trying to implement polymorphism with go interfaces - go

I'm trying to create a layer on top of a third party library, in this case libchan. Here's an interface I've defined:
type ReceiverStream interface {
Receive(msg interface{}) error
}
type InboundTransport interface {
WaitReceiveChannel() (ReceiverStream, error)
}
The InboundTransport is meant to be a stand-in for a type Transport:
// libchan.go
type Transport interface {
// NewSendChannel creates and returns a new send channel. The receive
// end will get picked up on the remote end of the transport through
// the remote calling WaitReceiveChannel.
NewSendChannel() (Sender, error)
// WaitReceiveChannel waits for a new channel be created by the
// remote end of the transport calling NewSendChannel.
WaitReceiveChannel() (Receiver, error)
}
Just for context, this is the libchan.Receiver definition (please note that it matches my ReceiverStream:
// libchan.go
type Receiver interface {
// Receive receives a message sent across the channel from
// a sender on the other side of the underlying transport.
// Receive is expected to receive the same object that was
// sent by the Sender, any differences between the
// receive and send type should be handled carefully. It is
// up to the application to determine type compatibility, if
// the receive object is incompatible, Receiver will
// throw an error.
Receive(message interface{}) error
}
The Transport is returned by the libchan library here:
// libchan/session.go:62
func NewTransport(provider StreamProvider) libchan.Transport {
...
}
Since libchan.Transport and InboundTransport share a WaitReceiveChannel() (ReceiverStream, error) method, I figured I should be able to sub one for the other, like so:
func (ln SpdyListener) Accept(addr string) InboundTransport {
var listener net.Listener
var err error
listener, err = net.Listen("tcp", addr)
if err != nil {
log.Fatal(err)
}
c, err := listener.Accept()
if err != nil {
log.Fatal(err)
}
p, err := spdy.NewSpdyStreamProvider(c, true)
if err != nil {
log.Fatal(err)
}
return spdy.NewTransport(p)
}
But I get an error:
cannot use spdy.NewTransport(p) (type libchan.Transport) as type InboundTransport in return argument:
libchan.Transport does not implement InboundTransport (wrong type for WaitReceiveChannel method)
have WaitReceiveChannel() (libchan.Receiver, error)
want WaitReceiveChannel() (ReceiverStream, error)
I assume that what this error means is that a type of ReceiverStream does not match libchan.Receiver, but I thought that golang interfaces were implicit, meaning that as long as the return type implements the same methods as the expected interface, it would pass compilation. Is there anything I can change so that I can superimpose a self-defined interface onto one returned by a third part library?
TLDR: A third party lib is returning an object of interface Transport. The Transport interface specifies a method WaitReceiveChannel(). I have a self-defined interface InboundTransport that also specifies WaitReceiveChannel(). The third-party method I'm calling returns an object that implements Transport by way of method WaitReceiveChannel(). I assumed that it would also implement InboundTransport since the latter also specifies a WaitReceiveChannel() of the same type. This isn't working. Why not?

As you already know interfaces in Go are satisfied implicitly.
But, as the error states,
WaitReceiveChannel() (libchan.Receiver, error)
and
WaitReceiveChannel() (ReceiverStream, error)
are two different method types, resulting in libchan.Transport not implicitly implementing InboundTransport.
To work around this you have to write a thin wrapper around libchan.Transport that implements the InboundTransport properly.
type TransportWrapper struct {
t *libchan.Transport
}
func (w *TransportWrapper) WaitReceiveChannel() (Receiver, error) {
return w.t.WaitReceiveChannel()
}
// ...
func (ln SpdyListener) Accept(addr string) InboundTransport {
var listener net.Listener
var err error
listener, err = net.Listen("tcp", addr)
if err != nil {
log.Fatal(err)
}
c, err := listener.Accept()
if err != nil {
log.Fatal(err)
}
p, err := spdy.NewSpdyStreamProvider(c, true)
if err != nil {
log.Fatal(err)
}
return &TransportWrapper{spdy.NewTransport(p)}
}

