Fix hypothetical bug in ExprState building for hashing

david-rowley · david-rowley · commit 87f81a556321 · 2024-11-06T09:16:00.000+13:00
adf97c1 gave ExprStates the ability to hash expressions and return a single hash value. That commit supports seeding the hash value with an initial value to have that blended into the final hash value. Here we fix a hypothetical bug where if there are zero expressions to hash, the initial value is stored in the wrong location. The existing code stored the initial value in an intermediate location expecting that when the expressions were hashed that those steps would store the final hash value in the ExprState.resvalue field. However, that wouldn't happen when there are zero expressions to hash. The correct thing to do instead is to have a special case for zero expressions and when we hit that case, store the initial value directly in the ExprState.resvalue. The reason that this is a hypothetical bug is that no code currently calls ExecBuildHash32Expr passing a non-zero initial value. Discussion: https://postgr.es/m/CAApHDvpMAL_zxbMRr1LOex3O7Y7R7ZN2i8iUFLQhqQiJMAg3qw@mail.gmail.com
diff --git a/src/backend/executor/execExpr.c b/src/backend/executor/execExpr.c
@@ -4004,20 +4004,21 @@ ExecBuildHash32Expr(TupleDesc desc, const TupleTableSlotOps *ops,
 	ListCell   *lc2;
 	intptr_t	strict_opcode;
 	intptr_t	opcode;
+	int			num_exprs = list_length(hash_exprs);
 
-	Assert(list_length(hash_exprs) == list_length(collations));
+	Assert(num_exprs == list_length(collations));
 
 	state->parent = parent;
 
 	/* Insert setup steps as needed. */
 	ExecCreateExprSetupSteps(state, (Node *) hash_exprs);
 
 	/*
-	 * When hashing more than 1 expression or if we have an init value, we
-	 * need somewhere to store the intermediate hash value so that it's
-	 * available to be combined with the result of subsequent hashing.
+	 * Make a place to store intermediate hash values between subsequent
+	 * hashing of individual expressions.  We only need this if there is more
+	 * than one expression to hash or an initial value plus one expression.
 	 */
-	if (list_length(hash_exprs) > 1 || init_value != 0)
+	if ((int64) num_exprs + (init_value != 0) > 1)
 		iresult = palloc(sizeof(NullableDatum));
 
 	if (init_value == 0)
@@ -4032,11 +4033,15 @@ ExecBuildHash32Expr(TupleDesc desc, const TupleTableSlotOps *ops,
 	}
 	else
 	{
-		/* Set up operation to set the initial value. */
+		/*
+		 * Set up operation to set the initial value.  Normally we store this
+		 * in the intermediate hash value location, but if there are no exprs
+		 * to hash, store it in the ExprState's result field.
+		 */
 		scratch.opcode = EEOP_HASHDATUM_SET_INITVAL;
 		scratch.d.hashdatum_initvalue.init_value = UInt32GetDatum(init_value);
-		scratch.resvalue = &iresult->value;
-		scratch.resnull = &iresult->isnull;
+		scratch.resvalue = num_exprs > 0 ? &iresult->value : &state->resvalue;
+		scratch.resnull = num_exprs > 0 ? &iresult->isnull : &state->resnull;
 
 		ExprEvalPushStep(state, &scratch);
 
@@ -4074,7 +4079,7 @@ ExecBuildHash32Expr(TupleDesc desc, const TupleTableSlotOps *ops,
 						&fcinfo->args[0].value,
 						&fcinfo->args[0].isnull);
 
-		if (i == list_length(hash_exprs) - 1)
+		if (i == num_exprs - 1)
 		{
 			/* the result for hashing the final expr is stored in the state */
 			scratch.resvalue = &state->resvalue;
