lightningd: implement --daemon.

Includes closing off stdout and stderr.  We don't do it directly in the
arg parser, as we want to interact normally (eg with other errors) before
we turn off stdout/stderr.

Fixes: #986
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/lightningd.c

@@ -7,6 +7,7 @@
 #include <ccan/array_size/array_size.h>
 #include <ccan/cast/cast.h>
 #include <ccan/crypto/hkdf_sha256/hkdf_sha256.h>
+#include <ccan/daemonize/daemonize.h>
 #include <ccan/err/err.h>
 #include <ccan/io/fdpass/fdpass.h>
 #include <ccan/io/io.h>
@@ -22,6 +23,7 @@
 #include <common/utils.h>
 #include <common/version.h>
 #include <common/wireaddr.h>
+#include <errno.h>
 #include <lightningd/bitcoind.h>
 #include <lightningd/chaintopology.h>
 #include <lightningd/invoice.h>
@@ -68,6 +70,7 @@ static struct lightningd *new_lightningd(const tal_t *ctx,
 	timers_init(&ld->timers, time_mono());
 	ld->topology = new_topology(ld, ld->log);
 	ld->debug_subdaemon_io = NULL;
+	ld->daemon = false;
 
 	return ld;
 }
@@ -230,6 +233,24 @@ static void init_txfilter(struct wallet *w, struct txfilter *filter)
 	}
 }
 
+static void daemonize_but_keep_dir(void)
+{
+	/* daemonize moves us into /, but we want to be here */
+	const char *cwd = path_cwd(NULL);
+
+	if (!cwd)
+		fatal("Could not get current directory: %s", strerror(errno));
+	if (!daemonize())
+		fatal("Could not become a daemon: %s", strerror(errno));
+
+	/* Move back: important, since lightning dir may be relative! */
+	if (chdir(cwd) != 0)
+		fatal("Could not return to directory %s: %s",
+		      cwd, strerror(errno));
+
+	tal_free(cwd);
+}
+
 int main(int argc, char *argv[])
 {
 	struct log_book *log_book;
@@ -331,6 +352,10 @@ int main(int argc, char *argv[])
 	/* Create RPC socket (if any) */
 	setup_jsonrpc(ld, ld->rpc_filename);
 
+	/* Now we're about to start, become daemon if desired. */
+	if (ld->daemon)
+		daemonize_but_keep_dir();
+
 	/* Mark ourselves live. */
 	log_info(ld->log, "Server started with public key %s, alias %s (color #%s) and lightningd %s",
 		 type_to_string(ltmp, struct pubkey, &ld->id),

lightningd/lightningd.h

@@ -77,6 +77,9 @@ struct lightningd {
 	/* The directory to find all the subdaemons. */
 	const char *daemon_dir;
 
+	/* Are we told to run in the background. */
+	bool daemon;
+
 	/* Our config dir, and rpc file */
 	char *config_dir;
 	char *rpc_filename;

lightningd/options.c

@@ -7,6 +7,7 @@
 #include <ccan/read_write_all/read_write_all.h>
 #include <ccan/short_types/short_types.h>
 #include <ccan/tal/grab_file/grab_file.h>
+#include <ccan/tal/path/path.h>
 #include <ccan/tal/str/str.h>
 #include <common/configdir.h>
 #include <common/memleak.h>
@@ -211,6 +212,8 @@ static char *opt_set_fee_rates(const char *arg, struct chain_topology *topo)
 
 static void config_register_opts(struct lightningd *ld)
 {
+	opt_register_noarg("--daemon", opt_set_bool, &ld->daemon,
+			 "Run in the background, suppress stdout/stderr");
 	opt_register_arg("--ignore-fee-limits", opt_set_bool_arg, opt_show_bool,
 			 &ld->config.ignore_fee_limits,
 			 "(DANGEROUS) allow peer to set any feerate");