Redesign the caching done by get_cached_rowtype().

Previously, get_cached_rowtype() cached a pointer to a reference-counted
tuple descriptor from the typcache, relying on the ExprContextCallback
mechanism to release the tupdesc refcount when the expression tree
using the tupdesc was destroyed.  This worked fine when it was designed,
but the introduction of within-DO-block COMMITs broke it.  The refcount
is logged in a transaction-lifespan resource owner, but plpgsql won't
destroy simple expressions made within the DO block (before its first
commit) until the DO block is exited.  That results in a warning about
a leaked tupdesc refcount when the COMMIT destroys the original resource
owner, and then an error about the active resource owner not holding a
matching refcount when the expression is destroyed.

To fix, get rid of the need to have a shutdown callback at all, by
instead caching a pointer to the relevant typcache entry.  Those
survive for the life of the backend, so we needn't worry about the
pointer becoming stale.  (For registered RECORD types, we can still
cache a pointer to the tupdesc, knowing that it won't change for the
life of the backend.)  This mechanism has been in use in plpgsql
and expandedrecord.c since commit 4b93f57, and seems to work well.

This change requires modifying the ExprEvalStep structs used by the
relevant expression step types, which is slightly worrisome for
back-patching.  However, there seems no good reason for extensions
to be familiar with the details of these particular sub-structs.

Per report from Rohit Bhogate.  Back-patch to v11 where within-DO-block
COMMITs became a thing.

Discussion: https://postgr.es/m/CAAV6ZkQRCVBh8qAY+SZiHnz+U+FqAGBBDaDTjF2yiKa2nJSLKg@mail.gmail.com

Author: tglsfdc

src/backend/executor/execExpr.c

@@ -1375,7 +1375,7 @@ ExecInitExprRec(Expr *node, ExprState *state,
 				scratch.opcode = EEOP_FIELDSELECT;
 				scratch.d.fieldselect.fieldnum = fselect->fieldnum;
 				scratch.d.fieldselect.resulttype = fselect->resulttype;
-				scratch.d.fieldselect.argdesc = NULL;
+				scratch.d.fieldselect.rowcache.cacheptr = NULL;
 
 				ExprEvalPushStep(state, &scratch);
 				break;
@@ -1385,7 +1385,7 @@ ExecInitExprRec(Expr *node, ExprState *state,
 			{
 				FieldStore *fstore = (FieldStore *) node;
 				TupleDesc	tupDesc;
-				TupleDesc  *descp;
+				ExprEvalRowtypeCache *rowcachep;
 				Datum	   *values;
 				bool	   *nulls;
 				int			ncolumns;
@@ -1401,17 +1401,17 @@ ExecInitExprRec(Expr *node, ExprState *state,
 				values = (Datum *) palloc(sizeof(Datum) * ncolumns);
 				nulls = (bool *) palloc(sizeof(bool) * ncolumns);
 
-				/* create workspace for runtime tupdesc cache */
-				descp = (TupleDesc *) palloc(sizeof(TupleDesc));
-				*descp = NULL;
+				/* create shared composite-type-lookup cache struct */
+				rowcachep = palloc(sizeof(ExprEvalRowtypeCache));
+				rowcachep->cacheptr = NULL;
 
 				/* emit code to evaluate the composite input value */
 				ExecInitExprRec(fstore->arg, state, resv, resnull);
 
 				/* next, deform the input tuple into our workspace */
 				scratch.opcode = EEOP_FIELDSTORE_DEFORM;
 				scratch.d.fieldstore.fstore = fstore;
-				scratch.d.fieldstore.argdesc = descp;
+				scratch.d.fieldstore.rowcache = rowcachep;
 				scratch.d.fieldstore.values = values;
 				scratch.d.fieldstore.nulls = nulls;
 				scratch.d.fieldstore.ncolumns = ncolumns;
@@ -1469,7 +1469,7 @@ ExecInitExprRec(Expr *node, ExprState *state,
 				/* finally, form result tuple */
 				scratch.opcode = EEOP_FIELDSTORE_FORM;
 				scratch.d.fieldstore.fstore = fstore;
-				scratch.d.fieldstore.argdesc = descp;
+				scratch.d.fieldstore.rowcache = rowcachep;
 				scratch.d.fieldstore.values = values;
 				scratch.d.fieldstore.nulls = nulls;
 				scratch.d.fieldstore.ncolumns = ncolumns;
@@ -1615,17 +1615,24 @@ ExecInitExprRec(Expr *node, ExprState *state,
 		case T_ConvertRowtypeExpr:
 			{
 				ConvertRowtypeExpr *convert = (ConvertRowtypeExpr *) node;
+				ExprEvalRowtypeCache *rowcachep;
+
+				/* cache structs must be out-of-line for space reasons */
+				rowcachep = palloc(2 * sizeof(ExprEvalRowtypeCache));
+				rowcachep[0].cacheptr = NULL;
+				rowcachep[1].cacheptr = NULL;
 
