Add new table function viewIfPermitted().

programs/client/Client.cpp

@@ -106,7 +106,9 @@ void Client::processError(const String & query) const
 std::vector<String> Client::loadWarningMessages()
 {
     std::vector<String> messages;
-    connection->sendQuery(connection_parameters.timeouts, "SELECT message FROM system.warnings", "" /* query_id */,
+    connection->sendQuery(connection_parameters.timeouts,
+                          "SELECT * FROM viewIfPermitted(SELECT message FROM system.warnings ELSE null('message String'))",
+                          "" /* query_id */,
                           QueryProcessingStage::Complete,
                           &global_context->getSettingsRef(),
                           &global_context->getClientInfo(), false, {});

src/Client/Suggest.cpp

@@ -50,52 +50,58 @@ static String getLoadSuggestionQuery(Int32 suggestion_limit, bool basic_suggesti
 {
     /// NOTE: Once you will update the completion list,
     /// do not forget to update 01676_clickhouse_client_autocomplete.sh
-    WriteBufferFromOwnString query;
-    query << "SELECT DISTINCT arrayJoin(extractAll(name, '[\\\\w_]{2,}')) AS res FROM ("
-        "SELECT name FROM system.functions"
-        " UNION ALL "
-        "SELECT name FROM system.table_engines"
-        " UNION ALL "
-        "SELECT name FROM system.formats"
-        " UNION ALL "
-        "SELECT name FROM system.table_functions"
-        " UNION ALL "
-        "SELECT name FROM system.data_type_families"
-        " UNION ALL "
-        "SELECT name FROM system.merge_tree_settings"
-        " UNION ALL "
-        "SELECT name FROM system.settings"
-        " UNION ALL ";
+    String query;
+
+    auto add_subquery = [&](std::string_view select, std::string_view result_column_name)
+    {
+        if (!query.empty())
+            query += " UNION ALL ";
+        query += fmt::format("SELECT * FROM viewIfPermitted({} ELSE null('{} String'))", select, result_column_name);
+    };
+
+    auto add_column = [&](std::string_view column_name, std::string_view table_name, bool distinct, std::optional<Int64> limit)
+    {
+        add_subquery(
+            fmt::format(
+                "SELECT {}{} FROM system.{}{}",
+                (distinct ? "DISTINCT " : ""),
+                column_name,
+                table_name,
+                (limit ? (" LIMIT " + std::to_string(*limit)) : "")),
+            column_name);
+    };
+
+    add_column("name", "functions", false, {});
+    add_column("name", "table_engines", false, {});
+    add_column("name", "formats", false, {});
+    add_column("name", "table_functions", false, {});
+    add_column("name", "data_type_families", false, {});
+    add_column("name", "merge_tree_settings", false, {});
+    add_column("name", "settings", false, {});
+
     if (!basic_suggestion)
     {
-        query << "SELECT cluster FROM system.clusters"
-                 " UNION ALL "
-                 "SELECT macro FROM system.macros"
-                 " UNION ALL "
-                 "SELECT policy_name FROM system.storage_policies"
-                 " UNION ALL ";
+        add_column("cluster", "clusters", false, {});
+        add_column("macro", "macros", false, {});
+        add_column("policy_name", "storage_policies", false, {});
     }
-    query << "SELECT concat(func.name, comb.name) FROM system.functions AS func CROSS JOIN system.aggregate_function_combinators AS comb WHERE is_aggregate";
+
+    add_subquery("SELECT concat(func.name, comb.name) AS x FROM system.functions AS func CROSS JOIN system.aggregate_function_combinators AS comb WHERE is_aggregate", "x");
+    
     /// The user may disable loading of databases, tables, columns by setting suggestion_limit to zero.
     if (suggestion_limit > 0)
     {
-        String limit_str = toString(suggestion_limit);
-        query << " UNION ALL "
-                 "SELECT name FROM system.databases LIMIT " << limit_str
-              << " UNION ALL "
-                 "SELECT DISTINCT name FROM system.tables LIMIT " << limit_str
-              << " UNION ALL ";
-
+        add_column("name", "databases", false, suggestion_limit);
+        add_column("name", "tables", true, suggestion_limit);
         if (!basic_suggestion)
         {
-            query << "SELECT DISTINCT name FROM system.dictionaries LIMIT " << limit_str
-                  << " UNION ALL ";
+            add_column("name", "dictionaries", true, suggestion_limit);
         }
-        query << "SELECT DISTINCT name FROM system.columns LIMIT " << limit_str;
+        add_column("name", "columns", true, suggestion_limit);
     }
-    query << ") WHERE notEmpty(res)";
 
