drivers/main.c: Split backgrounding driver into two stages

This is needed in particular with libusb on linux as it will create
a thread to handle either netlink or udev messages. If this is created
before we background, the driver will hang during exit trying to join
the non-existant thread in the new process. Shows this warning while
stopping the driver.

Stopping /run/nut/usbhid-ups-xxxx.pid failed, retrying harder: Success

Signed-off-by: Doug Nazar <nazard@nazar.ca>

Author: dougnazar

common/common.c

@@ -797,30 +797,25 @@ void open_syslog(const char *progname)
 #endif	/* WIN32 */
 }
 
-/* close ttys and become a daemon */
-void background(void)
+int background_fork(void)
 {
-	/* Normally we enable SYSLOG and disable STDERR,
-	 * unless NUT_DEBUG_SYSLOG envvar interferes as
-	 * interpreted in syslog_is_disabled() method: */
-	int	syslog_disabled = syslog_is_disabled(),
-		stderr_disabled = (syslog_disabled == 0 || syslog_disabled == 2);
+	int	pid = 0;
 
 #ifndef WIN32
-	int	pid;
-
 	if ((pid = fork()) < 0)
 		fatal_with_errno(EXIT_FAILURE, "Unable to enter background");
 #endif	/* !WIN32 */
 
-	if (!syslog_disabled)
-		/* not disabled: NUT_DEBUG_SYSLOG is unset or invalid */
-		xbit_set(&upslog_flags, UPSLOG_SYSLOG);
-	if (stderr_disabled)
-		/* NUT_DEBUG_SYSLOG="none" or unset/invalid */
-		xbit_clear(&upslog_flags, UPSLOG_STDERR);
+	return pid;
+}
 
+/* close ttys and become a daemon */
+void background(void)
+{
 #ifndef WIN32
+	int	pid;
+
+	pid = background_fork();
 	if (pid != 0) {
 		/* parent */
 		/* these are typically fds 0-2: */
@@ -829,9 +824,28 @@ void background(void)
 		close(STDERR_FILENO);
 		_exit(EXIT_SUCCESS);
 	}
+#else	/* WIN32 */
+	NUT_WIN32_INCOMPLETE_MAYBE_NOT_APPLICABLE();
+#endif	/* WIN32 */
+	background_child();
+}
+
+void background_child(void)
+{
+	/* Normally we enable SYSLOG and disable STDERR,
+	 * unless NUT_DEBUG_SYSLOG envvar interferes as
+	 * interpreted in syslog_is_disabled() method: */
+	int	syslog_disabled = syslog_is_disabled(),
+		stderr_disabled = (syslog_disabled == 0 || syslog_disabled == 2);
 
-	/* child */
+	if (!syslog_disabled)
+		/* not disabled: NUT_DEBUG_SYSLOG is unset or invalid */
+		xbit_set(&upslog_flags, UPSLOG_SYSLOG);
+	if (stderr_disabled)
+		/* NUT_DEBUG_SYSLOG="none" or unset/invalid */
+		xbit_clear(&upslog_flags, UPSLOG_STDERR);
 
+#ifndef WIN32
 	/* make fds 0-2 (typically) point somewhere defined */
 # ifdef HAVE_DUP2
 	/* system can close (if needed) and (re-)open a specific FD number */

drivers/main.c

@@ -2188,6 +2188,7 @@ int main(int argc, char **argv)
 # ifndef WIN32
 	int	cmd = 0;
 	pid_t	oldpid = -1;
+	int	background_pipefd[2];
 # else	/* WIN32 */
 /* FIXME NUT_WIN32_INCOMPLETE : *actually* handle WIN32 builds too */
 	const char	* cmd = NULL;
@@ -2995,6 +2996,33 @@ int main(int argc, char **argv)
 	 * when its a pdu! */
 	dstate_setinfo("device.type", "ups");
 
+#ifndef WIN32
+	if (foreground == 0) {
+		/* start backgrounding */
+		int ret, pid;
+
+		ret = pipe(background_pipefd);
+		if (ret)
+			fatal_with_errno(EXIT_FAILURE, "pipe creation failed");
+
+		pid = background_fork();
+		if (pid > 0) {
+			/* parent: wait for child to send success or exit */
+			char ch;
+
+			close(background_pipefd[1]);
+			// Wait for child
+			ret = read(background_pipefd[0], &ch, 1);
+
+			close(background_pipefd[0]);
+			close(STDIN_FILENO);
+			close(STDOUT_FILENO);
+			close(STDERR_FILENO);
+			_exit(ret == 1 ? EXIT_SUCCESS : EXIT_FAILURE);
+		}
+	}
+#endif
+
 	dstate_setinfo("driver.state", "init.device");
 	upsdrv_callbacks.upsdrv_initups();
 	dstate_setinfo("driver.state", "init.quiet");
@@ -3192,11 +3220,23 @@ int main(int argc, char **argv)
 
 	switch (foreground) {
 		case 0:
+#ifndef WIN32
+			/* close handles */
+			background_child();
+
+			/* notify parent of success */
+			i = write(background_pipefd[1], "G", 1);
+			if (i < 1)
+				upslogx(LOG_ERR, "Unable to call parent pipe for shutdown");
+			close(background_pipefd[1]);
+#else
 			background();
+#endif
 			/* We had saved a PID before backgrounding, but
 			 * it changes when backgrounding - so save again
 			 */
 			writepid(pidfn);
+
 			break;
 
 		/* >0: Keep the initial PID; don't care about "!dump_data" here

include/common.h

@@ -286,6 +286,10 @@ void open_syslog(const char *progname);
 /* close ttys and become a daemon */
 void background(void);
 
+/* Support functions for backgrounding */
+int background_fork(void);
+void background_child(void);
+
 /* allow tagging the (forked) process in logs to ease debugging */
 const char *getproctag(void);
 /* save a copy of tag, or call with NULL to clean and free the internal buffer;