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diff --git a/src/main/tools/process-wrapper-legacy.h b/src/main/tools/process-wrapper-legacy.h
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+// Copyright 2017 The Bazel Authors. All rights reserved.
+//
+// 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
+//
+//    http://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.
+
+#ifndef SRC_MAIN_TOOLS_PROCESS_WRAPPER_LEGACY_H_
+#define SRC_MAIN_TOOLS_PROCESS_WRAPPER_LEGACY_H_
+
+#include <signal.h>
+#include <vector>
+
+// The process-wrapper implementation that was used until and including Bazel
+// 0.4.5. Waits for the wrapped process to exit and then kills its process
+// group. Works on all POSIX operating systems (tested on Linux, macOS,
+// FreeBSD).
+//
+// Caveats:
+// - Killing just the process group of the spawned child means that daemons or
+//   other processes spawned by the child may not be killed if they change their
+//   process group.
+// - Does not wait for grandchildren to exit, thus processes spawned by the
+//   child that could not be killed will linger around in the background.
+// - Has a PID reuse race condition, because the kill() to the process group is
+//   sent after waitpid() was called on the main child.
+class LegacyProcessWrapper {
+ public:
+  // Run the command specified in the `opt.args` array and kill it after
+  // `opt.timeout_secs` seconds.
+  static void RunCommand();
+
+ private:
+  static void SpawnChild();
+  static void WaitForChild();
+  static void OnSignal(int sig);
+
+  static pid_t child_pid;
+  static volatile sig_atomic_t last_signal;
+};
+
+#endif