Related

How to create a mock inside the function under test

I have a set of functions, which uses the pool of objects. This pool has been mocked. It works fine in most of the cases. But in some functions i call the methods of objects from the pool. So i need to mock this objects too.
Lets say:
// ObjectGeter is a interface that is mocked
type ObjectGeter interface {
GetObject(id int) ObjectType, error
}
// this function is under test
func SomeFunc(og ObjectGeter,id int, otherArgument SomeType) error {
// some actions with otherArgument
// and may be return an error
obj, err := og.GetObject(id)
if err !=nil {
return errors.New("GetObject error")
}
rezult, err := obj.SomeMethod()
if err !=nil {
return errors.New("One of internal errors")
}
return rezult, nil
}
Is there a way to test whole this function? I can create interface SomeMethoder which wraps the SomeMethod(), but i can't find the way how to assign it to obj inside SomeFunc without changing the signature of GetObject to GetObject(id int) SomeMethoder,error.
Currently i see the one approach - testing by a parts.
The only solution i'v found without of changing of paradigm is a wrapper. It is pretty trivial but may be some one will need it once.
Originally i have some type:
type PoolType struct {...}
func (p *PoolType)GetObject(id int) (ObjectType, error) {...}
and interface, that wraps PoolType.GetObject and that i'v mocked.
Now i have the interface:
type SomeMethoder interface {
SomeMethod() (ResultType, error)
}
to wrap object returned by PoolType.GetObject().
To produce it i have interface:
type ObjectGeter interface {
GetObject(id int) (SomeMethoder, error)
}
and type
type MyObjectGeter struct {
pool *PoolType
}
func New(pool *PoolType) *MyObjectGeter {
return &MyObjectGeter{pool: pool}
}
func (p *MyObjectGeter)GetObject(id int) (SomeMethoder, error) {
return p.pool.GetObject(id)
}
that implements it.
So:
// this function is under test
func SomeFunc(og ObjectGeter,id int, otherArgument SomeType) error {
// some actions with otherArgument
// and may be return an error
iface, err := og.GetObject(id)
if err !=nil {
return errors.New("GetObject error")
}
rezult, err := iface.SomeMethod()
if err !=nil {
return errors.New("One of internal errors")
}
return rezult, nil
}
is called by
og := New(pool)
SomeFunc(og,id,otherArgument)
in real work.
After all to test whole SomeFunc i have to:
func TestSomeFuncSuccess (t *testing.T) {
controller := gomock.NewController(t)
defer controller.Finish()
objectGeter := mocks.NewMockObjectGeter(controller)
someMethoder := mocks.NewMockSomeMethoder(controller)
gomock.InOrder(
args.objectGeter.EXPECT().
GetObject(correctIdCOnst).
Return(someMethoder, nil),
args.someMethoder.EXPECT().
SomeMethod().
Return(NewResultType(...),nil).
Times(args.test.times[1]),
)
result, err := SomeFunc(objectGeter,correctIdCOnst,otherArgumentConst)
// some checks
}
So, the only untested part is MyObjectGeter.GetObject that is enough for me.

Golang Cast interface to struct

Hi I'm trying to retrieve the function/method of one struct but I'm using an interface as parameter and using this interface I'm trying to access the function of the struct. To demonstrate what I want below is my code
// Here I'm trying to use "GetValue" a function of RedisConnection but since "c" is an interface it doesn't know that I'm trying to access the RedisConnection function. How Do I fix this?
func GetRedisValue(c Connection, key string) (string, error) {
value, err := c.GetValue(key)
return value, err
}
// Connection ...
type Connection interface {
GetClient() (*redis.Client, error)
}
// RedisConnection ...
type RedisConnection struct {}
// NewRedisConnection ...
func NewRedisConnection() Connection {
return RedisConnection{}
}
// GetClient ...
func (r RedisConnection) GetClient() (*redis.Client, error) {
redisHost := "localhost"
redisPort := "6379"
if os.Getenv("REDIS_HOST") != "" {
redisHost = os.Getenv("REDIS_HOST")
}
if os.Getenv("REDIS_PORT") != "" {
redisPort = os.Getenv("REDIS_PORT")
}
client := redis.NewClient(&redis.Options{
Addr: redisHost + ":" + redisPort,
Password: "", // no password set
DB: 0, // use default DB
})
return client, nil
}
// GetValue ...
func (r RedisConnection) GetValue(key string) (string, error) {
client, e := r.GetClient()
result, err := client.Ping().Result()
return result, nil
}
To answer the question directly, i.e., to cast an interface into a concrete type, you do:
v = i.(T)
where i is the interface and T is the concrete type. It will panic if the underlying type is not T. To have a safe cast, you use:
v, ok = i.(T)
and if the underlying type is not T, ok is set to false, otherwise true. Note that T can also be an interface type and if it is, the code cast i into a new interface instead of a concrete type.
And please be noted, casting an interface is likely a symbol of bad design. As in your code, you should ask yourself, does your custom interface Connection solely requires GetClient or does it always requires a GetValue? Does your GetRedisValue function requires a Connection or does it always wants a concrete struct?
Change your code accordingly.
Your Connection interface:
type Connection interface {
GetClient() (*redis.Client, error)
}
only says that there is a GetClient method, it says nothing about supporting GetValue.
If you want to call GetValue on a Connection like this:
func GetRedisValue(c Connection, key string) (string, error) {
value, err := c.GetValue(key)
return value, err
}
then you should include GetValue in the interface:
type Connection interface {
GetClient() (*redis.Client, error)
GetValue(string) (string, error) // <-------------------
}
Now you're saying that all Connections will support the GetValue method that you want to use.

