fix: correct token bucket delays when experiencing system time jumps

.changes/9e04fd2c-136d-490b-9ecf-d6072a6f34c1.json

@@ -0,0 +1,8 @@
+{
+    "id": "9e04fd2c-136d-490b-9ecf-d6072a6f34c1",
+    "type": "bugfix",
+    "description": "Fix a bug where system time jumps could cause unexpected retry behavior",
+    "issues": [
+        "awslabs/smithy-kotlin#805"
+    ]
+}

runtime/runtime-core/common/src/aws/smithy/kotlin/runtime/retries/delay/StandardRetryTokenBucket.kt

@@ -6,30 +6,31 @@
 package aws.smithy.kotlin.runtime.retries.delay
 
 import aws.smithy.kotlin.runtime.retries.policy.RetryErrorType
-import aws.smithy.kotlin.runtime.time.Clock
-import aws.smithy.kotlin.runtime.time.epochMilliseconds
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.sync.Mutex
 import kotlinx.coroutines.sync.withLock
 import kotlin.math.ceil
 import kotlin.math.floor
 import kotlin.math.min
+import kotlin.time.ExperimentalTime
+import kotlin.time.TimeSource
 
 private const val MS_PER_S = 1_000
 
 /**
  * The standard implementation of a [RetryTokenBucket].
  * @param options The configuration to use for this bucket.
- * @param clock A clock to use for time calculations.
+ * @param timeSource A monotonic time source to use for calculating the temporal token fill of the bucket.
  */
-public class StandardRetryTokenBucket(
+@OptIn(ExperimentalTime::class)
+public class StandardRetryTokenBucket constructor(
     public val options: StandardRetryTokenBucketOptions = StandardRetryTokenBucketOptions.Default,
-    private val clock: Clock = Clock.System,
+    private val timeSource: TimeSource = TimeSource.Monotonic,
 ) : RetryTokenBucket {
     internal var capacity = options.maxCapacity
         private set
 
-    private var lastTimestamp = now()
+    private var lastTimeMark = timeSource.markNow()
     private val mutex = Mutex()
 
     /**
@@ -58,13 +59,11 @@ public class StandardRetryTokenBucket(
             capacity = 0
         }
 
-        lastTimestamp = now()
+        lastTimeMark = timeSource.markNow()
     }
 
-    private fun now(): Long = clock.now().epochMilliseconds
-
     private fun refillCapacity() {
-        val refillMs = now() - lastTimestamp
+        val refillMs = lastTimeMark.elapsedNow().inWholeMilliseconds
         val refillSize = floor(options.refillUnitsPerSecond.toDouble() / MS_PER_S * refillMs).toInt()
         capacity = min(options.maxCapacity, capacity + refillSize)
     }
@@ -73,7 +72,7 @@ public class StandardRetryTokenBucket(
         refillCapacity()
 
         capacity = min(options.maxCapacity, capacity + size)
-        lastTimestamp = now()
+        lastTimeMark = timeSource.markNow()
     }
 
     /**

runtime/runtime-core/common/test/aws/smithy/kotlin/runtime/retries/delay/StandardRetryTokenBucketTest.kt

@@ -6,14 +6,14 @@
 package aws.smithy.kotlin.runtime.retries.delay
 
 import aws.smithy.kotlin.runtime.retries.policy.RetryErrorType
-import aws.smithy.kotlin.runtime.time.ManualClock
 import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.test.runTest
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertIs
 import kotlin.time.Duration.Companion.milliseconds
 import kotlin.time.ExperimentalTime
+import kotlin.time.TestTimeSource
 
 class StandardRetryTokenBucketTest {
     companion object {
@@ -88,17 +88,17 @@ class StandardRetryTokenBucketTest {
     @OptIn(ExperimentalCoroutinesApi::class, ExperimentalTime::class)
     @Test
     fun testRefillOverTime() = runTest {
-        val clock = ManualClock()
+        val timeSource = TestTimeSource()
 
         // A bucket that costs capacity for an initial try
-        val bucket = StandardRetryTokenBucket(DefaultOptions.copy(initialTryCost = 5), clock)
+        val bucket = StandardRetryTokenBucket(DefaultOptions.copy(initialTryCost = 5), timeSource)
 
         assertEquals(10, bucket.capacity)
         assertTime(0) { bucket.acquireToken() }
         assertEquals(5, bucket.capacity)
 
         // Refill rate is 10/s == 1/100ms so after 250ms we should have 2 more tokens.
-        clock.advance(250.milliseconds)
+        timeSource += 250.milliseconds
 
         assertTime(0) { bucket.acquireToken() }
         assertEquals(2, bucket.capacity) // We had 5, 2 refilled, and then we decremented 5 more.