 				/* evaluate argument into step's result area */
 				ExecInitExprRec(convert->arg, state, resv, resnull);
 
 				/* and push conversion step */
 				scratch.opcode = EEOP_CONVERT_ROWTYPE;
-				scratch.d.convert_rowtype.convert = convert;
-				scratch.d.convert_rowtype.indesc = NULL;
-				scratch.d.convert_rowtype.outdesc = NULL;
+				scratch.d.convert_rowtype.inputtype =
+					exprType((Node *) convert->arg);
+				scratch.d.convert_rowtype.outputtype = convert->resulttype;
+				scratch.d.convert_rowtype.incache = &rowcachep[0];
+				scratch.d.convert_rowtype.outcache = &rowcachep[1];
 				scratch.d.convert_rowtype.map = NULL;
-				scratch.d.convert_rowtype.initialized = false;
 
 				ExprEvalPushStep(state, &scratch);
 				break;
@@ -2250,7 +2257,7 @@ ExecInitExprRec(Expr *node, ExprState *state,
 						 (int) ntest->nulltesttype);
 				}
 				/* initialize cache in case it's a row test */
-				scratch.d.nulltest_row.argdesc = NULL;
+				scratch.d.nulltest_row.rowcache.cacheptr = NULL;
 
 				/* first evaluate argument into result variable */
 				ExecInitExprRec(ntest->arg, state,

src/backend/executor/execExprInterp.c

@@ -145,8 +145,8 @@ static void ExecInitInterpreter(void);
 static void CheckVarSlotCompatibility(TupleTableSlot *slot, int attnum, Oid vartype);
 static void CheckOpSlotCompatibility(ExprEvalStep *op, TupleTableSlot *slot);
 static TupleDesc get_cached_rowtype(Oid type_id, int32 typmod,
-									TupleDesc *cache_field, ExprContext *econtext);
-static void ShutdownTupleDescRef(Datum arg);
+									ExprEvalRowtypeCache *rowcache,
+									bool *changed);
 static void ExecEvalRowNullInt(ExprState *state, ExprEvalStep *op,
 							   ExprContext *econtext, bool checkisnull);
 
@@ -1959,56 +1959,78 @@ CheckOpSlotCompatibility(ExprEvalStep *op, TupleTableSlot *slot)
  * get_cached_rowtype: utility function to lookup a rowtype tupdesc
  *
  * type_id, typmod: identity of the rowtype
- * cache_field: where to cache the TupleDesc pointer in expression state node
- *		(field must be initialized to NULL)
- * econtext: expression context we are executing in
+ * rowcache: space for caching identity info
+ *		(rowcache->cacheptr must be initialized to NULL)
+ * changed: if not NULL, *changed is set to true on any update
  *
- * NOTE: because the shutdown callback will be called during plan rescan,
- * must be prepared to re-do this during any node execution; cannot call
- * just once during expression initialization.
+ * The returned TupleDesc is not guaranteed pinned; caller must pin it
+ * to use it across any operation that might incur cache invalidation.
+ * (The TupleDesc is always refcounted, so just use IncrTupleDescRefCount.)
+ *
+ * NOTE: because composite types can change contents, we must be prepared
+ * to re-do this during any node execution; cannot call just once during
+ * expression initialization.
  */
 static TupleDesc
 get_cached_rowtype(Oid type_id, int32 typmod,
-				   TupleDesc *cache_field, ExprContext *econtext)
+				   ExprEvalRowtypeCache *rowcache,
+				   bool *changed)
 {
-	TupleDesc	tupDesc = *cache_field;
-
-	/* Do lookup if no cached value or if requested type changed */
-	if (tupDesc == NULL ||
-		type_id != tupDesc->tdtypeid ||
-		typmod != tupDesc->tdtypmod)
+	if (type_id != RECORDOID)
 	{
-		tupDesc = lookup_rowtype_tupdesc(type_id, typmod);
+		/*
+		 * It's a named composite type, so use the regular typcache.  Do a
+		 * lookup first time through, or if the composite type changed.  Note:
+		 * "tupdesc_id == 0" may look redundant, but it protects against the
+		 * admittedly-theoretical possibility that type_id was RECORDOID the
+		 * last time through, so that the cacheptr isn't TypeCacheEntry *.
+		 */
+		TypeCacheEntry *typentry = (TypeCacheEntry *) rowcache->cacheptr;
 
