Support imaps with oauthbearer authentication (Gmail)

    imaps+oauthbearer://user:token@host?token_endpoint=...

 - the config Source password is used as access token if
   no token_endpoint parameter is set
 - the config Source password is used as refresh token if
   token_endpoint parameter is set, and used to exchange
   with an access token

The implementation has only been tested with Gmail.

    source = imaps+oauthbearer://{username}:{refersh_token}@imap.gmail.com:993? \
    client_id=XX&\
    client_secret=XX&\
    token_endpoint=https%3A%2F%2Faccounts.google.com%2Fo%2Foauth2%2Ftoken

client credentials created with

    https://console.developers.google.com/apis/credentials

refresh token created with

    https://github.com/google/gmail-oauth2-tools/blob/master/python/oauth2.py

rel: https://todo.sr.ht/~sircmpwn/aerc2/42

1 files changed, 42 insertions, 0 deletions

diff --git a/lib/oauthbearer.go b/lib/oauthbearer.go
new file mode 100644
index 0000000..5bcba60
--- /dev/null
+++ b/lib/oauthbearer.go
@@ -0,0 +1,42 @@
+package lib
+
+import (
+	"context"
+	"fmt"
+	"github.com/emersion/go-imap/client"
+	"github.com/emersion/go-sasl"
+	"golang.org/x/oauth2"
+)
+
+type OAuthBearer struct {
+	OAuth2  *oauth2.Config
+	Enabled bool
+}
+
+func (c *OAuthBearer) exchangeRefreshToken(refreshToken string) (*oauth2.Token, error) {
+	token := new(oauth2.Token)
+	token.RefreshToken = refreshToken
+	token.TokenType = "Bearer"
+	return c.OAuth2.TokenSource(context.TODO(), token).Token()
+}
+
+func (c *OAuthBearer) Authenticate(username string, password string, client *client.Client) error {
+	if ok, err := client.SupportAuth(sasl.OAuthBearer); err != nil || !ok {
+		return fmt.Errorf("OAuthBearer not supported %v", err)
+	}
+
+	if c.OAuth2.Endpoint.TokenURL != "" {
+		token, err := c.exchangeRefreshToken(password)
+		if err != nil {
+			return err
+		}
+		password = token.AccessToken
+	}
+
+	saslClient := sasl.NewOAuthBearerClient(&sasl.OAuthBearerOptions{
+		Username: username,
+		Token:    password,
+	})
+
+	return client.Authenticate(saslClient)
+}