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Fix ancient memory leak in contrib/auto_explain.

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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
This commit is contained in:
Tom Lane 2021-02-02 13:49:08 -05:00
parent 179775135b
commit 2671125c75
1 changed files with 9 additions and 2 deletions
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11
contrib/auto_explain/auto_explain.c
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@ -321,8 +321,15 @@ explain_ExecutorEnd(QueryDesc *queryDesc)
{ {
if (queryDesc->totaltime && auto_explain_enabled()) if (queryDesc->totaltime && auto_explain_enabled())
{ {
MemoryContext oldcxt;
double msec; 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 * Make sure stats accumulation is done. (Note: it's okay if several
* levels of hook all do this.) * levels of hook all do this.)
@ -370,9 +377,9 @@ explain_ExecutorEnd(QueryDesc *queryDesc)
(errmsg("duration: %.3f ms plan:\n%s", (errmsg("duration: %.3f ms plan:\n%s",
msec, es->str->data), msec, es->str->data),
errhidestmt(true))); errhidestmt(true)));
pfree(es->str->data);
} }
MemoryContextSwitchTo(oldcxt);
} }
if (prev_ExecutorEnd) if (prev_ExecutorEnd)