-    return query.str();
+    query = "SELECT DISTINCT arrayJoin(extractAll(name, '[\\\\w_]{2,}')) AS res FROM (" + query + ") WHERE notEmpty(res)";
+    return query;
 }
 
 template <typename ConnectionType>

src/Parsers/ASTFunction.cpp

@@ -509,6 +509,25 @@ void ASTFunction::formatImplWithoutAlias(const FormatSettings & settings, Format
                     settings.ostr << ')';
                 written = true;
             }
+
+            if (!written && name == "viewIfPermitted"sv)
+            {
+                /// viewIfPermitted() needs special formatting: ELSE instead of comma between arguments, and better indents too.
+                const auto * nl_or_nothing = settings.one_line ? "" : "\n";
+                auto indent0 = settings.one_line ? "" : String(4u * frame.indent, ' ');
+                auto indent1 = settings.one_line ? "" : String(4u * (frame.indent + 1), ' ');
+                auto indent2 = settings.one_line ? "" : String(4u * (frame.indent + 2), ' ');
+                settings.ostr << (settings.hilite ? hilite_function : "") << name << "(" << (settings.hilite ? hilite_none : "") << nl_or_nothing;
+                FormatStateStacked frame_nested = frame;
+                frame_nested.need_parens = false;
+                frame_nested.indent += 2;
+                arguments->children[0]->formatImpl(settings, state, frame_nested);
+                settings.ostr << nl_or_nothing << indent1 << (settings.hilite ? hilite_keyword : "") << (settings.one_line ? " " : "")
+                              << "ELSE " << (settings.hilite ? hilite_none : "") << nl_or_nothing << indent2;
+                arguments->children[1]->formatImpl(settings, state, frame_nested);
+                settings.ostr << nl_or_nothing << indent0 << ")";
+                return;
+            }
         }
 
         if (!written && arguments->children.size() >= 2)

src/Parsers/ExpressionElementParsers.cpp

@@ -1068,13 +1068,16 @@ bool ParserFunction::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
 bool ParserTableFunctionView::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
 {
     ParserIdentifier id_parser;
-    ParserKeyword view("VIEW");
     ParserSelectWithUnionQuery select;
 
     ASTPtr identifier;
     ASTPtr query;
 
-    if (!view.ignore(pos, expected))
+    bool if_permitted = false;
+
+    if (ParserKeyword{"VIEWIFPERMITTED"}.ignore(pos, expected))
+        if_permitted = true;
+    else if (!ParserKeyword{"VIEW"}.ignore(pos, expected))
         return false;
 
     if (pos->type != TokenType::OpeningRoundBracket)
@@ -1094,15 +1097,30 @@ bool ParserTableFunctionView::parseImpl(Pos & pos, ASTPtr & node, Expected & exp
         return false;
     }
 
+    ASTPtr else_ast;
+    if (if_permitted)
+    {
+        if (!ParserKeyword{"ELSE"}.ignore(pos, expected))
+            return false;
+
+        if (!ParserWithOptionalAlias{std::make_unique<ParserFunction>(true, true), true}.parse(pos, else_ast, expected))
+            return false;
+    }
+
     if (pos->type != TokenType::ClosingRoundBracket)
         return false;
+
     ++pos;
+
+    auto expr_list = std::make_shared<ASTExpressionList>();
+    expr_list->children.push_back(query);
+    if (if_permitted)
+        expr_list->children.push_back(else_ast);
+
     auto function_node = std::make_shared<ASTFunction>();
     tryGetIdentifierNameInto(identifier, function_node->name);
-    auto expr_list_with_single_query = std::make_shared<ASTExpressionList>();
-    expr_list_with_single_query->children.push_back(query);
-    function_node->name = "view";
-    function_node->arguments = expr_list_with_single_query;
+    function_node->name = if_permitted ? "viewIfPermitted" : "view";
+    function_node->arguments = expr_list;
     function_node->children.push_back(function_node->arguments);
     node = function_node;
     return true;
@@ -1971,6 +1989,7 @@ const char * ParserAlias::restricted_keywords[] =
     "WITH",
     "INTERSECT",
     "EXCEPT",
+    "ELSE",
     nullptr
 };
 

src/Parsers/ExpressionElementParsers.h

@@ -162,7 +162,7 @@ protected:
     bool is_table_function;
 };
 
