askrene: fork before calling the route solver.

This is fairly simple.  We do all the prep work, fire off the child,
and it continues all the way to producing JSON output (or an error).
The parent then forwards it.

Limitations (fixed in successive patches):

1. Child logging currently gets lost.
2. We wait for the child, so this code is not a speedup.

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

Author: rustyrussell

plugins/askrene/askrene.c

@@ -8,6 +8,10 @@
  */
 #include "config.h"
 #include <ccan/array_size/array_size.h>
+#include <ccan/json_out/json_out.h>
+#include <ccan/noerr/noerr.h>
+#include <ccan/read_write_all/read_write_all.h>
+#include <ccan/tal/grab_file/grab_file.h>
 #include <ccan/tal/str/str.h>
 #include <common/clock_time.h>
 #include <common/dijkstra.h>
@@ -24,6 +28,8 @@
 #include <plugins/askrene/layer.h>
 #include <plugins/askrene/mcf.h>
 #include <plugins/askrene/reserve.h>
+#include <sys/wait.h>
+#include <unistd.h>
 
 /* "spendable" for a channel assumes a single HTLC: for additional HTLCs,
  * the need to pay for fees (if we're the owner) reduces it */
@@ -561,18 +567,106 @@ void get_constraints(const struct route_query *rq,
 	reserve_sub(rq->reserved, &scidd, rq->layers, max);
 }
 
+/* Returns fd to child */
+static int fork_router_child(struct route_query *rq,
+			     enum algorithm algo,
+			     struct timemono deadline,
+			     const struct gossmap_node *srcnode,
+			     const struct gossmap_node *dstnode,
+			     struct amount_msat amount,
+			     struct amount_msat maxfee,
+			     u32 finalcltv, u32 maxdelay,
+			     const char *cmd_id,
+			     struct json_filter *cmd_filter,
+			     int *child_pid)
+{
+	int replyfds[2];
+	double probability;
+	struct flow **flows;
+	struct route **routes;
+	struct amount_msat *amounts;
+	const char *err, *p;
+	size_t len;
+
+	if (pipe(replyfds) != 0)
+		return -1;
+	*child_pid = fork();
+	if (*child_pid < 0) {
+		close_noerr(replyfds[0]);
+		close_noerr(replyfds[1]);
+		return -1;
+	}
+	if (*child_pid != 0) {
+		close(replyfds[1]);
+		return replyfds[0];
+	}
+
+	/* We are the child.  Run the algo */
+	close(replyfds[0]);
+	if (algo == ALGO_SINGLE_PATH) {
+		err = single_path_routes(rq, rq, deadline, srcnode, dstnode,
+					 amount, maxfee, finalcltv,
+					 maxdelay, &flows, &probability);
+	} else {
+		assert(algo == ALGO_DEFAULT);
+		err = default_routes(rq, rq, deadline, srcnode, dstnode,
+				     amount, maxfee, finalcltv, maxdelay,
+				     &flows, &probability);
+	}
+	if (err) {
+		write_all(replyfds[1], err, strlen(err));
+		/* Non-zero exit tells parent this is an error string. */
+		exit(1);
+	}
+
+	/* otherwise we continue */
+	assert(tal_count(flows) > 0);
+	rq_log(tmpctx, rq, LOG_DBG, "Final answer has %zu flows",
+	       tal_count(flows));
+
+	/* convert flows to routes */
+	routes = convert_flows_to_routes(rq, rq, finalcltv, flows,
+					 &amounts);
+	assert(tal_count(routes) == tal_count(flows));
+	assert(tal_count(amounts) == tal_count(flows));
+
+	/* output the results */
+	struct json_stream *js = new_json_stream(tmpctx, NULL, NULL);
+	json_object_start(js, NULL);
+	json_add_string(js, "jsonrpc", "2.0");
+	json_add_id(js, cmd_id);
+	json_object_start(js, "result");
+	if (cmd_filter)
+		json_stream_attach_filter(js, cmd_filter);
+	json_add_getroutes(js, routes, amounts, probability, finalcltv);
+
+	/* Detach filter before it complains about closing object it never saw */
+	if (cmd_filter) {
+		err = json_stream_detach_filter(tmpctx, js);
+		if (err)
+			json_add_string(js, "warning_parameter_filter", err);
+	}
+	/* "result" object */
+	json_object_end(js);
+	/* Global object */
+	json_object_end(js);
+	json_stream_close(js, NULL);
+
+	p = json_out_contents(js->jout, &len);
+	if (!write_all(replyfds[1], p, len))
+		abort();
+	exit(0);
+}
+
 static struct command_result *do_getroutes(struct command *cmd,
 					   struct gossmap_localmods *localmods,
 					   struct getroutes_info *info)
 {
 	struct askrene *askrene = get_askrene(cmd->plugin);
 	struct route_query *rq = tal(cmd, struct route_query);
-	const char *err;
-	double probability;
-	struct amount_msat *amounts;
-	struct route **routes;
-	struct flow **flows;
-	struct json_stream *response;
+	const char *err, *json;
+	struct timemono time_start, deadline;
+	int child_fd, child_pid, child_status;
 
