Don't crash when hitting long backoffs. (swift-server#458)

Motivation: If we backoff sufficiently far we can overflow Int64, which will cause us to crash. Modifications: Clamp the backoff value before we convert to Int64. Results: No crashes!

Author: Lukasa

Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+Backoff.swift

@@ -46,9 +46,12 @@ extension HTTPConnectionPool{
  // - 29 failed attempts: ~60s (max out)
 
 letstart=Double(TimeAmount.milliseconds(100).nanoseconds)
-letbackoffNanoseconds=Int64(start * pow(1.25,Double(attempts -1)))
+letbackoffNanosecondsDouble= start * pow(1.25,Double(attempts -1))
 
-letbackoff:TimeAmount=min(.nanoseconds(backoffNanoseconds),.seconds(60))
+ // Cap to 60s _before_ we convert to Int64, to avoid trapping in the Int64 initializer.
+letbackoffNanoseconds=Int64(min(backoffNanosecondsDouble,Double(TimeAmount.seconds(60).nanoseconds)))
+
+letbackoff=TimeAmount.nanoseconds(backoffNanoseconds)
 
  // Calculate a 3% jitter range
 letjitterRange=(backoff.nanoseconds /100)*3

Tests/AsyncHTTPClientTests/HTTPConnectionPoolTests+XCTest.swift

@@ -33,6 +33,7 @@ extension HTTPConnectionPoolTests{
 ("testConnectionCreationIsRetriedUntilRequestIsCancelled", testConnectionCreationIsRetriedUntilRequestIsCancelled),
 ("testConnectionShutdownIsCalledOnActiveConnections", testConnectionShutdownIsCalledOnActiveConnections),
 ("testConnectionPoolStressResistanceHTTP1", testConnectionPoolStressResistanceHTTP1),
+("testBackoffBehavesSensibly", testBackoffBehavesSensibly),
 ]
 }
 }

Tests/AsyncHTTPClientTests/HTTPConnectionPoolTests.swift

@@ -453,6 +453,32 @@ class HTTPConnectionPoolTests: XCTestCase{
  pool.shutdown()
 XCTAssertNoThrow(try poolDelegate.future.wait())
 }
+
+func testBackoffBehavesSensibly()throws{
+varbackoff=HTTPConnectionPool.calculateBackoff(failedAttempt:1)
+
+ // The value should be 100ms±3ms
+XCTAssertLessThanOrEqual((backoff -.milliseconds(100)).nanoseconds.magnitude,TimeAmount.milliseconds(3).nanoseconds.magnitude)
+
+ // Should always increase
+ // We stop when we get within the jitter of 60s, which is 1.8s
+varattempt=1
+while backoff <(.seconds(60)-.milliseconds(1800)){
+ attempt +=1
+letnewBackoff=HTTPConnectionPool.calculateBackoff(failedAttempt: attempt)
+
+XCTAssertGreaterThan(newBackoff, backoff)
+ backoff = newBackoff
+}
+
+ // Ok, now we should be able to do a hundred increments, and always hit 60s, plus or minus 1.8s of jitter.
+foroffsetin0..<100{
+XCTAssertLessThanOrEqual(
+(HTTPConnectionPool.calculateBackoff(failedAttempt: attempt + offset)-.seconds(60)).nanoseconds.magnitude,
+TimeAmount.milliseconds(1800).nanoseconds.magnitude
+)
+}
+}
 }
 
 classTestDelegate:HTTPConnectionPoolDelegate{