-// A special function parser for view table function.
+// A special function parser for view and viewIfPermitted table functions.
 // It parses an SELECT query as its argument and doesn't support getColumnName().
 class ParserTableFunctionView : public IParserBase
 {

src/Storages/StorageView.cpp

@@ -180,9 +180,13 @@ void StorageView::replaceWithSubquery(ASTSelectQuery & outer_query, ASTPtr view_
     if (!table_expression->database_and_table_name)
     {
         // If it's a view table function, add a fake db.table name.
-        if (table_expression->table_function && table_expression->table_function->as<ASTFunction>()->name == "view")
-            table_expression->database_and_table_name = std::make_shared<ASTTableIdentifier>("__view");
-        else
+        if (table_expression->table_function)
+        {
+            auto table_function_name = table_expression->table_function->as<ASTFunction>()->name;
+            if ((table_function_name == "view") || (table_function_name == "viewIfPermitted"))
+                table_expression->database_and_table_name = std::make_shared<ASTTableIdentifier>("__view");
+        }
+        if (!table_expression->database_and_table_name)
             throw Exception("Logical error: incorrect table expression", ErrorCodes::LOGICAL_ERROR);
     }
 

src/TableFunctions/TableFunctionViewIfPermitted.cpp

@@ -0,0 +1,113 @@
+#include <Interpreters/InterpreterSelectWithUnionQuery.h>
+#include <Parsers/ASTFunction.h>
+#include <Parsers/ASTLiteral.h>
+#include <Parsers/ASTSelectWithUnionQuery.h>
+#include <Storages/StorageNull.h>
+#include <Storages/StorageView.h>
+#include <Storages/checkAndGetLiteralArgument.h>
+#include <TableFunctions/ITableFunction.h>
+#include <TableFunctions/TableFunctionFactory.h>
+#include <TableFunctions/TableFunctionViewIfPermitted.h>
+#include <TableFunctions/parseColumnsListForTableFunction.h>
+#include <Interpreters/evaluateConstantExpression.h>
+#include "registerTableFunctions.h"
+
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int NUMBER_OF_ARGUMENTS_DOESNT_MATCH;
+    extern const int BAD_ARGUMENTS;
+    extern const int ACCESS_DENIED;
+}
+
+
+const ASTSelectWithUnionQuery & TableFunctionViewIfPermitted::getSelectQuery() const
+{
+    return *create.select;
+}
+
+void TableFunctionViewIfPermitted::parseArguments(const ASTPtr & ast_function, ContextPtr context)
+{
+    const auto * function = ast_function->as<ASTFunction>();
+    if (!function || !function->arguments || (function->arguments->children.size() != 2))
+        throw Exception(
+            ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH,
+            "Table function '{}' requires two arguments: a SELECT query and a table function",
+            getName());
+
+    const auto & arguments = function->arguments->children;
+    auto * select = arguments[0]->as<ASTSelectWithUnionQuery>();
+    if (!select)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Table function '{}' requires a SELECT query as its first argument", getName());
+    create.set(create.select, select->clone());
+
+    else_ast = arguments[1];
+    if (!else_ast->as<ASTFunction>())
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Table function '{}' requires a table function as its second argument", getName());
+    else_table_function = TableFunctionFactory::instance().get(else_ast, context);
+}
+
+ColumnsDescription TableFunctionViewIfPermitted::getActualTableStructure(ContextPtr context) const
+{
+    return else_table_function->getActualTableStructure(context);
+}
+
+StoragePtr TableFunctionViewIfPermitted::executeImpl(
+    const ASTPtr & /* ast_function */, ContextPtr context, const std::string & table_name, ColumnsDescription /* cached_columns */) const
+{
+    StoragePtr storage;
+    auto columns = getActualTableStructure(context);
+
+    if (isPermitted(context, columns))
+    {
+        storage = std::make_shared<StorageView>(StorageID(getDatabaseName(), table_name), create, columns, "");
+    }
+    else
+    {
+        storage = else_table_function->execute(else_ast, context, table_name);
+    }
+
+    storage->startup();
+    return storage;
+}
+
+bool TableFunctionViewIfPermitted::isPermitted(const ContextPtr & context, const ColumnsDescription & else_columns) const
+{
+    Block sample_block;
+
+    try
+    {
+        /// Will throw ACCESS_DENIED if the current user is not allowed to execute the SELECT query.
+        sample_block = InterpreterSelectWithUnionQuery::getSampleBlock(create.children[0], context);
+    }
+    catch (Exception & e)
+    {
+        if (e.code() == ErrorCodes::ACCESS_DENIED)
+            return {};
+        throw;
+    }
+
+    /// We check that columns match only if permitted (otherwise we could reveal the structure to an user who must not know it).
+    ColumnsDescription columns{sample_block.getNamesAndTypesList()};
+    if (columns != else_columns)
+    {
+        throw Exception(
+            ErrorCodes::BAD_ARGUMENTS,
+            "Table function '{}' requires a SELECT query with the result columns matching a table function after 'ELSE'. "
+            "Currently the result columns of the SELECT query are {}, and the table function after 'ELSE' gives {}",
+            getName(),
+            columns.toString(),
+            else_columns.toString());
+    }
+
+    return true;
+}
+
+void registerTableFunctionViewIfPermitted(TableFunctionFactory & factory)
+{
+    factory.registerFunction<TableFunctionViewIfPermitted>();
+}
+
+}