Wrapping a db object in Go and running two methods in the same transaction

In the effort of learning Go a bit better, I am trying to refactor a series of functions which accept a DB connection as the first argument into struct methods and something a bit more "idiomatically" Go.
Right now my "data store" methods are something like this:
func CreateA(db orm.DB, a *A) error {
db.Exec("INSERT...")
}
func CreateB(db orm.DB, b *B) error {
db.Exec("INSERT...")
}
These the functions work perfectly fine. orm.DB is the DB interface of go-pg.
Since the two functions accept a db connection I can either pass an actual connection or a transaction (which implements the same interface). I can be sure that both functions issuing SQL INSERTs run in the same transaction, avoiding having inconsistent state in the DB in case either one of them fails.
The trouble started when I decided to read more about how to structure the code a little better and to make it "mockable" in case I need to.
So I googled a bit, read the article Practical Persistence in Go: Organising Database Access and tried to refactor the code to use proper interfaces.
The result is something like this:
type Store {
CreateA(a *A) error
CreateB(a *A) error
}
type DB struct {
orm.DB
}
func NewDBConnection(p *ConnParams) (*DB, error) {
.... create db connection ...
return &DB{db}, nil
}
func (db *DB) CreateA(a *A) error {
...
}
func (db *DB) CreateB(b *B) error {
...
}
which allows me to write code like:
db := NewDBConnection()
DB.CreateA(a)
DB.CreateB(b)
instead of:
db := NewDBConnection()
CreateA(db, a)
CreateB(db, b)
The actual issue is that I lost the ability to run the two functions in the same transaction. Before I could do:
pgDB := DB.DB.(*pg.DB) // convert the interface to an actual connection
pgDB.RunInTransaction(func(tx *pg.Tx) error {
CreateA(tx, a)
CreateB(tx, b)
})
or something like:
tx := db.DB.Begin()
err = CreateA(tx, a)
err = CreateB(tx, b)
if err != nil {
tx.Rollback()
} else {
tx.Commit()
}
which is more or less the same thing.
Since the functions were accepting the common interface between a connection and a transaction I could abstract from my model layer the transaction logic sending down either a full connection or a transaction. This allowed me to decide in the "HTTP handler" when to create a trasaction and when I didn't need to.
Keep in mind that the connection is a global object representing a pool of connections handled automatically by go, so the hack I tried:
pgDB := DB.DB.(*pg.DB) // convert the interface to an actual connection
err = pgDB.RunInTransaction(func(tx *pg.Tx) error {
DB.DB = tx // replace the connection with a transaction
DB.CreateA(a)
DB.CreateB(a)
})
it's clearly a bad idea, because although it works, it works only once because we replace the global connection with a transaction. The following request breaks the server.
Any ideas? I can't find information about this around, probably because I don't know the right keywords being a noob.
I've done something like this in the past (using the standard sql package, you may need to adapt it to your needs):
var ErrNestedTransaction = errors.New("nested transactions are not supported")
// abstraction over sql.TX and sql.DB
// a similar interface seems to be already defined in go-pg. So you may not need this.
type executor interface {
Exec(query string, args ...interface{}) (sql.Result, error)
Query(query string, args ...interface{}) (*sql.Rows, error)
QueryRow(query string, args ...interface{}) *sql.Row
}
type Store struct {
// this is the actual connection(pool) to the db which has the Begin() method
db *sql.DB
executor executor
}
func NewStore(dsn string) (*Store, error) {
db, err := sql.Open("sqlite3", dsn)
if err != nil {
return nil, err
}
// the initial store contains just the connection(pool)
return &Store{db, db}, nil
}
func (s *Store) RunInTransaction(f func(store *Store) error) error {
if _, ok := s.executor.(*sql.Tx); ok {
// nested transactions are not supported!
return ErrNestedTransaction
}
tx, err := s.db.Begin()
if err != nil {
return err
}
transactedStore := &Store{
s.db,
tx,
}
err = f(transactedStore)
if err != nil {
tx.Rollback()
return err
}
return tx.Commit()
}
func (s *Store) CreateA(thing A) error {
// your implementation
_, err := s.executor.Exec("INSERT INTO ...", ...)
return err
}
And then you use it like
// store is a global object
store.RunInTransaction(func(store *Store) error {
// this instance of Store uses a transaction to execute the methods
err := store.CreateA(a)
if err != nil {
return err
}
return store.CreateB(b)
})
The trick is to use the executor instead of the *sql.DB in your CreateX methods, which allows you to dynamically change the underlying implementation (tx vs. db). However, since there is very little information out there on how to deal with this issue, I can't assure you that this is the "best" solution. Other suggestions are welcome!

