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Fix ancient memory leak in contrib/auto_explain.
Tue, 2 Feb 2021 18:49:08 +0000 (13:49 -0500)
Tue, 2 Feb 2021 18:49:08 +0000 (13:49 -0500)
The ExecutorEnd hook is invoked in a context that could be quite
long-lived, not the executor's own per-query context as I think
we were sort of assuming. Thus, any cruft generated while producing
the EXPLAIN output could accumulate over multiple queries. This can
result in spectacular leakage if log_nested_statements is on, and
even without that I'm surprised nobody complained before.

To fix, just switch into the executor's context so that anything we
allocate will be released when standard_ExecutorEnd frees the executor
state. We might as well nuke the code's retail pfree of the explain
output string, too; that's laughably inadequate to the need.

Japin Li, per report from Jeff Janes. This bug is old, so
back-patch to all supported branches.

Discussion: https://postgr.es/m/CAMkU=1wCVtbeRn0s9gt12KwQ7PLXovbpM8eg25SYocKW3BT4hg@mail.gmail.com


diff --git a/contrib/auto_explain/auto_explain.c b/contrib/auto_explain/auto_explain.c
index faa6231d8722d315d7441aa7522eb5b94cfc4cdd..445bb371912171fcf67553a05b7570e7b79a657f 100644 (file)
--- a/contrib/auto_explain/auto_explain.c
+++ b/contrib/auto_explain/auto_explain.c
@@ -371,8 +371,15 @@ explain_ExecutorEnd(QueryDesc *queryDesc)
{
if (queryDesc->totaltime && auto_explain_enabled())
{
+ MemoryContext oldcxt;
double msec;
+ /*
+ * Make sure we operate in the per-query context, so any cruft will be
+ * discarded later during ExecutorEnd.
+ */
+ oldcxt = MemoryContextSwitchTo(queryDesc->estate->es_query_cxt);
+
/*
* Make sure stats accumulation is done. (Note: it's okay if several
* levels of hook all do this.)
@@ -424,9 +431,9 @@ explain_ExecutorEnd(QueryDesc *queryDesc)
(errmsg("duration: %.3f ms plan:\n%s",
msec, es->str->data),
errhidestmt(true)));
-
- pfree(es->str->data);
}
+
+ MemoryContextSwitchTo(oldcxt);
}
if (prev_ExecutorEnd)
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