chore: import upstream snapshot with attribution

Sources/ContainerizationOS/Reaper.swift

@@ -0,0 +1,60 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025-2026 Apple Inc. and the Containerization project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+#if canImport(Musl)
+import Musl
+#elseif canImport(Glibc)
+import Glibc
+#elseif canImport(Darwin)
+import Darwin
+#endif
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
+import Foundation
+#endif
+
+/// A process reaper that returns exited processes along
+/// with their exit status.
+public struct Reaper {
+    /// Process's pid and exit status.
+    typealias Exit = (pid: Int32, status: Int32)
+
+    /// Reap all pending processes and return the pid and exit status.
+    public static func reap() -> [Int32: Int32] {
+        var reaped = [Int32: Int32]()
+        while true {
+            guard let exit = wait() else {
+                return reaped
+            }
+            reaped[exit.pid] = exit.status
+        }
+        return reaped
+    }
+
+    /// Returns the exit status of the last process that exited.
+    /// nil is returned when no pending processes exist.
+    private static func wait() -> Exit? {
+        var rus = rusage()
+        var ws = Int32()
+
+        let pid = wait4(-1, &ws, WNOHANG, &rus)
+        if pid <= 0 {
+            return nil
+        }
+        return (pid: pid, status: Command.toExitStatus(ws))
+    }
+}