Source: conn_go18.go in package github.com/lib/pq

package pq

import (
	"context"
	"database/sql"
	"database/sql/driver"
	"fmt"
	"io"
	"io/ioutil"
	"time"
)

Implement the "QueryerContext" interface

func (cn *conn) QueryContext(ctx context.Context, query string, args []driver.NamedValue) (driver.Rows, error) {
	list := make([]driver.Value, len(args))
	for i, nv := range args {
		list[i] = nv.Value
	}
	finish := cn.watchCancel(ctx)
	r, err := cn.query(query, list)
	if err != nil {
		if finish != nil {
			finish()
		}
		return nil, err
	}
	r.finish = finish
	return r, nil
}

Implement the "ExecerContext" interface

func (cn *conn) ExecContext(ctx context.Context, query string, args []driver.NamedValue) (driver.Result, error) {
	list := make([]driver.Value, len(args))
	for i, nv := range args {
		list[i] = nv.Value
	}

	if finish := cn.watchCancel(ctx); finish != nil {
		defer finish()
	}

	return cn.Exec(query, list)
}

Implement the "ConnBeginTx" interface

func (cn *conn) BeginTx(ctx context.Context, opts driver.TxOptions) (driver.Tx, error) {
	var mode string

	switch sql.IsolationLevel(opts.Isolation) {

Don't touch mode: use the server's default

	case sql.LevelReadUncommitted:
		mode = " ISOLATION LEVEL READ UNCOMMITTED"
	case sql.LevelReadCommitted:
		mode = " ISOLATION LEVEL READ COMMITTED"
	case sql.LevelRepeatableRead:
		mode = " ISOLATION LEVEL REPEATABLE READ"
	case sql.LevelSerializable:
		mode = " ISOLATION LEVEL SERIALIZABLE"
	default:
		return nil, fmt.Errorf("pq: isolation level not supported: %d", opts.Isolation)
	}

	if opts.ReadOnly {
		mode += " READ ONLY"
	} else {
		mode += " READ WRITE"
	}

	tx, err := cn.begin(mode)
	if err != nil {
		return nil, err
	}
	cn.txnFinish = cn.watchCancel(ctx)
	return tx, nil
}

func (cn *conn) Ping(ctx context.Context) error {
	if finish := cn.watchCancel(ctx); finish != nil {
		defer finish()
	}
	rows, err := cn.simpleQuery(";")
	if err != nil {
		return driver.ErrBadConn // https://golang.org/pkg/database/sql/driver/#Pinger
	}
	rows.Close()
	return nil
}

func (cn *conn) watchCancel(ctx context.Context) func() {
	if done := ctx.Done(); done != nil {
		finished := make(chan struct{})
		go func() {
			select {

At this point the function level context is canceled, so it must not be used for the additional network request to cancel the query. Create a new context to pass into the dial.

				ctxCancel, cancel := context.WithTimeout(context.Background(), time.Second*10)
				defer cancel()

				_ = cn.cancel(ctxCancel)
				finished <- struct{}{}
			case <-finished:
			}
		}()
		return func() {
			select {
			case <-finished:
			case finished <- struct{}{}:
			}
		}
	}
	return nil
}

func (cn *conn) cancel(ctx context.Context) error {
	c, err := dial(ctx, cn.dialer, cn.opts)
	if err != nil {
		return err
	}
	defer c.Close()

	{
		can := conn{
			c: c,
		}
		err = can.ssl(cn.opts)
		if err != nil {
			return err
		}

		w := can.writeBuf(0)
		w.int32(80877102) // cancel request code
		w.int32(cn.processID)
		w.int32(cn.secretKey)

		if err := can.sendStartupPacket(w); err != nil {
			return err
		}
	}

Read until EOF to ensure that the server received the cancel.

	{
		_, err := io.Copy(ioutil.Discard, c)
		return err
	}