Valuer interface is not respected for nil values

[SajanAlexander-okta]

Describe the bug
When using a struct that implements driver.Valuer but is backed by a nilable type, Value is never called when the value is nil. This is used by some types where a default should be stored within the DB. The default converter for database/sql will call this on a nil.

I'm using the stdlib compatibility layer. When tracing through the code, the database/sql code will call the CheckNamedValue method on *stdlib.Conn, which will then just pass through the values directly.

Later on, when the prepared statement is being built, anynil.NormalizeSlice is called on the arguments, which then sets the value of the arg as nil and erases any type information about the arg.

The nil value hits a nil check, preventing any Encode plan from being executed to convert the value / call Value on it.

To Reproduce
See code below. The first insert will result in a NULL value to be inserted, while the second one will insert an empty object of {}.

package main

import (
	"database/sql"
	"database/sql/driver"
	"encoding/json"
	"fmt"
	"os"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/stdlib"
)

// JSONText code was copied from github.com/jmoiron/sqlx types.JSONText so there's not another import required outside of pgx and the Go stdlib
type JSONText json.RawMessage

var emptyJSON = JSONText("{}")

func (j JSONText) Value() (driver.Value, error) {
	var m json.RawMessage
	var err = j.Unmarshal(&m)
	if err != nil {
		return []byte{}, err
	}
	return []byte(j), nil
}

func (j *JSONText) Unmarshal(v interface{}) error {
	if len(*j) == 0 {
		*j = emptyJSON
	}
	return json.Unmarshal([]byte(*j), v)
}

func main() {
	connConfig, err := pgx.ParseConfig(os.Getenv("PGX_TEST_DATABASE"))
	if err != nil {
		fmt.Println("error parsing config: ", err)
		return
	}

	dn := stdlib.RegisterConnConfig(connConfig)

	db, err := sql.Open("pgx", dn)
	if err != nil {
		fmt.Println("error opening database: ", err)
		return
	}

	_, err = db.Exec("CREATE TABLE IF NOT EXISTS pgx_nil_test_table (data jsonb)")
	if err != nil {
		fmt.Println("error creating table: ", err)
	}

	var data JSONText

	_, err = db.Exec("INSERT INTO pgx_nil_test_table (data) VALUES ($1)", data)
	if err != nil {
		fmt.Println("error inserting data: ", err)
	}

	data = JSONText(make([]byte, 0))
	_, err = db.Exec("INSERT INTO pgx_nil_test_table (data) VALUES ($1)", data)
	if err != nil {
		fmt.Println("error inserting data: ", err)
	}
}

Please run your example with the race detector enabled. For example, go run -race main.go or go test -race.

Expected behavior
The Value method is called on a nil type that implements driver.Valuer to allow for any default value

Actual behavior
The Value method is not called / the value is not converted

Version

• Go: $ go version -> go1.20.4 darwin/arm64
• PostgreSQL: $ psql --no-psqlrc --tuples-only -c 'select version()' -> [e.g. PostgreSQL 14.4 on x86_64-apple-darwin21.5.0, compiled by Apple clang version 13.1.6 (clang-1316.0.21.2.5), 64-bit]
• pgx: $ grep 'github.com/jackc/pgx/v[0-9]' go.mod -> v5.5.1

Additional context
Add any other context about the problem here.

[jackc]

Duplicate of #1566.

This was changed in v5. There's a few possible changes that would bring back that behavior from v4, but progress seems stalled at the moment.