-		if (*cache_field)
+		if (unlikely(typentry == NULL ||
+					 rowcache->tupdesc_id == 0 ||
+					 typentry->tupDesc_identifier != rowcache->tupdesc_id))
 		{
-			/* Release old tupdesc; but callback is already registered */
-			ReleaseTupleDesc(*cache_field);
-		}
-		else
-		{
-			/* Need to register shutdown callback to release tupdesc */
-			RegisterExprContextCallback(econtext,
-										ShutdownTupleDescRef,
-										PointerGetDatum(cache_field));
-		}
-		*cache_field = tupDesc;
+			typentry = lookup_type_cache(type_id, TYPECACHE_TUPDESC);
+			if (typentry->tupDesc == NULL)
+				ereport(ERROR,
+						(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+						 errmsg("type %s is not composite",
+								format_type_be(type_id))));
+			rowcache->cacheptr = (void *) typentry;
+			rowcache->tupdesc_id = typentry->tupDesc_identifier;
+			if (changed)
+				*changed = true;
+		}
+		return typentry->tupDesc;
+	}
+	else
+	{
+		/*
+		 * A RECORD type, once registered, doesn't change for the life of the
+		 * backend.  So we don't need a typcache entry as such, which is good
+		 * because there isn't one.  It's possible that the caller is asking
+		 * about a different type than before, though.
+		 */
+		TupleDesc	tupDesc = (TupleDesc) rowcache->cacheptr;
+
+		if (unlikely(tupDesc == NULL ||
+					 rowcache->tupdesc_id != 0 ||
+					 type_id != tupDesc->tdtypeid ||
+					 typmod != tupDesc->tdtypmod))
+		{
+			tupDesc = lookup_rowtype_tupdesc(type_id, typmod);
+			/* Drop pin acquired by lookup_rowtype_tupdesc */
+			ReleaseTupleDesc(tupDesc);
+			rowcache->cacheptr = (void *) tupDesc;
+			rowcache->tupdesc_id = 0;	/* not a valid value for non-RECORD */
+			if (changed)
+				*changed = true;
+		}
+		return tupDesc;
 	}
-	return tupDesc;
 }
 
-/*
- * Callback function to release a tupdesc refcount at econtext shutdown
- */
-static void
-ShutdownTupleDescRef(Datum arg)
-{
-	TupleDesc  *cache_field = (TupleDesc *) DatumGetPointer(arg);
-
-	if (*cache_field)
-		ReleaseTupleDesc(*cache_field);
-	*cache_field = NULL;
-}
 
 /*
  * Fast-path functions, for very simple expressions
@@ -2600,8 +2622,7 @@ ExecEvalRowNullInt(ExprState *state, ExprEvalStep *op,
 
 	/* Lookup tupdesc if first time through or if type changes */
 	tupDesc = get_cached_rowtype(tupType, tupTypmod,
-								 &op->d.nulltest_row.argdesc,
-								 econtext);
+								 &op->d.nulltest_row.rowcache, NULL);
 
 	/*
 	 * heap_attisnull needs a HeapTuple not a bare HeapTupleHeader.
@@ -3034,8 +3055,7 @@ ExecEvalFieldSelect(ExprState *state, ExprEvalStep *op, ExprContext *econtext)
 
 		/* Lookup tupdesc if first time through or if type changes */
 		tupDesc = get_cached_rowtype(tupType, tupTypmod,
-									 &op->d.fieldselect.argdesc,
-									 econtext);
+									 &op->d.fieldselect.rowcache, NULL);
 
 		/*
 		 * Find field's attr record.  Note we don't support system columns
@@ -3093,9 +3113,9 @@ ExecEvalFieldStoreDeForm(ExprState *state, ExprEvalStep *op, ExprContext *econte
 {
 	TupleDesc	tupDesc;
 
-	/* Lookup tupdesc if first time through or after rescan */
+	/* Lookup tupdesc if first time through or if type changes */
 	tupDesc = get_cached_rowtype(op->d.fieldstore.fstore->resulttype, -1,
-								 op->d.fieldstore.argdesc, econtext);
+								 op->d.fieldstore.rowcache, NULL);
 
