Advanced Testing Techniques

Alan Braithwaite

@caust1c

https://abraithwaite.net

Segment Inc

Why test at all?

@caust1c

Roadmap

Structuring Code for Testing
Writing effective test suites
The last HTTP test you'll ever write

@caust1c

Not Covering

go vet, golint, etc
test coverage
race detection
testing frameworks

@caust1c

Testable Code

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Dependency Injection
Channels and Tests
Interfaces and Tests

Testable Code

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Dependency Injection

A software design pattern whereby dependencies are passed as an argument to a constructor.

Testable Code

@caust1c

func NewService(db *sql.DB) *Service {
    // ...
}

func NewService() *Service {
    db := sql.Open("postgres", "postgres://user:pass@localhost/myapp")
}

func NewService(DSN string) *Service {
    db := sql.Open("postgres", DSN)
    // ...
}

type Store interface {
    //...
}

func NewService(store Store) *Service {
    // ...
}

Testable Code

@caust1c

// expiration.go

var now = time.Now

func calcExpiry(d time.Duration) time.Time {
    return now() + d
}

// expiration_test.go

func TestExpiration(t *testing.T) {
    oldnow := now
    defer func() { now = oldnow }()
    now = func() time.Time {
        return time.Date(1985, 10, 26, 1, 22, 0, 0, time.UTC)
    }
    // ... run test
}

Channels and Tests

@caust1c

What sends data on this channel?
Who owns the channel?
What is the lifecycle of this channel?
How do we handle errors?
Is the channel buffered or unbuffered?

Channels and Tests

@caust1c

package queuelib

type Reader struct {...}
func (r *Reader) ReadChan() <-chan Msg {...}

package consumer_test

func TestReader(t *testing.T) {
    r := queuelib.NewReader()
    
    for _, v := range r.ReadChan() {
        // test v
    }
}

Channels and Tests

@caust1c

package consumer_test

func TestReader(t *testing.T) {
    r := queuelib.NewReader()
    // timeouts?
  
    for {
        select {
        case r := <-r.ReadChan():
            // test thing
        case e := <-r.ErrChan():
            // what triggered this?  How do we debug?
        }
    }
}

Channels and Tests

@caust1c

package consumer_test

func TestReader(t *testing.T) {
    r := queuelib.NewReader()
    msg, err := r.ReadMessage()
    assert.NoError(t, err, "should not error reading message")
    // ... rest of test
}

Interfaces and Testing

@caust1c

Library authors: don't export interfaces without a strong case to do so
Program authors: create internal interfaces for external dependencies

Interfaces and Testing

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Sarama

@caust1c

Exports 22 Interfaces
Provides a special mocks package to satisfy those interfaces for testing
Tight coupling to Kafka's implementation
What are we testing when using this?

Interfaces and Testing

@caust1c

package consumer

type Message struct {
    Key   []byte
    Value []byte
    Topic string
    // additional metadata
}

type Consumer interface {
    Receive(context.Context) (Message, AckFunc, error)
}

type Producer interface {
    Send(context.Context, AckFunc, ...Message) error
}

Why This Works

@caust1c

Consumers define what they depend on
Consumers can implement stubs
Reduce complexity for maintaining library
Better debuggability and tooling
(e.g. goto definition)

https://rakyll.org/interface-pollution/
https://www.ardanlabs.com/blog/2016/10/avoid-interface-pollution.html

Parameterized Tests Suites

@caust1c

Start with a single implementation
Test it
Abstract away the implementation
Write new implementation(s) to satisfy interface
Refactor tests to use new interface

Single Implementation

@caust1c

type Cache struct {
    rp *redis.Pool
}

func (c Cache) Get(key string) (string, error) {
    conn := c.rp.Get()
    defer conn.Close()
    res, err := conn.Do("GET", key)
    if ret, ok := res.([]byte); ok {
        return string(ret), err
    }
    return "", err
}

func (c Cache) Set(key, value string) error {
    conn := c.rp.Get()
    defer conn.Close()
    _, err := conn.Do("SET", key, value)
    return err
}

Single Test

@caust1c

func TestCache(t *testing.T) {
    c := NewCache(&redis.Pool{
        Dial: func() (redis.Conn, error) {
            return redis.Dial("tcp", "localhost:6379")
        },
    })
    assert.NoError(t, c.Set("foo", "bar"), "setting should not error")
    act, err := c.Get("foo")
    assert.NoError(t, err, "getting should not error")
    assert.Equal(t, "bar", act)
}

Abstraction

@caust1c

type Store interface {
    Get(key string) (string, error)
    Set(key, value string) error
}

type Cache struct {
    store Store
}

func (c Cache) Get(key string) (string, error) {
    ret, err := c.store.Get(key)
    // ...
    return ret, err
}

func (c Cache) Set(key, value string) error {
    err := c.store.Set(key, value)
    // ...
    return err
}

Multiple Implementations

@caust1c

type Memcache struct {
    mc *memcache.Client
}

func (mc Memcache) Get(key string) (string, error) {
    i, err := mc.mc.Get(key)
    // ...
    return string(i.Value), err
}

func (mc Memcache) Set(key, value string) error {
    err := mc.mc.Set(&memcache.Item{
        Key:   key,
        Value: []byte(value),
    })
    // ...
    return err
}

@caust1c

func TestCacheRedis(t *testing.T) {
    store := NewRedis(&redis.Pool{
        Dial: func() (redis.Conn, error) {
            return redis.Dial("tcp", "localhost:6379")
        },
    })
    SuiteCache(t, store)
}

func TestCacheMemcache(t *testing.T) {
    store := NewMemcache(memcache.New("localhost:11211"))
    SuiteCache(t, store)
}

func SuiteCache(t *testing.T, s Store) {
    c := Cache{
        store: s,
    }
    assert.NoError(t, c.Set("foo", "bar"), "setting should not error")
    act, err := c.Get("foo")
    assert.NoError(t, err, "getting should not error")
    assert.Equal(t, "bar", act)
}

Test Suite

Why?

