db: Add IS NULL and IS NOT NULL unary operators

If you try to (mysql) prepare a statement with "... column IS ?" you'd
get an error.

For mysql, you could fix this by using the "... column <=> ?" special
operator ('IS NOT DISTINCT FROM'), but that's not standard SQL.

Instead, you can now do:

    update_cols[0] = &some_column;
    update_ops[0] = OP_IS_NULL;
    update_vals[0].nul = 1;

You still need to set .nul=1 because prepared statements have to consume
a single value.

db/db_op.c

@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2021 OpenSIPS
+ *
+ * This file is part of opensips, a free SIP server.
+ *
+ * opensips is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version
+ *
+ * opensips is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301  USA
+ */
+
+#include "db_op.h"
+
+
+const char OP_IS_NULL[] = " IS NULL";
+const char OP_IS_NOT_NULL[] = " IS NOT NULL";

db/db_op.h

@@ -40,6 +40,17 @@
 /** operator negation */
 #define OP_NEQ "!="
 
+/* Special unary operators here, because mysql prepared statements do
+ * not cope with 'column IS ?' (they would cope with 'column <=> ?' but
+ * that's not standard SQL).
+ * (Declared as char array instead of define, so they can be pointer
+ * compared in code.) */
+
+/** unary operator: IS NULL */
+extern const char OP_IS_NULL[]; /* " IS NULL" */
+/** unary operator: IS NOT NULL */
+extern const char OP_IS_NOT_NULL[]; /* " IS NOT NULL" */
+
 
 /**
  * This type represents an expression operator uses for SQL queries.

db/db_ut.c

@@ -336,16 +336,33 @@ int db_print_where(const db_con_t* _c, char* _b, const int _l, const db_key_t* _
 
 	for(i = 0; i < _n; i++) {
 		if (_o) {
-			ret = snprintf(_b + len, _l - len, "%.*s%s",
-				_k[i]->len, _k[i]->s, _o[i]);
-			if (ret < 0 || ret >= (_l - len)) goto error;
-			len += ret;
+			/* Special case: if there is unary operator and
+			 * we use a prepared statement, we must still
+			 * consume one value, even though we're not
+			 * using it. Otherwise the number of values will
+			 * be incorrect. */
+			if ((_o[i] == OP_IS_NULL || _o[i] == OP_IS_NOT_NULL) &&
+					CON_HAS_PS(_c)) {
+				/* ?=NULL will never be true; so we're
+				 * safely consuming a single argument */
+				ret = snprintf(_b + len, _l - len, "(%.*s%s OR ?=NULL)",
+						_k[i]->len, _k[i]->s, _o[i]);
+				if (ret < 0 || ret >= (_l - len)) goto error;
+				len += ret;
+			} else {
+				ret = snprintf(_b + len, _l - len, "%.*s%s",
+						_k[i]->len, _k[i]->s, _o[i]);
+				if (ret < 0 || ret >= (_l - len)) goto error;
+				len += ret;
+			}
 		} else {
 			ret = snprintf(_b + len, _l - len, "%.*s=", _k[i]->len, _k[i]->s);
 			if (ret < 0 || ret >= (_l - len)) goto error;
 			len += ret;
 		}
-		if (CON_HAS_PS(_c)) {
+		if (_o && (_o[i] == OP_IS_NULL || _o[i] == OP_IS_NOT_NULL)) {
+			;
+		} else if (CON_HAS_PS(_c)) {
 			*(_b+len++) = '?';
 		} else {
 			l = _l - len;