LibSQL: Parse and execute sequential placeholder values

This partially implements SQLite's bind-parameter expression to support
indicating placeholder values in a SQL statement. For example:

    INSERT INTO table VALUES (42, ?);

In the above statement, the '?' identifier is a placeholder. This will
allow clients to compile statements a single time while running those
statements any number of times with different placeholder values.

Further, this will help mitigate SQL injection attacks.

Userland/Libraries/LibSQL/AST/AST.h

@@ -300,7 +300,8 @@ private:
 
 struct ExecutionContext {
     NonnullRefPtr<Database> database;
-    class Statement const* statement;
+    Statement const* statement { nullptr };
+    Span<Value const> placeholder_values {};
     Tuple* current_row { nullptr };
 };
 
@@ -361,6 +362,21 @@ public:
     virtual ResultOr<Value> evaluate(ExecutionContext&) const override;
 };
 
+class Placeholder : public Expression {
+public:
+    explicit Placeholder(size_t parameter_index)
+        : m_parameter_index(parameter_index)
+    {
+    }
+
+    size_t parameter_index() const { return m_parameter_index; }
+
+    virtual ResultOr<Value> evaluate(ExecutionContext&) const override;
+
+private:
+    size_t m_parameter_index { 0 };
+};
+
 class NestedExpression : public Expression {
 public:
     NonnullRefPtr<Expression> const& expression() const { return m_expression; }
@@ -729,7 +745,7 @@ private:
 
 class Statement : public ASTNode {
 public:
-    ResultOr<ResultSet> execute(AK::NonnullRefPtr<Database> database) const;
+    ResultOr<ResultSet> execute(AK::NonnullRefPtr<Database> database, Span<Value const> placeholder_values = {}) const;
 
     virtual ResultOr<ResultSet> execute(ExecutionContext&) const
     {

Userland/Libraries/LibSQL/AST/Expression.cpp

@@ -29,6 +29,13 @@ ResultOr<Value> NullLiteral::evaluate(ExecutionContext&) const
     return Value {};
 }
 
+ResultOr<Value> Placeholder::evaluate(ExecutionContext& context) const
+{
+    if (parameter_index() >= context.placeholder_values.size())
+        return Result { SQLCommand::Unknown, SQLErrorCode::InvalidNumberOfPlaceholderValues };
+    return context.placeholder_values[parameter_index()];
+}
+
 ResultOr<Value> NestedExpression::evaluate(ExecutionContext& context) const
 {
     return expression()->evaluate(context);

Userland/Libraries/LibSQL/AST/Parser.cpp

@@ -401,7 +401,6 @@ NonnullRefPtr<Expression> Parser::parse_expression()
     if (match_secondary_expression())
         expression = parse_secondary_expression(move(expression));
 
-    // FIXME: Parse 'bind-parameter'.
     // FIXME: Parse 'function-name'.
     // FIXME: Parse 'raise-function'.
 
@@ -414,6 +413,9 @@ NonnullRefPtr<Expression> Parser::parse_primary_expression()
     if (auto expression = parse_literal_value_expression())
         return expression.release_nonnull();
 
+    if (auto expression = parse_bind_parameter_expression())
+        return expression.release_nonnull();
+
     if (auto expression = parse_column_name_expression())
         return expression.release_nonnull();
 
@@ -528,6 +530,21 @@ RefPtr<Expression> Parser::parse_literal_value_expression()
     return {};
 }
 
+// https://sqlite.org/lang_expr.html#varparam
+RefPtr<Expression> Parser::parse_bind_parameter_expression()
+{
+    // FIXME: Support ?NNN, :AAAA, @AAAA, and $AAAA forms.
+    if (consume_if(TokenType::Placeholder)) {
+        auto parameter = m_parser_state.m_bound_parameters;
+        if (++m_parser_state.m_bound_parameters > Limits::maximum_bound_parameters)
+            syntax_error(DeprecatedString::formatted("Exceeded maximum number of bound parameters {}", Limits::maximum_bound_parameters));
+
+        return create_ast_node<Placeholder>(parameter);
+    }
+
+    return {};
+}
+
 RefPtr<Expression> Parser::parse_column_name_expression(DeprecatedString with_parsed_identifier, bool with_parsed_period)
 {
     if (with_parsed_identifier.is_null() && !match(TokenType::Identifier))

Userland/Libraries/LibSQL/AST/Parser.h

@@ -19,6 +19,7 @@ namespace Limits {
 // https://www.sqlite.org/limits.html
 constexpr size_t maximum_expression_tree_depth = 1000;
 constexpr size_t maximum_subquery_depth = 100;
+constexpr size_t maximum_bound_parameters = 1000;
 }
 
 class Parser {
@@ -52,6 +53,7 @@ private:
         Vector<Error> m_errors;
         size_t m_current_expression_depth { 0 };
         size_t m_current_subquery_depth { 0 };
+        size_t m_bound_parameters { 0 };
     };
 
     NonnullRefPtr<Statement> parse_statement();
@@ -71,6 +73,7 @@ private:
     NonnullRefPtr<Expression> parse_secondary_expression(NonnullRefPtr<Expression> primary);
     bool match_secondary_expression() const;
     RefPtr<Expression> parse_literal_value_expression();
+    RefPtr<Expression> parse_bind_parameter_expression();
     RefPtr<Expression> parse_column_name_expression(DeprecatedString with_parsed_identifier = {}, bool with_parsed_period = false);
     RefPtr<Expression> parse_unary_operator_expression();
     RefPtr<Expression> parse_binary_operator_expression(NonnullRefPtr<Expression> lhs);

Userland/Libraries/LibSQL/AST/Statement.cpp

@@ -11,9 +11,9 @@
 
 namespace SQL::AST {
 
-ResultOr<ResultSet> Statement::execute(AK::NonnullRefPtr<Database> database) const
+ResultOr<ResultSet> Statement::execute(AK::NonnullRefPtr<Database> database, Span<Value const> placeholder_values) const
 {
-    ExecutionContext context { move(database), this, nullptr };
+    ExecutionContext context { move(database), this, placeholder_values, nullptr };
     auto result = TRY(execute(context));
 
     // FIXME: When transactional sessions are supported, don't auto-commit modifications.

Userland/Libraries/LibSQL/AST/Token.h

@@ -171,6 +171,7 @@ namespace SQL::AST {
     __ENUMERATE_SQL_TOKEN("_blob_", BlobLiteral, Blob)                    \
     __ENUMERATE_SQL_TOKEN("_eof_", Eof, Invalid)                          \
     __ENUMERATE_SQL_TOKEN("_invalid_", Invalid, Invalid)                  \
+    __ENUMERATE_SQL_TOKEN("?", Placeholder, Operator)                     \
     __ENUMERATE_SQL_TOKEN("&", Ampersand, Operator)                       \
     __ENUMERATE_SQL_TOKEN("*", Asterisk, Operator)                        \
     __ENUMERATE_SQL_TOKEN(",", Comma, Punctuation)                        \