@caust1c

Write less code (fewer tests!)
Be confident in the behavior when switching implementations
Define the behavior in crystal clear terms
Communicating Intent

Last HTTP test ever

@caust1c

func NewServer() *http.ServeMux {
        mux := http.NewServeMux()
        mux.HandleFunc("/one", HandlerOne)
        mux.HandleFunc("/two", HandlerTwo)
        return mux
}

func HandlerOne(rw http.ResponseWriter, req *http.Request) {
        if req.Method != "GET" {
                rw.WriteHeader(http.StatusMethodNotAllowed)
                return
        }
        rw.Write([]byte("One!"))
}

func HandlerTwo(rw http.ResponseWriter, req *http.Request) {
        if req.Method != "POST" {
                rw.WriteHeader(http.StatusInternalServerError)
                return
        }
        rw.Write([]byte("Two!"))
}

Last HTTP test ever

@caust1c

func TestHandlers(t *testing.T) {
    go func() {
        http.ListenAndServe(":8080", NewServer())
    }()
    res, err := http.Get("http://localhost:8080/one")
    assert.NoError(t, err, "getting url should not error")
    v, err := ioutil.ReadAll(res.Body)
    assert.Equal(t, "One!", string(v), "respsonse should match")
    assert.Equal(t, 200, res.StatusCode, "respsonse should 200")
    res, _ = http.Head("http://localhost:8080/one")
    assert.Equal(t, 405, res.StatusCode, "should be method not allowed")

}

Last HTTP test ever

@caust1c

func TestHandlers(t *testing.T) {
    s := NewServer()
    req := httptest.NewRequest(http.MethodGet, "http://localhost:8080/one", nil)
    res := httptest.NewRecorder()
    s.ServeHTTP(res, req)
    assert.Equal(t, "One!", res.Body.String(), "respsonse should match")
    assert.Equal(t, 200, res.Code, "respsonse should 200")
    req = httptest.NewRequest(http.MethodHead, "http://localhost:8080/one", nil)
    res = httptest.NewRecorder()
    s.ServeHTTP(res, req)
    assert.Equal(t, 405, res.Code, "should be method not allowed")
}

@caust1c

func TestHandlers(t *testing.T) {
    tests := []struct {
        Name, Method, URL string
        ExpectBody        string
        ExpectCode        int
    }{
        {"basic GET", http.MethodGet, "http://localhost/one", "One!", 200},
        {"HEAD not allowed", http.MethodHead, "http://localhost/one", "", 405},
    }
    s := NewServer()
    for _, c := range tests {
        t.Run(c.Name, func(t *testing.T) {
            req := httptest.NewRequest(c.Method, c.URL, nil)
            res := httptest.NewRecorder()
            s.ServeHTTP(res, req)
            assert.Equal(t, c.ExpectCode, res.Code, "respsonse should 200")
            if c.ExpectCode != 200 {
                assert.Equal(t, c.ExpectBody, res.Body.String())
            }
        })
    }
}

Last HTTP test ever

@caust1c

func TestHandlers(t *testing.T) {
    // ...
    s := NewServer()
    jar, err := cookiejar.New(&cookiejar.Options{
        PublicSuffixList: publicsuffix.List
    })
    for _, c := range tests {
        t.Run(c.Name, func(t *testing.T) {
            req := httptest.NewRequest(c.Method, c.URL, nil)
            res := httptest.NewRecorder()
            for _, cookie := range jar.Cookies(req.URL) {
                req.AddCookie(cookie)
            }
            s.ServeHTTP(res, req)
            assert.Equal(t, c.ExpectCode, res.Code, "respsonse should 200")
            if c.ExpectCode == 200 {
                assert.Equal(t, c.ExpectBody, res.Body.String())
            }
            jar.SetCookies(req.URL, res.Cookies())

        })
    }
}

Last HTTP test ever

@caust1c

# Dockerfile-postgresql
FROM postgres:9.6
COPY data/migrations/*.up.sql /docker-entrypoint-initdb.d/

# docker-compose.yml
---
version: "3"
services:

  postgres:
    restart: always
    ports:
      - 5432:5432
    build:
      context: .
      dockerfile: Dockerfile-postgresql
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: password

Last HTTP test ever

@caust1c

func TestHandlers(t *testing.T) {
    // ...
    db := NewDB("postgres://postgres:password@localhost:5432/mydb")
    s := NewServer(db)
    jar, err := cookiejar.New(&cookiejar.Options{
        PublicSuffixList: publicsuffix.List
    })
    for _, c := range tests {
        t.Run(c.Name, func(t *testing.T) {
            req := httptest.NewRequest(c.Method, c.URL, nil)
            res := httptest.NewRecorder()
            for _, cookie := range jar.Cookies(req.URL) {
                req.AddCookie(cookie)
            }
            s.ServeHTTP(res, req)
            assert.Equal(t, c.ExpectCode, res.Code, "respsonse should 200")
            if c.ExpectCode == 200 {
                assert.Equal(t, c.ExpectBody, res.Body.String())
            }
            jar.SetCookies(req.URL, res.Cookies())

        })
    }
    truncateTables(t, db)
}

Wrap

@caust1c

Testing is communication
Testing should make writing code easier
Consider testing strategies in context

https://xkcd.com/979/