api/v2: name the token's tags in the token answer

Only a token that can assign tags names them, as Tailscale's answer
does.

hscontrol/api/v2/oauth.go

@@ -35,6 +35,9 @@ type tokenResponse struct {
 	TokenType   string `json:"token_type"`
 	ExpiresIn   int    `json:"expires_in"`
 	Scope       string `json:"scope,omitempty"`
+	// Tags names the tags of a token that can assign them, as Tailscale's
+	// answer does; a reader token's answer has none.
+	Tags string `json:"tags,omitempty"`
 }
 
 // oauthTokenHandler implements the client-credentials grant (RFC 6749 §4.4):
@@ -110,12 +113,20 @@ func oauthTokenHandler(b Backend) http.HandlerFunc {
 			return
 		}
 
-		writeJSON(w, http.StatusOK, tokenResponse{
+		resp := tokenResponse{
 			AccessToken: tokenStr,
 			TokenType:   "Bearer",
 			ExpiresIn:   int(accessTokenTTL.Seconds()),
 			Scope:       strings.Join(token.Scopes, " "),
-		})
+		}
+
+		granted := scope.Parse(token.Scopes)
+		if scope.Grants(granted, scope.AuthKeys) || scope.Grants(granted, scope.DevicesCore) ||
+			scope.Grants(granted, scope.OAuthKeys) {
+			resp.Tags = strings.Join(token.Tags, " ")
+		}
+
+		writeJSON(w, http.StatusOK, resp)
 	}
 }
 

hscontrol/apiv2_compat_test.go

@@ -56,20 +56,9 @@ var apiv2CompatSkipReasons = map[string]string{
 
 	"oauth/O3/0/status": mintScopeRefusalStatus,
 	"oauth/O4/0/scope":  unknownScopeGranted,
+	"oauth/O4/0/tags":   unknownScopeGranted,
 
-	"oauth/O4/0/tags":  mintTagsNotEchoed,
-	"oauth/E1/0/tags":  mintTagsNotEchoed,
-	"oauth/E9a/0/tags": mintTagsNotEchoed,
-	"oauth/T1/0/tags":  mintTagsNotEchoed,
-	"oauth/D0/6/tags":  mintTagsNotEchoed,
-	"oauth/B1/0/tags":  mintTagsNotEchoed,
-	"oauth/B3/0/tags":  mintTagsNotEchoed,
-	"oauth/B11/1/tags": mintTagsNotEchoed,
-
-	"key-list/setup-token-keys/0/tags": mintTagsNotEchoed,
-	"key-list/setup-token-keys/2/tags": mintTagsNotEchoed,
-	"key-list/setup-token-keys/4/tags": mintTagsNotEchoed,
-	"key-list/setup-token-keys/6/tags": mintTagsNotEchoed,
+	"key-list/setup-token-keys/4/tags": mintTagsSorted,
 
 	"oauth/K5/0/status": unknownKeyType,
 
@@ -151,7 +140,7 @@ const (
 // Differences no planned change removes yet.
 const (
 	mintScopeRefusalStatus      = "TOKEN_SCOPE_REFUSAL_STATUS: SaaS refuses a token scope the client lacks with 403 and a message; headscale with 400 invalid_scope"
-	mintTagsNotEchoed           = "TOKEN_TAGS_NOT_ECHOED: SaaS's token answer names the token's tags; headscale's has no tags"
+	mintTagsSorted              = "TOKEN_TAGS_SORTED: SaaS's token answer names the tags in the order its client was made with; headscale stores a client's tags sorted"
 	unknownKeyType              = "UNKNOWN_KEY_TYPE: SaaS refuses an unknown keyType with 400 before it checks scopes; headscale checks the scope first and answers 403"
 	tagRefusalStatus            = "TAG_REFUSAL_STATUS: SaaS refuses a tag outside the token's grant with 400 requested tags are invalid or not permitted; headscale with 403"
 	emptyKeyListNull            = "EMPTY_KEY_LIST_NULL: SaaS answers a listing that holds no key with keys null; headscale with []"

hscontrol/apiv2_oauth_test.go

@@ -215,6 +215,15 @@ func TestAPIv2OAuth_TokenEndpoint(t *testing.T) {
 	require.Equalf(t, http.StatusOK, statusDup, "%v", mDup)
 	assert.Equal(t, "auth_keys", mDup["scope"])
 
+	// It names the tags of a token that can assign them; a reader token's
+	// answer has no tags field, whatever its client's tags.
+	assert.Equal(t, "tag:ci", m["tags"])
+
+	_, readerSecret := createClient(t, baseURL, admin, []string{"auth_keys:read"}, []string{"tag:ci"})
+	statusReader, mReader := mintToken(t, baseURL, "", readerSecret, "", false)
+	require.Equalf(t, http.StatusOK, statusReader, "%v", mReader)
+	assert.NotContains(t, mReader, "tags")
+
 	// Bad credentials → RFC 6749 invalid_client.
 	statusBad, mBad := mintToken(t, baseURL, clientID, "hskey-client-deadbeefdead-"+stringOf("0", 64), "", false)
 	assert.Equal(t, http.StatusUnauthorized, statusBad)