feat: add support for RLS in cloudsync_changes virtual table

if the cloudsync_col_value stmt returns 0 rows, it means that the value from the original table related to the current row in the augmented table is not found because of a RLS restriction, otherwise it always returns one value (maybe null)

Author: andinux

src/cloudsync.c

@@ -2247,7 +2247,7 @@ void cloudsync_col_value (sqlite3_context *context, int argc, sqlite3_value **ar
     rc = sqlite3_step(vm);
     if (rc == SQLITE_DONE) {
         rc = SQLITE_OK;
-        sqlite3_result_null(context);
+        sqlite3_result_text(context, CLOUDSYNC_RLS_RESTRICTED_VALUE, -1, SQLITE_STATIC);
     } else if (rc == SQLITE_ROW) {
         // store value result
         rc = SQLITE_OK;

src/cloudsync.h

@@ -18,6 +18,7 @@
 
 #define CLOUDSYNC_VERSION                       "0.7.1"
 #define CLOUDSYNC_TOMBSTONE_VALUE               "__[RIP]__"
+#define CLOUDSYNC_RLS_RESTRICTED_VALUE          "__[RLS]__"
 #define CLOUDSYNC_DISABLE_ROWIDONLY_TABLES      1
 
 typedef struct cloudsync_context cloudsync_context;

src/vtab.c

@@ -127,6 +127,8 @@ char *build_changes_sql (sqlite3 *db, const char *idxs) {
      * cloud sync tables, filtered and ordered based on the `db_version` and
      * `seq` fields.
      */
+    
+    //  tbl,pk,col_name,col_value,col_version,db_version,site_id,cl,seq
 
     const char *query =
     "WITH table_names AS ( "
@@ -148,7 +150,7 @@ char *build_changes_sql (sqlite3 *db, const char *idxs) {
     "        COALESCE(t2.col_version, 1) AS cl "
     "     FROM \"' || \"table_meta\" || '\" AS t1 "
     "     LEFT JOIN cloudsync_site_id AS site_tbl ON t1.site_id = site_tbl.rowid "
-    "     LEFT JOIN \"' || \"table_meta\" || '\" AS t2 ON t1.pk = t2.pk AND t2.col_name = ''" CLOUDSYNC_TOMBSTONE_VALUE "''' "
+    "     LEFT JOIN \"' || \"table_meta\" || '\" AS t2 ON t1.pk = t2.pk AND t2.col_name = ''" CLOUDSYNC_TOMBSTONE_VALUE "'' WHERE col_value != ''" CLOUDSYNC_RLS_RESTRICTED_VALUE "''' "
     "    AS query_string FROM table_names "
     "), "
     "union_query AS ( "