb=20
NOTICE:  error i=3, failures=20, total_kb=22704, diff_kb=8
NOTICE:  error i=4, failures=20, total_kb=22700, diff_kb=-8
NOTICE:  error i=5, failures=20, total_kb=22704, diff_kb=4
NOTICE:  error i=6, failures=20, total_kb=22704, diff_kb=0
NOTICE:  error i=7, failures=20, total_kb=22708, diff_kb=4
NOTICE:  error i=8, failures=20, total_kb=22704, diff_kb=-8
NOTICE:  error i=9, failures=20, total_kb=22708, diff_kb=4
NOTICE:  error i=10, failures=20, total_kb=22708, diff_kb=-4

--
Andrey Chernyy