Use of Execution and teardown sequences race with process termination

Process termination monitoring is started asynchronously with the closure which receives the Execution, meaning any use of Execution in the body to send signals, etc., may send them to a pid which no longer refers to the original process. The cleanup handler may also run after the process has already terminated.

Author: jakepetroules

Sources/Subprocess/Configuration.swift

@@ -74,30 +74,29 @@ public struct Configuration: Sendable {
         let pid = spawnResults.execution.processIdentifier
 
         var spawnResultBox: SpawnResult?? = consume spawnResults
+        var _spawnResult = spawnResultBox!.take()!
 
-        return try await withAsyncTaskCleanupHandler {
-            var _spawnResult = spawnResultBox!.take()!
+        let processIdentifier = _spawnResult.execution.processIdentifier
+
+        let result = try await withAsyncTaskCleanupHandler {
             let inputIO = _spawnResult.inputWriteEnd()
             let outputIO = _spawnResult.outputReadEnd()
             let errorIO = _spawnResult.errorReadEnd()
-            let processIdentifier = _spawnResult.execution.processIdentifier
 
-            async let terminationStatus = try monitorProcessTermination(
-                forProcessWithIdentifier: processIdentifier
-            )
             // Body runs in the same isolation
-            let result = try await body(_spawnResult.execution, inputIO, outputIO, errorIO)
-            return ExecutionResult(
-                terminationStatus: try await terminationStatus,
-                value: result
-            )
+            return try await body(_spawnResult.execution, inputIO, outputIO, errorIO)
         } onCleanup: {
             // Attempt to terminate the child process
             await Execution.runTeardownSequence(
                 self.platformOptions.teardownSequence,
                 on: pid
             )
         }
+
+        return ExecutionResult(
+            terminationStatus: try await monitorProcessTermination(forProcessWithIdentifier: processIdentifier),
+            value: result
+        )
     }
 }