storage: add etcd storage

This patch adds etcd storage implementation. This should be useful in
environments where
- we dont want to depends on a separate, hard to maintain SQL cluster
- we dont want to incur the overhead of talking to kubernetes apiservers
- kubernetes is not available yet, or if kubernetes depends on dex
to perform authentication and the operator would like to remove any
circular dependency if possible.

storage/etcd/etcd_test.go

@@ -0,0 +1,94 @@
+package etcd
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/coreos/dex/storage"
+	"github.com/coreos/dex/storage/conformance"
+	"github.com/coreos/etcd/clientv3"
+	"github.com/sirupsen/logrus"
+)
+
+func withTimeout(t time.Duration, f func()) {
+	c := make(chan struct{})
+	defer close(c)
+
+	go func() {
+		select {
+		case <-c:
+		case <-time.After(t):
+			// Dump a stack trace of the program. Useful for debugging deadlocks.
+			buf := make([]byte, 2<<20)
+			fmt.Fprintf(os.Stderr, "%s\n", buf[:runtime.Stack(buf, true)])
+			panic("test took too long")
+		}
+	}()
+
+	f()
+}
+
+func cleanDB(c *conn) error {
+	ctx := context.TODO()
+	for _, prefix := range []string{
+		clientPrefix,
+		authCodePrefix,
+		refreshTokenPrefix,
+		authRequestPrefix,
+		passwordPrefix,
+		offlineSessionPrefix,
+		connectorPrefix,
+	} {
+		_, err := c.db.Delete(ctx, prefix, clientv3.WithPrefix())
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+var logger = &logrus.Logger{
+	Out:       os.Stderr,
+	Formatter: &logrus.TextFormatter{DisableColors: true},
+	Level:     logrus.DebugLevel,
+}
+
+func TestEtcd(t *testing.T) {
+	testEtcdEnv := "DEX_ETCD_ENDPOINTS"
+	endpointsStr := os.Getenv(testEtcdEnv)
+	if endpointsStr == "" {
+		t.Skipf("test environment variable %q not set, skipping", testEtcdEnv)
+		return
+	}
+	endpoints := strings.Split(endpointsStr, ",")
+
+	newStorage := func() storage.Storage {
+		s := &Etcd{
+			Endpoints: endpoints,
+		}
+		conn, err := s.open(logger)
+		if err != nil {
+			fmt.Fprintln(os.Stdout, err)
+			t.Fatal(err)
+		}
+
+		if err := cleanDB(conn); err != nil {
+			fmt.Fprintln(os.Stdout, err)
+			t.Fatal(err)
+		}
+		return conn
+	}
+
+	withTimeout(time.Second*10, func() {
+		conformance.RunTests(t, newStorage)
+	})
+
+	withTimeout(time.Minute*1, func() {
+		conformance.RunTransactionTests(t, newStorage)
+	})
+}