Limit the number of simultanious children

Make sure minidlna cannot fork more than 5 children, to avoid becoming a
fork bomb.

Reported-by: Rinat Ibragimov <ibragimovrinat@mail.ru>

process.h

@@ -0,0 +1,66 @@
+/* Process handling
+ *
+ * Copyright © 2013, Benoît Knecht <benoit.knecht@fsfe.org>
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * The name of the author may not be used to endorse or promote products
+ *       derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+#ifndef __PROCESS_H__
+#define __PROCESS_H__
+
+#include <unistd.h>
+
+/**
+ * Fork a new child (just like fork()) but keep track of how many childs are
+ * already running, and refuse fo fork if there are too many.
+ * @return -1 if it couldn't fork, 0 in the child process, the pid of the
+ *         child process in the parent process.
+ */
+pid_t process_fork(void);
+
+/**
+ * Handler to be called upon receiving SIGCHLD. This signal is received by the
+ * parent process when a child terminates, and this handler updates the number
+ * of running childs accordingly.
+ * @param signal The signal number.
+ */
+void process_handle_child_termination(int signal);
+
+/**
+ * Daemonize the current process by forking itself and redirecting standard
+ * input, standard output and standard error to /dev/null.
+ * @return The pid of the process.
+ */ 
+int process_daemonize(void);
+
+/**
+ * Check if the process corresponding to the pid found in the pid file is
+ * running.
+ * @param fname The path to the pid file.
+ * @return 0 if no other instance is running, -1 if the file name is invalid,
+ *         -2 if another instance is running.
+ */
+int process_check_if_running(const char *fname);
+
+#endif // __PROCESS_H__