use array instead of map

internal/api/routes/auth.go

@@ -34,8 +34,8 @@ import (
 	"stereo.cat/backend/internal/types"
 )
 
-var oauthStates = make(map[string]struct{})
-var oauthStatesMu sync.Mutex
+var states []string
+var statesMutex sync.Mutex
 
 func generateState(length int) (string, error) {
 	b := make([]byte, length)
@@ -54,9 +54,9 @@ func RegisterAuthRoutes(cfg *types.StereoConfig, api *gin.RouterGroup) {
 			return
 		}
 
-		oauthStatesMu.Lock()
-		oauthStates[state] = struct{}{}
-		oauthStatesMu.Unlock()
+		statesMutex.Lock()
+		states = append(states, state)
+		statesMutex.Unlock()
 
 		discordURL := fmt.Sprintf(
 			"https://discord.com/oauth2/authorize?client_id=%s&response_type=code&redirect_uri=%s&scope=identify%%20email&state=%s",
@@ -77,13 +77,20 @@ func RegisterAuthRoutes(cfg *types.StereoConfig, api *gin.RouterGroup) {
 		code := c.Query("code")
 		state := c.Query("state")
 
-		oauthStatesMu.Lock()
-		_, ok := oauthStates[state]
-		if ok {
-			delete(oauthStates, state)
+		statesMutex.Lock()
+
+		found := false
+		for i, s := range states {
+			if s == state {
+				states = append(states[:i], states[i+1:]...)
+				found = true
+				break
+			}
 		}
-		oauthStatesMu.Unlock()
-		if !ok {
+
+		statesMutex.Unlock()
+
+		if !found {
 			c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "Invalid state"})
 			return
 		}