 	/* update the gossmap */
 	if (gossmap_refresh(askrene->gossmap)) {
@@ -655,50 +749,54 @@ static struct command_result *do_getroutes(struct command *cmd,
 		       "maxparts == 1: switching to a single path algorithm.");
 	}
 
-	/* Compute the routes. At this point we might select between multiple
-	 * algorithms. Right now there is only one algorithm available. */
-	struct timemono time_start = time_mono();
-	struct timemono deadline = timemono_add(time_start,
-						time_from_sec(askrene->route_seconds));
-	if (info->dev_algo == ALGO_SINGLE_PATH) {
-		err = single_path_routes(rq, rq, deadline, srcnode, dstnode, info->amount,
-					 info->maxfee, info->finalcltv,
-					 info->maxdelay, &flows, &probability);
-	} else {
-		assert(info->dev_algo == ALGO_DEFAULT);
-		err = default_routes(rq, rq, deadline, srcnode, dstnode, info->amount,
-				     info->maxfee, info->finalcltv,
-				     info->maxdelay, &flows, &probability);
+	time_start = time_mono();
+	deadline = timemono_add(time_start,
+				time_from_sec(askrene->route_seconds));
+	child_fd = fork_router_child(rq, info->dev_algo,
+				     deadline, srcnode, dstnode, info->amount,
+				     info->maxfee, info->finalcltv, info->maxdelay,
+				     cmd->id, cmd->filter, &child_pid);
+	if (child_fd == -1) {
+		err = tal_fmt(tmpctx, "failed to fork: %s", strerror(errno));
+		goto fail_broken;
 	}
+
+	/* FIXME: Go async! */
+	json = grab_fd_str(cmd, child_fd);
+	close(child_fd);
+	waitpid(child_pid, &child_status, 0);
+
 	struct timerel time_delta = timemono_between(time_mono(), time_start);
 
 	/* log the time of computation */
 	rq_log(tmpctx, rq, LOG_DBG, "get_routes %s %" PRIu64 " ms",
-	       err ? "failed after" : "completed in",
+	       WEXITSTATUS(child_status) != 0 ? "failed after" : "completed in",
 	       time_to_msec(time_delta));
-	if (err)
-		goto fail;
 
-	/* otherwise we continue */
-	assert(tal_count(flows) > 0);
-	rq_log(tmpctx, rq, LOG_DBG, "Final answer has %zu flows",
-	       tal_count(flows));
-
-	/* convert flows to routes */
-	routes = convert_flows_to_routes(rq, rq, info->finalcltv, flows,
-					 &amounts);
-	assert(tal_count(routes) == tal_count(flows));
-	assert(tal_count(amounts) == tal_count(flows));
+	if (WIFSIGNALED(child_status)) {
+		err = tal_fmt(tmpctx, "child died with signal %u",
+			      WTERMSIG(child_status));
+		goto fail_broken;
+	}
+	/* This is how it indicates an error message */
+	if (WEXITSTATUS(child_status) != 0 && json) {
+		err = json;
+		goto fail;
+	}
+	if (!json) {
+		err = tal_fmt(tmpctx, "child produced no output (exited %i)?",
+			      WEXITSTATUS(child_status));
+		goto fail_broken;
+	}
 
 	/* At last we remove the localmods from the gossmap. */
 	gossmap_remove_localmods(askrene->gossmap, localmods);
 
-	/* output the results */
-	response = jsonrpc_stream_success(cmd);
-	json_add_getroutes(response, routes, amounts, probability,
-			   info->finalcltv);
-	return command_finished(cmd, response);
+	/* Child already created this fully formed.  We just paste it */
+	return command_finish_rawstr(cmd, json, strlen(json));
 
+fail_broken:
+	plugin_log(cmd->plugin, LOG_BROKEN, "%s", err);
 fail:
 	assert(err);
 	gossmap_remove_localmods(askrene->gossmap, localmods);