Avoid caching expression state trees for domain constraints across queries.

In commit 8abb3cda0d I attempted to cache
the expression state trees constructed for domain CHECK constraints for
the life of the backend (assuming the domain's constraints don't get
redefined).  However, this turns out not to work very well, because
execQual.c will run those state trees with ecxt_per_query_memory pointing
to a query-lifespan context, and in some situations we'll end up with
pointers into that context getting stored into the state trees.  This
happens in particular with SQL-language functions, as reported by
Emre Hasegeli, but there are many other cases.

To fix, keep only the expression plan trees for domain CHECK constraints
in the typcache's data structure, and revert to performing ExecInitExpr
(at least) once per query to set up expression state trees in the query's
context.

Eventually it'd be nice to undo this, but that will require some careful
thought about memory management for expression state trees, and it seems
far too late for any such redesign in 9.5.  This way is still much more
efficient than what happened before 8abb3cda0.

src/backend/utils/cache/typcache.c

@@ -94,6 +94,13 @@ static TypeCacheEntry *firstDomainTypeEntry = NULL;
  * Data stored about a domain type's constraints.  Note that we do not create
  * this struct for the common case of a constraint-less domain; we just set
  * domainData to NULL to indicate that.
+ *
+ * Within a DomainConstraintCache, we abuse the DomainConstraintState node
+ * type a bit: check_expr fields point to expression plan trees, not plan
+ * state trees.  When needed, expression state trees are built by flat-copying
+ * the DomainConstraintState nodes and applying ExecInitExpr to check_expr.
+ * Such a state tree is not part of the DomainConstraintCache, but is
+ * considered to belong to a DomainConstraintRef.
  */
 struct DomainConstraintCache
 {
@@ -152,6 +159,7 @@ static void load_domaintype_info(TypeCacheEntry *typentry);
 static int	dcs_cmp(const void *a, const void *b);
 static void decr_dcc_refcount(DomainConstraintCache *dcc);
 static void dccref_deletion_callback(void *arg);
+static List *prep_domain_constraints(List *constraints, MemoryContext execctx);
 static bool array_element_has_equality(TypeCacheEntry *typentry);
 static bool array_element_has_compare(TypeCacheEntry *typentry);
 static bool array_element_has_hashing(TypeCacheEntry *typentry);
@@ -762,13 +770,14 @@ load_domaintype_info(TypeCacheEntry *typentry)
 
 			check_expr = (Expr *) stringToNode(constring);
 
-			/* ExecInitExpr assumes we've planned the expression */
+			/* ExecInitExpr will assume we've planned the expression */
 			check_expr = expression_planner(check_expr);
 
 			r = makeNode(DomainConstraintState);
 			r->constrainttype = DOM_CONSTRAINT_CHECK;
 			r->name = pstrdup(NameStr(c->conname));
-			r->check_expr = ExecInitExpr(check_expr, NULL);
+			/* Must cast here because we're not storing an expr state node */
+			r->check_expr = (ExprState *) check_expr;
 
 			MemoryContextSwitchTo(oldcxt);
 
@@ -913,6 +922,40 @@ dccref_deletion_callback(void *arg)
 	}
 }
 
+/*
+ * prep_domain_constraints --- prepare domain constraints for execution
+ *
+ * The expression trees stored in the DomainConstraintCache's list are
+ * converted to executable expression state trees stored in execctx.
+ */
+static List *
+prep_domain_constraints(List *constraints, MemoryContext execctx)
+{
+	List	   *result = NIL;
+	MemoryContext oldcxt;
+	ListCell   *lc;
+
+	oldcxt = MemoryContextSwitchTo(execctx);
+
+	foreach(lc, constraints)
+	{
+		DomainConstraintState *r = (DomainConstraintState *) lfirst(lc);
+		DomainConstraintState *newr;
+
+		newr = makeNode(DomainConstraintState);
+		newr->constrainttype = r->constrainttype;
+		newr->name = r->name;
+		/* Must cast here because cache items contain expr plan trees */
+		newr->check_expr = ExecInitExpr((Expr *) r->check_expr, NULL);
+
+		result = lappend(result, newr);
+	}
+
+	MemoryContextSwitchTo(oldcxt);
+
+	return result;
+}
+
 /*
  * InitDomainConstraintRef --- initialize a DomainConstraintRef struct
  *
@@ -926,6 +969,7 @@ InitDomainConstraintRef(Oid type_id, DomainConstraintRef *ref,
 	/* Look up the typcache entry --- we assume it survives indefinitely */
 	ref->tcache = lookup_type_cache(type_id, TYPECACHE_DOMAIN_INFO);
 	/* For safety, establish the callback before acquiring a refcount */
+	ref->refctx = refctx;
 	ref->dcc = NULL;
 	ref->callback.func = dccref_deletion_callback;
 	ref->callback.arg = (void *) ref;
@@ -935,7 +979,8 @@ InitDomainConstraintRef(Oid type_id, DomainConstraintRef *ref,
 	{
 		ref->dcc = ref->tcache->domainData;
 		ref->dcc->dccRefCount++;
-		ref->constraints = ref->dcc->constraints;
+		ref->constraints = prep_domain_constraints(ref->dcc->constraints,
+												   ref->refctx);
 	}
 	else
 		ref->constraints = NIL;
@@ -969,6 +1014,14 @@ UpdateDomainConstraintRef(DomainConstraintRef *ref)
 
