Change 'unsafeDowncast' to 'as!' (#115)

glbrntt · web-flow · commit a6dc9869b241 · 2024-07-25T05:18:49.000-07:00
Motivation:

The 'unsafeDowncast' can cause a miscompile leading to unexpected
runtime behaviour.

Modifications:

- Use 'as!' instead

Result:

No miscompiles on 5.10
diff --git a/Sources/Prometheus/NIOLock.swift b/Sources/Prometheus/NIOLock.swift
@@ -141,7 +141,8 @@ final class LockStorage<Value>: ManagedBuffer<Value, LockPrimitive> {
         let buffer = Self.create(minimumCapacity: 1) { _ in
             return value
         }
-        let storage = unsafeDowncast(buffer, to: Self.self)
+        // Avoid 'unsafeDowncast' as there is a miscompilation on 5.10.
+        let storage = buffer as! Self
 
         storage.withUnsafeMutablePointers { _, lockPtr in
             LockOperations.create(lockPtr)

Original file line number	Diff line number	Diff line change
`@@ -141,7 +141,8 @@ final class LockStorage<Value>: ManagedBuffer<Value, LockPrimitive> {`
`141`	`141`	`let buffer = Self.create(minimumCapacity: 1) { _ in`
`142`	`142`	`return value`
`143`	`143`	`}`
`144`		`- let storage = unsafeDowncast(buffer, to: Self.self)`
	`144`	`+ // Avoid 'unsafeDowncast' as there is a miscompilation on 5.10.`
	`145`	`+ let storage = buffer as! Self`
`145`	`146`
`146`	`147`	`storage.withUnsafeMutablePointers { _, lockPtr in`
`147`	`148`	`LockOperations.create(lockPtr)`