`dbname` connection parameter incorrectly sent as startup parameter in v1.11.0+

[yunliaw]

Description

We found this bug after attempting to upgrade lib/pq from v1.10.9 to v1.11.1. We got connections fail with:

pq: unrecognized configuration parameter "dbname" (42704)

This occurs because dbname is being included in the PostgreSQL startup message as a runtime parameter, rather than being used only client-side to populate the database field.

Reproduction

package main

import (
    "database/sql"
    "fmt"
    _ "github.com/lib/pq"
)

func main() {
    // Using "database=" in DSN
    db, err := sql.Open("postgres", "host=localhost user=postgres password=secret port=5432 database=mydb sslmode=disable")
    if err != nil {
        fmt.Println("Open error:", err)
        return
    }
    err = db.Ping()
    if err != nil {
        fmt.Println("Ping error:", err) // pq: unrecognized configuration parameter "dbname" (42704)
    }
}

Root Cause

From the preliminary investigation it looks like that the regression is introduced in #1240. Here's the break down:

v1.10.9

1. DSN parsed: o["database"] = "mydb"
2. startup() has special handling for dbname → sends correct database field to server

v1.11.1

1. DSN parsed: o["database"] = "mydb"
2. setFromTag() loop looks for o["dbname"] (based on struct tag) - not found
3. o["database"] remains in the map
4. After loop: database → dbname conversion creates o["dbname"] = "mydb"
5. o["dbname"] goes into cfg.Runtime (leftover keys)
6. startup() sends cfg.Runtime parameters to server
7. PostgreSQL rejects dbname as unknown startup parameter → error 42704

Fix

database -> dbname alias conversion should happen before the setFromTag

[arp242]

database=mydb should be dbname=mydb. Previously database= kind-of worked by accident, but this was undocumented, untested, and doesn't work with libpq or psql.

I guess I can add a compatibility thing, but everyone should really be using dbname=.

[yunliaw]

We can change from database to dbname. Nonetheless this is either a regression or a breaking change.

If we want to stop people from using database, it will be great that we can have a clear error message and announce it as a breaking change in the changelog.