Raw "SELECT ?" sql query results in a panic for the mysql backend

[weiznich]

Setup

Versions

• Rust: 1.71
• Diesel: 2.1.0
• Database: mysql
• Operating System: fedora

Feature Flags

• diesel: mysql

Problem Description

Raw select queries with binds as select clause fail at runtime with the following error:

thread 'multiconnection::nullable_type_checks' panicked at 'internal error: entered unreachable code: We ensure at the call side that we do not hit this type here. If you ever see this error, something has gone very wrong. Please open an issue at the diesel github repo in this case', diesel/src/mysql/connection/bind.rs:685:50
stack backtrace:
   0: rust_begin_unwind
             at /rustc/2c8cc343237b8f7d5a3c3703e3a87f2eb2c54a74/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/2c8cc343237b8f7d5a3c3703e3a87f2eb2c54a74/library/core/src/panicking.rs:64:14
   2: diesel::mysql::connection::bind::<impl core::convert::From<(mysqlclient_sys::enum_field_types,diesel::mysql::connection::bind::Flags)> for diesel::mysql::backend::MysqlType>::from
             at /home/weiznich/Documents/rust/diesel/diesel/src/mysql/connection/bind.rs:685:50
   3: <T as core::convert::Into<U>>::into
             at /rustc/2c8cc343237b8f7d5a3c3703e3a87f2eb2c54a74/library/core/src/convert/mod.rs:726:9
   4: diesel::mysql::connection::bind::BindData::value
             at /home/weiznich/Documents/rust/diesel/diesel/src/mysql/connection/bind.rs:410:23
   5: <diesel::mysql::connection::stmt::iterator::MysqlField as diesel::row::Field<diesel::mysql::backend::Mysql>>::value
             at /home/weiznich/Documents/rust/diesel/diesel/src/mysql/connection/stmt/iterator.rs:223:72

What are you trying to accomplish?

let mut conn = MysqlConnection::establish("…").unwrap();

#[derive(QueryableByName)]
struct Res {
    #[diesel(sql_type = sql_types::Integer)]
    bar: i32
}

diesel::sql_query("SELECT ? as bar").bind::<sql_types::Integer, i32>(1).get_result::<Res>(&mut conn);

What is the expected output?

The query runs successful

What is the actual output?

The panic message from above

Are you seeing any additional errors?

Steps to reproduce

let mut conn = MysqlConnection::establish("…").unwrap();

#[derive(QueryableByName)]
struct Res {
    #[diesel(sql_type = sql_types::Integer)]
    bar: i32
}

diesel::sql_query("SELECT ? as bar").bind::<sql_types::Integer, i32>(1).get_result::<Res>(&mut conn);

Preliminary debugging

This seems to happen because mysql returns MYSQL_TYPE_NULL in such cases and we don't have any type information on our own there. I've tried to just ignore the missing type information but we need the type information for automatically converting to the right numeric type here:

diesel/diesel/src/mysql/value.rs

Lines 60 to 84 in 02a37df

	pub(crate) fn numeric_value(&self) -> deserialize::Result<NumericRepresentation<'_>> {
	Ok(match self.tpe {
	MysqlType::UnsignedTiny | MysqlType::Tiny => {
	NumericRepresentation::Tiny(self.raw[0] as i8)
	}
	MysqlType::UnsignedShort | MysqlType::Short => {
	NumericRepresentation::Small(i16::from_ne_bytes((&self.raw[..2]).try_into()?))
	}
	MysqlType::UnsignedLong | MysqlType::Long => {
	NumericRepresentation::Medium(i32::from_ne_bytes((&self.raw[..4]).try_into()?))
	}
	MysqlType::UnsignedLongLong | MysqlType::LongLong => {
	NumericRepresentation::Big(i64::from_ne_bytes(self.raw.try_into()?))
	}
	MysqlType::Float => {
	NumericRepresentation::Float(f32::from_ne_bytes(self.raw.try_into()?))
	}
	MysqlType::Double => {
	NumericRepresentation::Double(f64::from_ne_bytes(self.raw.try_into()?))
	}
	MysqlType::Numeric => NumericRepresentation::Decimal(self.raw),
	_ => return Err(self.invalid_type_code("number")),
	})
	}

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.

• This issue can be reproduced on Rust's stable channel. (Your issue will be
  closed if this is not the case)
• This issue can be reproduced without requiring a third party crate

[pxp9]

it panic! because it hits this unreachable code

[weiznich]

@pxp9 Thanks for that note. I'm well aware of that fact as I'm already done some debugging there + wrote the corresponding panic! call on my own. This issue mostly exists to document that issue for now.