diff --git a/Sources/Subprocess/Configuration.swift b/Sources/Subprocess/Configuration.swift
index d67eede..6c1c41d 100644
--- a/Sources/Subprocess/Configuration.swift
+++ b/Sources/Subprocess/Configuration.swift
@@ -752,14 +752,15 @@ extension Optional where Wrapped == String {
     }
 }
 
-internal func withAsyncTaskCleanupHandler<Result>(
-    _ body: () async throws -> Result,
+internal func withAsyncTaskCleanupHandler<Success>(
+    _ body: () async throws -> Success,
     onCleanup handler: @Sendable @escaping () async -> Void,
     isolation: isolated (any Actor)? = #isolation
-) async rethrows -> Result {
+) async rethrows -> Success {
+    let runCancellationHandlerPromise = Promise<Bool, Never>()
     return try await withThrowingTaskGroup(
         of: Void.self,
-        returning: Result.self
+        returning: Success.self
     ) { group in
         group.addTask {
             // Keep this task sleep indefinitely until the parent task is cancelled.
@@ -767,16 +768,140 @@ internal func withAsyncTaskCleanupHandler<Result>(
             // before the time ends. We then run the cancel handler.
             do { while true { try await Task.sleep(nanoseconds: 1_000_000_000) } } catch {}
             // Run task cancel handler
-            await handler()
+            runCancellationHandlerPromise.fulfill(with: true)
+        }
+
+        group.addTask {
+            if await runCancellationHandlerPromise.value {
+                await handler()
+            }
+        }
+
+        defer {
+            group.cancelAll()
         }
 
         do {
             let result = try await body()
-            group.cancelAll()
+            runCancellationHandlerPromise.fulfill(with: false)
             return result
         } catch {
-            await handler()
+            runCancellationHandlerPromise.fulfill(with: true)
             throw error
         }
     }
 }
+
+#if canImport(Darwin)
+import os
+#else
+import Synchronization
+#endif
+
+/// Represents a placeholder for a value which will be provided by synchronous code running in another execution context.
+internal final class Promise<Success: Sendable, Failure: Swift.Error>: Sendable {
+    private struct State {
+        var waiters: [CheckedContinuation<Success, Failure>] = []
+        var value: Result<Success, Failure>?
+    }
+
+    #if canImport(Darwin)
+    private let state: OSAllocatedUnfairLock<State>
+    #else
+    private let state: Mutex<State>
+    #endif
+
+    /// Creates a new promise in the unfulfilled state.
+    public init() {
+        #if canImport(Darwin)
+        state = OSAllocatedUnfairLock(initialState: State(value: nil))
+        #else
+        state = Mutex(State(value: nil))
+        #endif
+    }
+
+    deinit {
+        state.withLock { state in
+            precondition(state.waiters.isEmpty, "Deallocated with remaining waiters")
+        }
+    }
+
+    /// Fulfills the promise with the specified result.
+    ///
+    /// - returns: Whether the promise was already fulfilled.
+    @discardableResult
+    public func fulfill(with result: Result<Success, Failure>) -> Bool {
+        let (waiters, alreadyFulfilled): ([CheckedContinuation<Success, Failure>], Bool) = state.withLock { state in
+            if state.value != nil {
+                return ([], true)
+            }
+            state.value = result
+            let waiters = state.waiters
+            state.waiters.removeAll()
+            return (waiters, false)
+        }
+
+        // Resume the continuations outside the lock to avoid potential deadlock if invoked in a cancellation handler.
+        for waiter in waiters {
+            waiter.resume(with: result)
+        }
+
+        return alreadyFulfilled
+    }
+
+    /// Fulfills the promise with the specified value.
+    ///
+    /// - returns: Whether the promise was already fulfilled.
+    @discardableResult
+    public func fulfill(with value: Success) -> Bool {
+        fulfill(with: .success(value))
+    }
+
+    /// Fulfills the promise with the specified error.
+    ///
+    /// - returns: Whether the promise was already fulfilled.
+    @discardableResult
+    public func fail(throwing error: Failure) -> Bool {
+        fulfill(with: .failure(error))
+    }
+}
+
+extension Promise where Success == Void {
+    /// Fulfills the promise.
+    ///
+    /// - returns: Whether the promise was already fulfilled.
+    @discardableResult
+    internal func fulfill() -> Bool {
+        fulfill(with: ())
+    }
+}
+
+extension Promise where Failure == Never {
+    /// Suspends if the promise is not yet fulfilled, and returns the value once it is.
+    internal var value: Success {
+        get async {
+            await withCheckedContinuation { continuation in
+                let value: Result<Success, Never>? = state.withLock { state in
+                    if let value = state.value {
+                        return value
+                    } else {
+                        state.waiters.append(continuation)
+                        return nil
+                    }
+                }
+
+                // Resume the continuations outside the lock to avoid potential deadlock if invoked in a cancellation handler.
+                if let value {
+                    continuation.resume(with: value)
+                }
+            }
+        }
+    }
+
+    /// Suspends if the promise is not yet fulfilled, and returns the result once it is.
+    internal var result: Result<Success, Failure> {
+        get async {
+            await .success(value)
+        }
+    }
+}