 	/* Check that current tupdesc doesn't have more fields than we allocated */
 	if (unlikely(tupDesc->natts > op->d.fieldstore.ncolumns))
@@ -3137,10 +3157,14 @@ ExecEvalFieldStoreDeForm(ExprState *state, ExprEvalStep *op, ExprContext *econte
 void
 ExecEvalFieldStoreForm(ExprState *state, ExprEvalStep *op, ExprContext *econtext)
 {
+	TupleDesc	tupDesc;
 	HeapTuple	tuple;
 
-	/* argdesc should already be valid from the DeForm step */
-	tuple = heap_form_tuple(*op->d.fieldstore.argdesc,
+	/* Lookup tupdesc (should be valid already) */
+	tupDesc = get_cached_rowtype(op->d.fieldstore.fstore->resulttype, -1,
+								 op->d.fieldstore.rowcache, NULL);
+
+	tuple = heap_form_tuple(tupDesc,
 							op->d.fieldstore.values,
 							op->d.fieldstore.nulls);
 
@@ -3157,13 +3181,13 @@ ExecEvalFieldStoreForm(ExprState *state, ExprEvalStep *op, ExprContext *econtext
 void
 ExecEvalConvertRowtype(ExprState *state, ExprEvalStep *op, ExprContext *econtext)
 {
-	ConvertRowtypeExpr *convert = op->d.convert_rowtype.convert;
 	HeapTuple	result;
 	Datum		tupDatum;
 	HeapTupleHeader tuple;
 	HeapTupleData tmptup;
 	TupleDesc	indesc,
 				outdesc;
+	bool		changed = false;
 
 	/* NULL in -> NULL out */
 	if (*op->resnull)
@@ -3172,24 +3196,19 @@ ExecEvalConvertRowtype(ExprState *state, ExprEvalStep *op, ExprContext *econtext
 	tupDatum = *op->resvalue;
 	tuple = DatumGetHeapTupleHeader(tupDatum);
 
-	/* Lookup tupdescs if first time through or after rescan */
-	if (op->d.convert_rowtype.indesc == NULL)
-	{
-		get_cached_rowtype(exprType((Node *) convert->arg), -1,
-						   &op->d.convert_rowtype.indesc,
-						   econtext);
-		op->d.convert_rowtype.initialized = false;
-	}
-	if (op->d.convert_rowtype.outdesc == NULL)
-	{
-		get_cached_rowtype(convert->resulttype, -1,
-						   &op->d.convert_rowtype.outdesc,
-						   econtext);
-		op->d.convert_rowtype.initialized = false;
-	}
-
-	indesc = op->d.convert_rowtype.indesc;
-	outdesc = op->d.convert_rowtype.outdesc;
+	/*
+	 * Lookup tupdescs if first time through or if type changes.  We'd better
+	 * pin them since type conversion functions could do catalog lookups and
+	 * hence cause cache invalidation.
+	 */
+	indesc = get_cached_rowtype(op->d.convert_rowtype.inputtype, -1,
+								op->d.convert_rowtype.incache,
+								&changed);
+	IncrTupleDescRefCount(indesc);
+	outdesc = get_cached_rowtype(op->d.convert_rowtype.outputtype, -1,
+								 op->d.convert_rowtype.outcache,
+								 &changed);
+	IncrTupleDescRefCount(outdesc);
 
 	/*
 	 * We used to be able to assert that incoming tuples are marked with
@@ -3200,8 +3219,8 @@ ExecEvalConvertRowtype(ExprState *state, ExprEvalStep *op, ExprContext *econtext
 	Assert(HeapTupleHeaderGetTypeId(tuple) == indesc->tdtypeid ||
 		   HeapTupleHeaderGetTypeId(tuple) == RECORDOID);
 
-	/* if first time through, initialize conversion map */
-	if (!op->d.convert_rowtype.initialized)
+	/* if first time through, or after change, initialize conversion map */
+	if (changed)
 	{
 		MemoryContext old_cxt;
 
@@ -3210,7 +3229,6 @@ ExecEvalConvertRowtype(ExprState *state, ExprEvalStep *op, ExprContext *econtext
 
 		/* prepare map from old to new attribute numbers */
 		op->d.convert_rowtype.map = convert_tuples_by_name(indesc, outdesc);
-		op->d.convert_rowtype.initialized = true;
 
 		MemoryContextSwitchTo(old_cxt);
 	}
@@ -3240,6 +3258,9 @@ ExecEvalConvertRowtype(ExprState *state, ExprEvalStep *op, ExprContext *econtext
 		 */
 		*op->resvalue = heap_copy_tuple_as_datum(&tmptup, outdesc);
 	}
+
+	DecrTupleDescRefCount(indesc);
+	DecrTupleDescRefCount(outdesc);
 }
 
 /*