runbg: new helper function inspired by the Infix project

The runbg() function is a delayed execvp() that has double-forked to the
background (to not create zombies).  It is useful for running commands
in the background for APIs that need to return a reply, e.g. a NETCONF
RPC to reboot the system.

Compared to the original in Infix, this version double forks to reparent
itself to PID 1 (init), closes all open files, including stdio.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/Makefile.am

@@ -11,12 +11,12 @@ libite_la_SOURCES     = chomp.c		copyfile.c			\
 			ifconfig.c	lfile.c				\
 			makepath.c	procval.c	progress.c	\
 			pidfile.c	pidfilefn.c	popenf.c	\
-			reallocarray.c	rsync.c		strtrim.c	\
+			reallocarray.c	rsync.c		runbg.c		\
 			strlcpy.c 	strlcat.c 	strtonum.c	\
 			strdupa.h	strndupa.h	strnlen.h	\
-			strmatch.c	systemf.c	touchf.c	\
+			strmatch.c	systemf.c	strtrim.c	\
 			telnet.c	tempfile.c	truncatef.c	\
-			yorn.c		which.c				\
+			touchf.c	yorn.c		which.c		\
 			lite.h		strlite.h			\
 			queue.h		tree.h
 

src/lite.h

@@ -128,6 +128,7 @@ void   *reallocarray(void *optr, size_t nmemb, size_t size);
 void    progress       (int percent, int max_width);
 void    progress_simple(int percent);
 
+int     runbg          (char *const cmd[], int delay);
 int     systemf        (const char *fmt, ...) __attribute__ ((format (printf, 1, 2)));
 
 sdbuf_t*telnet_open    (int addr, short port);

src/runbg.c

@@ -0,0 +1,93 @@
+/* Run a command in the background: fork() + usleep() + execvp()
+ *
+ * Copyright (c) 2023  Joachim Wiberg <troglobit@gmail.com>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/**
+ * @file runbg.c
+ * @author Joachim Wiberg
+ * @date 2023
+ * @copyright ISC License
+ */
+
+#include <errno.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+/**
+ * Run command in the background
+ * @param cmd    NULL terminated list of command and optional arguments
+ * @param delay  Microseconds after fork() to wait before calling cmd
+ *
+ * This function forks to run the given command @p cmd in the background.
+ * To ensure there are no lingering zombies the function actuall forks
+ * twice, and also calls setsid(), before it calls usleep() with the @p
+ * delay argument.  The command @p cmd is handed over to execvp().
+ *
+ * Since it runs in the background it is not possible to get the return
+ * code of the command.
+ *
+ * @returns on successful (first) fork(), this function returns POSIX
+ * OK(0), otherwise -1 and @a errno is set to indicate the error.
+ */
+int runbg(char *const cmd[], int delay)
+{
+	int pid = fork();
+	int rc;
+
+	if (!pid) {
+		int fd, maxfd;
+
+		if (setsid() == -1)
+			_exit(errno);
+
+		/* reparent to init */
+		pid = fork();
+		if (pid == -1)
+			_exit(errno);
+		if (pid > 0)
+			_exit(0);
+
+		maxfd = sysconf(_SC_OPEN_MAX);
+		if (maxfd == -1)
+			maxfd = 8192;
+		for (fd = 0; fd < maxfd; fd++)
+			close(fd);
+
+		usleep(delay);
+		_exit(execvp(cmd[0], cmd));
+	}
+
+	if (pid == -1)
+		return -1;
+
+	if (waitpid(pid, &rc, 0) == -1)
+		return -1;
+
+	if (WIFEXITED(rc)) {
+		errno = WEXITSTATUS(rc);
+		if (errno)
+			rc = -1;
+		else
+			rc = 0;
+	} else if (WIFSIGNALED(rc)) {
+		errno = EINTR;
+		rc = -1;
+	} else
+		rc = -1;
+
+	return rc;
+}