static const int PIPE_IN = -3;
static const int PIPE_OUT = -4;
+ /**
+ * See Subprocess::Options::dangerousPostForkPreExecCallback() for usage.
+ * Every derived class should include the following warning:
+ *
+ * DANGER: This class runs after fork in a child processes. Be fast, the
+ * parent thread is waiting, but remember that other parent threads are
+ * running and may mutate your state. Avoid mutating any data belonging to
+ * the parent. Avoid interacting with non-POD data that originated in the
+ * parent. Avoid any libraries that may internally reference non-POD data.
+ * Especially beware parent mutexes -- for example, glog's LOG() uses one.
+ */
+ struct DangerousPostForkPreExecCallback {
+ virtual ~DangerousPostForkPreExecCallback() {}
+ // This must return 0 on success, or an `errno` error code.
+ virtual int operator()() = 0;
+ };
+
/**
* Class representing various options: file descriptor behavior, and
* whether to use $PATH for searching for the executable,
return *this;
}
+ /**
+ * *** READ THIS WHOLE DOCBLOCK BEFORE USING ***
+ *
+ * Run this callback in the child after the fork, just before the
+ * exec(), and after the child's state has been completely set up:
+ * - signal handlers have been reset to default handling and unblocked
+ * - the working directory was set
+ * - closed any file descriptors specified via Options()
+ * - set child process flags (see code)
+ *
+ * This is EXTREMELY DANGEROUS. For example, this innocuous-looking code
+ * can cause a fraction of your Subprocess launches to hang forever:
+ *
+ * LOG(INFO) << "Hello from the child";
+ *
+ * The reason is that glog has an internal mutex. If your fork() happens
+ * when the parent has the mutex locked, the child will wait forever.
+ *
+ * == GUIDELINES ==
+ *
+ * - Be quick -- the parent thread is blocked until you exit.
+ * - Remember that other parent threads are running, and may mutate your
+ * state.
+ * - Avoid mutating any data belonging to the parent.
+ * - Avoid interacting with non-POD data that came from the parent.
+ * - Avoid any libraries that may internally reference non-POD state.
+ * - Especially beware parent mutexes, e.g. LOG() uses a global mutex.
+ * - Avoid invoking the parent's destructors (you can accidentally
+ * delete files, terminate network connections, etc).
+ * - Read http://ewontfix.com/7/
+ */
+ Options& dangerousPostForkPreExecCallback(
+ DangerousPostForkPreExecCallback* cob) {
+ dangerousPostForkPreExecCallback_ = cob;
+ return *this;
+ }
+
/**
* Helpful way to combine Options.
*/
int parentDeathSignal_{0};
#endif
bool processGroupLeader_{false};
+ DangerousPostForkPreExecCallback*
+ dangerousPostForkPreExecCallback_{nullptr};
};
static Options pipeStdin() { return Options().stdin(PIPE); }