Function that accepts generic struct

Is it possible to have my function definition below accept any type of struct?
I've tried to refactor like so:
// This method should accept any type of struct
// Once I receive my response from the database,
// I scan the rows to create a slice of type struct.
func generateResponse(rows *sqlx.Rows, structSlice []struct{}, structBody struct{}) ([]struct{}, error) {
for rows.Next() {
err := rows.StructScan(&structBody)
if err != nil {
return nil, err
}
structSlice = append(structSlice, structBody)
}
err := rows.Err()
if err != nil {
return nil, err
}
return structSlice, nil
}
Assume my struct is of type OrderRevenue.
When I call the function above:
structSlice, err := generateResponse(rows, []OrderRevenue{}, OrderRevenue{})
The error I get is:
cannot use []OrderRevenue literal as type []struct{} in argument...
Am I going about this the wrong way?
This is considered the cornerstone (or more of a limitation) of Go's type system. struct{} is an unnamed type that is different from struct{ field1 int } and of course is not the same as OrderRevenue{}.
Go emphasizes abstraction through interfaces, and perhaps you should try that. Here is the first take:
type OrderRevenue interface {
MarshalMyself() ([]byte, error)
}
type Anonymous struct {}
func (a Anonymous) MarshalMyself() ([]byte, error) {
// implementation's up to you
return []byte{}, nil
}
// the function signature
generateResponse(rows *sqlx.Rows, structSlice []OrderRevenue, structBody Body) ([]Body, error) {
// ...
}
In this case you can also use empty interface interface{}, which all types implement, but you'll have to recursively go through the structure to do manual type assertion. The best approach in Go is to know the shape of your data in advance, at least partially.

Extend struct in Go

Suppose object A has a field of type net.Dialer. I'd like to provide object A with a custom implementation of net.Dialer that augments the Dial method. Is this doable in Go? I'm trying to use embedded fields like so:
package main
import (
"net"
"fmt"
)
type dialerConsumer struct {
dialer net.Dialer
}
func (dc *dialerConsumer) use() error {
conn, e := dc.dialer.Dial("tcp", "golang.org:http")
if e != nil {
return e
}
fmt.Printf("conn: %s\n", conn)
return nil
}
type customDialer struct {
net.Dialer
}
func main() {
standardDialer := net.Dialer{}
consumer := &dialerConsumer{
dialer: standardDialer,
}
consumer.use()
/*
customDialer := customDialer{
net.Dialer{},
}
consumer = &dialerConsumer{
dialer: customDialer,
}
consumer.use()
*/
}
However, when I uncomment the commented-out code in main, I get the following compilation error:
src/test.go:38: cannot use customDialer (type customDialer) as type net.Dialer in field value
You're getting the error because customDialer and net.Dialer are two different types and cannot be used interchangeably. Embedding in Go is not the same as class inheritance in other OO langauges so it won't help you with what you trying to do.
What you can do instead in this case is to use Go interfaces which give you something like polymorphism/duck-typing, and since interfaces in Go are satified implicitly you can define a new interface that an existing type will implement by virtue of having a method with the same signature as the newly defined interface.
// already implemented by net.Dialer
type Dialer interface {
Dial(network, address string) (net.Conn, error)
}
type customDialer struct {
*net.Dialer
}
func (cd *customDialer) Dial(network, address string) (net.Conn, error) {
conn, err := cd.Dialer.Dial(network, address)
if err != nil {
return nil, err
}
fmt.Printf("conn: %s\n", conn)
return conn, nil
}
// now the dialer field can be set to *customDialer and net.Dialer as well
type dialerConsumer struct {
dialer Dialer
}
https://play.golang.org/p/i3Vpsh3wii

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