fix: allow for alternate usernames on conflict (#4614)

coderd/userauth.go

@@ -13,6 +13,7 @@ import (
 	"github.com/coreos/go-oidc/v3/oidc"
 	"github.com/google/go-github/v43/github"
 	"github.com/google/uuid"
+	"github.com/moby/moby/pkg/namesgenerator"
 	"golang.org/x/oauth2"
 	"golang.org/x/xerrors"
 
@@ -390,6 +391,38 @@ func (api *API) oauthLogin(r *http.Request, params oauthLoginParams) (*http.Cook
 				organizationID = organizations[0].ID
 			}
 
+			_, err := tx.GetUserByEmailOrUsername(ctx, database.GetUserByEmailOrUsernameParams{
+				Username: params.Username,
+			})
+			if err == nil {
+				var (
+					original      = params.Username
+					validUsername bool
+				)
+				for i := 0; i < 10; i++ {
+					alternate := fmt.Sprintf("%s-%s", original, namesgenerator.GetRandomName(1))
+
+					params.Username = httpapi.UsernameFrom(alternate)
+
+					_, err := tx.GetUserByEmailOrUsername(ctx, database.GetUserByEmailOrUsernameParams{
+						Username: params.Username,
+					})
+					if xerrors.Is(err, sql.ErrNoRows) {
+						validUsername = true
+						break
+					}
+					if err != nil {
+						return xerrors.Errorf("get user by email/username: %w", err)
+					}
+				}
+				if !validUsername {
+					return httpError{
+						code: http.StatusConflict,
+						msg:  fmt.Sprintf("exhausted alternatives for taken username %q", original),
+					}
+				}
+			}
+
 			user, _, err = api.CreateUser(ctx, tx, CreateUserRequest{
 				CreateUserRequest: codersdk.CreateUserRequest{
 					Email:          params.Email,