 		if (dcc)
 		{
+			/*
+			 * Note: we just leak the previous list of executable domain
+			 * constraints.  Alternatively, we could keep those in a child
+			 * context of ref->refctx and free that context at this point.
+			 * However, in practice this code path will be taken so seldom
+			 * that the extra bookkeeping for a child context doesn't seem
+			 * worthwhile; we'll just allow a leak for the lifespan of refctx.
+			 */
 			ref->constraints = NIL;
 			ref->dcc = NULL;
 			decr_dcc_refcount(dcc);
@@ -978,7 +1031,8 @@ UpdateDomainConstraintRef(DomainConstraintRef *ref)
 		{
 			ref->dcc = dcc;
 			dcc->dccRefCount++;
-			ref->constraints = dcc->constraints;
+			ref->constraints = prep_domain_constraints(dcc->constraints,
+													   ref->refctx);
 		}
 	}
 }

src/include/utils/typcache.h

@@ -130,6 +130,7 @@ typedef struct TypeCacheEntry
 typedef struct DomainConstraintRef
 {
 	List	   *constraints;	/* list of DomainConstraintState nodes */
+	MemoryContext refctx;		/* context holding DomainConstraintRef */
 
 	/* Management data --- treat these fields as private to typcache.c */
 	TypeCacheEntry *tcache;		/* owning typcache entry */

src/test/regress/expected/domain.out

@@ -682,6 +682,31 @@ select dom_check(0);
 drop function dom_check(int);
 drop domain di;
 --
+-- Check use of a (non-inline-able) SQL function in a domain constraint;
+-- this has caused issues in the past
+--
+create function sql_is_distinct_from(anyelement, anyelement)
+returns boolean language sql
+as 'select $1 is distinct from $2 limit 1';
+create domain inotnull int
+  check (sql_is_distinct_from(value, null));
+select 1::inotnull;
+ inotnull 
+----------
+        1
+(1 row)
+
+select null::inotnull;
+ERROR:  value for domain inotnull violates check constraint "inotnull_check"
+create table dom_table (x inotnull);
+insert into dom_table values ('1');
+insert into dom_table values (1);
+insert into dom_table values (null);
+ERROR:  value for domain inotnull violates check constraint "inotnull_check"
+drop table dom_table;
+drop domain inotnull;
+drop function sql_is_distinct_from(anyelement, anyelement);
+--
 -- Renaming
 --
 create domain testdomain1 as int;

src/test/regress/sql/domain.sql

@@ -515,6 +515,30 @@ drop function dom_check(int);
 
 drop domain di;
 
+--
+-- Check use of a (non-inline-able) SQL function in a domain constraint;
+-- this has caused issues in the past
+--
+
+create function sql_is_distinct_from(anyelement, anyelement)
+returns boolean language sql
+as 'select $1 is distinct from $2 limit 1';
+
+create domain inotnull int
+  check (sql_is_distinct_from(value, null));
+
+select 1::inotnull;
+select null::inotnull;
+
+create table dom_table (x inotnull);
+insert into dom_table values ('1');
+insert into dom_table values (1);
+insert into dom_table values (null);
+
+drop table dom_table;
+drop domain inotnull;
+drop function sql_is_distinct_from(anyelement, anyelement);
+
 --
 -- Renaming
 --