Testing zap logging for a Logger built from a custom Config

Zap has a concept of sinks, destinations for log messages. For testing, implement a sink that simply remembers messages (for instance in a bytes.Buffer):

package main

import (
    "bytes"
    "net/url"
    "strings"
    "testing"
    "time"

    "go.uber.org/zap"
)

// MemorySink implements zap.Sink by writing all messages to a buffer.
type MemorySink struct {
    *bytes.Buffer
}

// Implement Close and Sync as no-ops to satisfy the interface. The Write 
// method is provided by the embedded buffer.

func (s *MemorySink) Close() error { return nil }
func (s *MemorySink) Sync() error  { return nil }


func TestLogger(t *testing.T) {
    // Create a sink instance, and register it with zap for the "memory" 
    // protocol.
    sink := &MemorySink{new(bytes.Buffer)}
    zap.RegisterSink("memory", func(*url.URL) (zap.Sink, error) {
        return sink, nil
    })

    conf := zap.NewProductionConfig() // TODO: replace with real config

    // Redirect all messages to the MemorySink.    
    conf.OutputPaths = []string{"memory://"}

    l, err := conf.Build()
    if err != nil {
        t.Fatal(err)
    }

    l.Info("failed to fetch URL",
        zap.String("url", "http://example.com"),
        zap.Int("attempt", 3),
        zap.Duration("backoff", time.Second),
    )

    // Assert sink contents

    output := sink.String()
    t.Logf("output = %s", output)

    if !strings.Contains(output, `"url":"http://example.com"`) {
        t.Error("output missing: url=http://example.com")
    }
}