[RateLimiter] fix incorrect retryAfter of FixedWindow

A FixedWindow always ends `intervalInSeconds` after the start time (`timer`). So when calculating the time to consume some tokens the tokens will always be available at `timer + intervalInSeconds`. But currently this is reported incorrectly as `calculateTimeForTokens()` calculates the time based on the desired amount of tokens (and cycles) while it doesn't take into account `maxSize` amount of tokens become available at the windows end. Furthermore calculating the amount of needed cycles is incorrect. This as all tokens become available at once (at the windows end) and you can't consume more tokens than `maxSize` (which is validated at the start of `FixedWindowLimiter::reserve`, in case of `tokens > limit` it throws).
symfony · fabpot · Jul 13, 2023 · Jan 23, 2023 · Jul 10, 2023 · 231693282f8143b8cc889df261404e12a1cd9cb2
commit 231693282f8143b8cc889df261404e12a1cd9cb2
diff --git a/src/Symfony/Component/RateLimiter/Policy/FixedWindowLimiter.php b/src/Symfony/Component/RateLimiter/Policy/FixedWindowLimiter.php
@@ -68,7 +68,7 @@ public function reserve(int $tokens = 1, float $maxTime = null): Reservation
 
                 $reservation = new Reservation($now, new RateLimit($window->getAvailableTokens($now), \DateTimeImmutable::createFromFormat('U', floor($now)), true, $this->limit));
             } else {
-                $waitDuration = $window->calculateTimeForTokens($tokens);
+                $waitDuration = $window->calculateTimeForTokens($tokens, $now);
 
                 if (null !== $maxTime && $waitDuration > $maxTime) {
                     // process needs to wait longer than set interval

diff --git a/src/Symfony/Component/RateLimiter/Policy/Window.php b/src/Symfony/Component/RateLimiter/Policy/Window.php
@@ -75,15 +75,13 @@ public function getAvailableTokens(float $now)
         return $this->maxSize - $this->hitCount;
     }
 
-    public function calculateTimeForTokens(int $tokens): int
+    public function calculateTimeForTokens(int $tokens, float $now): int
     {
         if (($this->maxSize - $this->hitCount) >= $tokens) {
             return 0;
         }
 
-        $cyclesRequired = ceil($tokens / $this->maxSize);
-
-        return $cyclesRequired * $this->intervalInSeconds;
+        return (int) ceil($this->timer + $this->intervalInSeconds - $now);
     }
 
     public function __serialize(): array

diff --git a/src/Symfony/Component/RateLimiter/Tests/Policy/FixedWindowLimiterTest.php b/src/Symfony/Component/RateLimiter/Tests/Policy/FixedWindowLimiterTest.php
@@ -37,6 +37,7 @@ protected function setUp(): void
 
     public function testConsume()
     {
+        $now = time();
         $limiter = $this->createLimiter();
 
         // fill 9 tokens in 45 seconds
@@ -51,6 +52,9 @@ public function testConsume()
         $rateLimit = $limiter->consume();
         $this->assertFalse($rateLimit->isAccepted());
         $this->assertSame(10, $rateLimit->getLimit());
+        // Window ends after 1 minute
+        $retryAfter = \DateTimeImmutable::createFromFormat('U', $now + 60);
+        $this->assertEquals($retryAfter, $rateLimit->getRetryAfter());
     }
 
     /**