src/TableFunctions/TableFunctionViewIfPermitted.h

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <TableFunctions/ITableFunction.h>
+#include <Parsers/ASTCreateQuery.h>
+#include <base/types.h>
+
+namespace DB
+{
+
+/* viewIfPermitted(query ELSE null('structure'))
+ * Works as "view(query)" if the current user has the permissions required to execute "query"; works as "null('structure')" otherwise.
+ */
+class TableFunctionViewIfPermitted : public ITableFunction
+{
+public:
+    static constexpr auto name = "viewIfPermitted";
+    std::string getName() const override { return name; }
+
+    const ASTSelectWithUnionQuery & getSelectQuery() const;
+
+private:
+    StoragePtr executeImpl(const ASTPtr & ast_function, ContextPtr context, const String & table_name, ColumnsDescription cached_columns) const override;
+    const char * getStorageTypeName() const override { return "ViewIfPermitted"; }
+
+    void parseArguments(const ASTPtr & ast_function, ContextPtr context) override;
+    ColumnsDescription getActualTableStructure(ContextPtr context) const override;
+
+    bool isPermitted(const ContextPtr & context, const ColumnsDescription & else_columns) const;
+
+    ASTCreateQuery create;
+    ASTPtr else_ast;
+    TableFunctionPtr else_table_function;
+};
+
+}

src/TableFunctions/registerTableFunctions.cpp

@@ -42,6 +42,7 @@ void registerTableFunctions()
     registerTableFunctionJDBC(factory);
 
     registerTableFunctionView(factory);
+    registerTableFunctionViewIfPermitted(factory);
 
 #if USE_MYSQL
     registerTableFunctionMySQL(factory);

src/TableFunctions/registerTableFunctions.h

@@ -40,6 +40,7 @@ void registerTableFunctionODBC(TableFunctionFactory & factory);
 void registerTableFunctionJDBC(TableFunctionFactory & factory);
 
 void registerTableFunctionView(TableFunctionFactory & factory);
+void registerTableFunctionViewIfPermitted(TableFunctionFactory & factory);
 
 #if USE_MYSQL
 void registerTableFunctionMySQL(TableFunctionFactory & factory);

tests/integration/test_table_functions_access_rights/test.py

@@ -65,3 +65,38 @@ def test_merge():
         "it's necessary to have grant SELECT ON default.table2"
         in instance.query_and_get_error(select_query, user="A")
     )
+
+
+def test_view_if_permitted():
+    assert (
+        instance.query(
+            "SELECT * FROM viewIfPermitted(SELECT * FROM table1 ELSE null('x UInt32'))"
+        )
+        == "1\n"
+    )
+
+    expected_error = "requires a SELECT query with the result columns matching a table function after 'ELSE'"
+    assert expected_error in instance.query_and_get_error(
+        "SELECT * FROM viewIfPermitted(SELECT * FROM table1 ELSE null('x Int32'))"
+    )
+    assert expected_error in instance.query_and_get_error(
+        "SELECT * FROM viewIfPermitted(SELECT * FROM table1 ELSE null('y UInt32'))"
+    )
+
+    instance.query("CREATE USER A")
+    assert (
+        instance.query(
+            "SELECT * FROM viewIfPermitted(SELECT * FROM table1 ELSE null('x UInt32'))",
+            user="A",
+        )
+        == ""
+    )
+
+    instance.query("GRANT SELECT ON table1 TO A")
+    assert (
+        instance.query(
+            "SELECT * FROM viewIfPermitted(SELECT * FROM table1 ELSE null('x UInt32'))",
+            user="A",
+        )
+        == "1\n"
+    )