First pass at microsoft provider.

Code to get email address is missing. Code to only authorise certain
groups is missing.

server/auth/microsoft/microsoft.go

+package microsoft
+
+import (
+	"errors"
+	"net/http"
+	"strings"
+
+	"github.com/nsheridan/cashier/server/auth"
+	"github.com/nsheridan/cashier/server/config"
+	"github.com/nsheridan/cashier/server/metrics"
+
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/microsoft"
+)
+
+const (
+	name = "microsoft"
+)
+
+// Config is an implementation of `auth.Provider` for authenticating using a
+// Office 365 account.
+type Config struct {
+	config    *oauth2.Config
+	tenant    string
+	whitelist map[string]bool
+}
+
+var _ auth.Provider = (*Config)(nil)
+
+// New creates a new Microsoft provider from a configuration.
+func New(c *config.Auth) (*Config, error) {
+	uw := make(map[string]bool)
+	for _, u := range c.UsersWhitelist {
+		uw[u] = true
+	}
+	if c.ProviderOpts["tenant"] == "" && len(uw) == 0 {
+		return nil, errors.New("either Office 365 tenant or users whitelist must be specified")
+	}
+
+	return &Config{
+		config: &oauth2.Config{
+			ClientID:     c.OauthClientID,
+			ClientSecret: c.OauthClientSecret,
+			RedirectURL:  c.OauthCallbackURL,
+			Endpoint:     microsoft.AzureADEndpoint(c.ProviderOpts["tenant"]),
+		},
+		tenant:    c.ProviderOpts["tenant"],
+		whitelist: uw,
+	}, nil
+}
+
+// A new oauth2 http client.
+func (c *Config) newClient(token *oauth2.Token) *http.Client {
+	return c.config.Client(oauth2.NoContext, token)
+}
+
+// Name returns the name of the provider.
+func (c *Config) Name() string {
+	return name
+}
+
+// Valid validates the oauth token.
+func (c *Config) Valid(token *oauth2.Token) bool {
+	if len(c.whitelist) > 0 && !c.whitelist[c.Email(token)] {
+		return false
+	}
+	if !token.Valid() {
+		return false
+	}
+	metrics.M.AuthValid.WithLabelValues("microsoft").Inc()
+	return true
+}
+
+// Revoke disables the access token.
+func (c *Config) Revoke(token *oauth2.Token) error {
+	return nil
+}
+
+// StartSession retrieves an authentication endpoint from Microsoft.
+func (c *Config) StartSession(state string) *auth.Session {
+	return &auth.Session{
+		AuthURL: c.config.AuthCodeURL(state, oauth2.SetAuthURLParam("hd", c.tenant)),
+	}
+}
+
+// Exchange authorizes the session and returns an access token.
+func (c *Config) Exchange(code string) (*oauth2.Token, error) {
+	t, err := c.config.Exchange(oauth2.NoContext, code)
+	if err == nil {
+		metrics.M.AuthExchange.WithLabelValues("microsoft").Inc()
+	}
+	return t, err
+}
+
+// Email retrieves the email address of the user.
+func (c *Config) Email(token *oauth2.Token) string {
+	return "nobody@nowhere"
+}
+
+// Username retrieves the username portion of the user's email address.
+func (c *Config) Username(token *oauth2.Token) string {
+	return strings.Split(c.Email(token), "@")[0]
+}

server/auth/microsoft/microsoft_test.go

+package microsoft
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/nsheridan/cashier/server/config"
+	"github.com/stretchr/testify/assert"
+)
+
+var (
+	oauthClientID     = "id"
+	oauthClientSecret = "secret"
+	oauthCallbackURL  = "url"
+	tenant            = "example.com"
+	users             = []string{"user"}
+)
+
+func TestNew(t *testing.T) {
+	a := assert.New(t)
+	p, err := newMicrosoft()
+	a.NoError(err)
+	a.Equal(p.config.ClientID, oauthClientID)
+	a.Equal(p.config.ClientSecret, oauthClientSecret)
+	a.Equal(p.config.RedirectURL, oauthCallbackURL)
+	a.Equal(p.tenant, tenant)
+	a.Equal(p.whitelist, map[string]bool{"user": true})
+}
+
+func TestWhitelist(t *testing.T) {
+	c := &config.Auth{
+		OauthClientID:     oauthClientID,
+		OauthClientSecret: oauthClientSecret,
+		OauthCallbackURL:  oauthCallbackURL,
+		ProviderOpts:      map[string]string{"tenant": ""},
+		UsersWhitelist:    []string{},
+	}
+	if _, err := New(c); err == nil {
+		t.Error("creating a provider without a tenant set should return an error")
+	}
+	// Set a user whitelist but no tenant
+	c.UsersWhitelist = users
+	if _, err := New(c); err != nil {
+		t.Error("creating a provider with users but no tenant should not return an error")
+	}
+	// Unset the user whitelist and set a tenant
+	c.UsersWhitelist = []string{}
+	c.ProviderOpts = map[string]string{"tenant": tenant}
+	if _, err := New(c); err != nil {
+		t.Error("creating a provider with a tenant set but without a user whitelist should not return an error")
+	}
+}
+
+func TestStartSession(t *testing.T) {
+	a := assert.New(t)
+
+	p, err := newMicrosoft()
+	a.NoError(err)
+	s := p.StartSession("test_state")
+	a.Contains(s.AuthURL, fmt.Sprintf("login.microsoftonline.com/%s/oauth2/v2.0/authorize", tenant))
+}
+
+func newMicrosoft() (*Config, error) {
+	c := &config.Auth{
+		OauthClientID:     oauthClientID,
+		OauthClientSecret: oauthClientSecret,
+		OauthCallbackURL:  oauthCallbackURL,
+		ProviderOpts:      map[string]string{"tenant": tenant},
+		UsersWhitelist:    users,
+	}
+	return New(c)
+}