db, tx in ctx

main.go

package main

import (
	"context"
	"database/sql"
	"io"
	"log"
	"net/http"
)

type ctxKey int

const (
	_ ctxKey = iota
	ctxKeyQueryer
	ctxKeyDB
)

type queryer interface {
	QueryRow(string, ...interface{}) *sql.Row
}

type txController interface {
	WithTx(context.Context, func(context.Context) error) error
}

func withDB(ctx context.Context, db *sql.DB) context.Context {
	ctx = context.WithValue(ctx, ctxKeyDB, db)
	ctx = context.WithValue(ctx, ctxKeyQueryer, db)
	return ctx
}

type txCtrl struct{}

func (txCtrl) WithTx(ctx context.Context, f func(context.Context) error) error {
	db := getDB(ctx)

	// can add retry logic here

	tx, err := db.Begin()
	if err != nil {
		return err
	}
	defer tx.Rollback()

	ctx = context.WithValue(ctx, ctxKeyQueryer, tx)
	if err = f(ctx); err != nil {
		return err
	}
	return tx.Commit()
}

func getQueryer(ctx context.Context) queryer {
	return ctx.Value(ctxKeyQueryer).(queryer)
}

func getDB(ctx context.Context) *sql.DB {
	return ctx.Value(ctxKeyQueryer).(*sql.DB)
}

func queryUsername(ctx context.Context, userID string) (username string, err error) {
	q := getQueryer(ctx)

	err = q.QueryRow("select username from users where id = $1", userID).Scan(&username)
	return
}

type handlers struct {
	txCtrl txController
}

func (c *handlers) h1(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()

	queryUsername(ctx, "id1")

	io.WriteString(w, "ok")
}

func (c *handlers) h2(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()

	err := c.txCtrl.WithTx(ctx, func(ctx context.Context) error {
		queryUsername(ctx, "id1")
		queryUsername(ctx, "id2")
		return nil
	})
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
	io.WriteString(w, "ok")
}

func injectDB(db *sql.DB) func(h http.Handler) http.Handler {
	return func(h http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			ctx := r.Context()
			ctx = withDB(ctx, db)
			r = r.WithContext(ctx)
			h.ServeHTTP(w, r)
		})
	}
}

func main() {
	db, err := sql.Open("postgres", "postgresql://postgres@localhost?sslmode=disable")
	if err != nil {
		log.Fatal(err)
	}

	hs := handlers{txCtrl{}}

	mux := http.NewServeMux()
	mux.HandleFunc("/h1", hs.h1)
	mux.HandleFunc("/h2", hs.h2)

	h := injectDB(db)(mux)
	http.ListenAndServe(":8080", h)
}