Clarify Windows process setup failures

internal/adapters/subprocess/process_tree_windows.go

@@ -43,25 +43,44 @@ func (tree *windowsProcessTree) Start(cmd *exec.Cmd) error {
 	withHandleErr := cmd.Process.WithHandle(func(handle uintptr) {
 		assignErr = windows.AssignProcessToJobObject(job, windows.Handle(handle))
 	})
-	if withHandleErr == nil && assignErr == nil {
-		assignErr = resumeInitialThread(uint32(cmd.Process.Pid))
+	if withHandleErr != nil {
+		return failWindowsProcessStart(cmd, job, fmt.Errorf("acquire stable process handle for job assignment: %w", withHandleErr))
 	}
-	if withHandleErr != nil || assignErr != nil {
-		killErr := cmd.Process.Kill()
-		waitErr := cmd.Wait()
-		_ = windows.CloseHandle(job)
-		return joinErrors(
-			fmt.Errorf("acquire process handle for job assignment: %w", withHandleErr),
-			fmt.Errorf("assign process to job object or resume initial thread: %w", assignErr),
-			killErr,
-			waitErr,
-		)
+	if assignErr != nil {
+		return failWindowsProcessStart(cmd, job, fmt.Errorf("assign process to job object: %w", assignErr))
+	}
+	if err := resumeInitialThread(uint32(cmd.Process.Pid)); err != nil {
+		return failWindowsProcessStart(cmd, job, fmt.Errorf("resume initial process thread: %w", err))
 	}
 
 	tree.job = job
 	return nil
 }
 
+func failWindowsProcessStart(cmd *exec.Cmd, job windows.Handle, setupErr error) error {
+	killErr := cmd.Process.Kill()
+	waitErr := cmd.Wait()
+	var exitErr *exec.ExitError
+	if killErr == nil && errors.As(waitErr, &exitErr) {
+		// A killed suspended child normally reports a non-zero exit from Wait;
+		// successful reaping is not an additional setup failure.
+		waitErr = nil
+	}
+	return joinErrors(
+		setupErr,
+		wrapWindowsProcessCleanupError("kill suspended child", killErr),
+		wrapWindowsProcessCleanupError("reap suspended child", waitErr),
+		wrapWindowsProcessCleanupError("close unowned job object", windows.CloseHandle(job)),
+	)
+}
+
+func wrapWindowsProcessCleanupError(operation string, err error) error {
+	if err == nil {
+		return nil
+	}
+	return fmt.Errorf("%s: %w", operation, err)
+}
+
 func resumeInitialThread(processID uint32) error {
 	snapshot, err := windows.CreateToolhelp32Snapshot(windows.TH32CS_SNAPTHREAD, 0)
 	if err != nil {