Avoid query-lifetime memory leaks in XMLTABLE (bug #15321)

Multiple calls to XMLTABLE in a query (e.g. laterally applying it to a
table with an xml column, an important use-case) were leaking large
amounts of memory into the per-query context, blowing up memory usage.

Repair by reorganizing memory context usage in nodeTableFuncscan; use
the usual per-tuple context for row-by-row evaluations instead of
perValueCxt, and use the explicitly created context -- renamed from
perValueCxt to perTableCxt -- for arguments and state for each
individual table-generation operation.

Backpatch to PG10 where this code was introduced.

Original report by IRC user begriffs; analysis and patch by me.
Reviewed by Tom Lane and Pavel Stehule.

Discussion: https://postgr.es/m/153394403528.10284.7530399040974170549@wrigleys.postgresql.org

diff --git a/src/backend/executor/nodeTableFuncscan.c b/src/backend/executor/nodeTableFuncscan.c
index fed6f2b3a53e4305d50d8e820bea29cdc7059532..abbf0e4d445a7795a71a8b852f6bfd96b32ff868 100644 (file)
--- a/src/backend/executor/nodeTableFuncscan.c
+++ b/src/backend/executor/nodeTableFuncscan.c
@@ -164,7 +164,7 @@ ExecInitTableFuncScan(TableFuncScan *node, EState *estate, int eflags)
    /* Only XMLTABLE is supported currently */
    scanstate->routine = &XmlTableRoutine;
 
-   scanstate->perValueCxt =
+   scanstate->perTableCxt =
        AllocSetContextCreate(CurrentMemoryContext,
                              "TableFunc per value context",
                              ALLOCSET_DEFAULT_SIZES);
@@ -282,6 +282,16 @@ tfuncFetchRows(TableFuncScanState *tstate, ExprContext *econtext)
    oldcxt = MemoryContextSwitchTo(econtext->ecxt_per_query_memory);
    tstate->tupstore = tuplestore_begin_heap(false, false, work_mem);
 
+   /*
+    * Each call to fetch a new set of rows - of which there may be very many
+    * if XMLTABLE is being used in a lateral join - will allocate a possibly
+    * substantial amount of memory, so we cannot use the per-query context
+    * here. perTableCxt now serves the same function as "argcontext" does in
+    * FunctionScan - a place to store per-one-call (i.e. one result table)
+    * lifetime data (as opposed to per-query or per-result-tuple).
+    */
+   MemoryContextSwitchTo(tstate->perTableCxt);
+
    PG_TRY();
    {
        routine->InitOpaque(tstate,
@@ -313,8 +323,7 @@ tfuncFetchRows(TableFuncScanState *tstate, ExprContext *econtext)
    }
    PG_END_TRY();
 
-   /* return to original memory context, and clean up */
-   MemoryContextSwitchTo(oldcxt);
+   /* clean up and return to original memory context */
 
    if (tstate->opaque != NULL)
    {
@@ -322,6 +331,9 @@ tfuncFetchRows(TableFuncScanState *tstate, ExprContext *econtext)
        tstate->opaque = NULL;
    }
 
+   MemoryContextSwitchTo(oldcxt);
+   MemoryContextReset(tstate->perTableCxt);
+
    return;
 }
 
@@ -428,7 +440,14 @@ tfuncLoadRows(TableFuncScanState *tstate, ExprContext *econtext)
 
    ordinalitycol =
        ((TableFuncScan *) (tstate->ss.ps.plan))->tablefunc->ordinalitycol;
-   oldcxt = MemoryContextSwitchTo(tstate->perValueCxt);
+
+   /*
+    * We need a short-lived memory context that we can clean up each time
+    * around the loop, to avoid wasting space. Our default per-tuple context
+    * is fine for the job, since we won't have used it for anything yet in
+    * this tuple cycle.
+    */
+   oldcxt = MemoryContextSwitchTo(econtext->ecxt_per_tuple_memory);
 
    /*
     * Keep requesting rows from the table builder until there aren't any.
@@ -493,7 +512,7 @@ tfuncLoadRows(TableFuncScanState *tstate, ExprContext *econtext)
 
        tuplestore_putvalues(tstate->tupstore, tupdesc, values, nulls);
 
-       MemoryContextReset(tstate->perValueCxt);
+       MemoryContextReset(econtext->ecxt_per_tuple_memory);
    }
 
    MemoryContextSwitchTo(oldcxt);

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 018f50bbb713abd99c561a702a4d71d846dcacf7..41fa2052a210c0e5757ebb8716d0198adad525ad 100644 (file)
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1580,7 +1580,7 @@ typedef struct TableFuncScanState
    FmgrInfo   *in_functions;   /* input function for each column */
    Oid        *typioparams;    /* typioparam for each column */
    int64       ordinal;        /* row number to be output next */
-   MemoryContext perValueCxt;  /* short life context for value evaluation */
+   MemoryContext perTableCxt;  /* per-table context */
    Tuplestorestate *tupstore;  /* output tuple store */
 } TableFuncScanState;