lightningd: add dev option to save hooks and notifications to/from plugins.

diff --git a/lightningd/options.c b/lightningd/options.c
index b55238c5..252fcb54 100644
--- a/lightningd/options.c
+++ b/lightningd/options.c
@@ -931,6 +931,10 @@ static void dev_register_opts(struct lightningd *ld)
 		       opt_set_bool,
 		       &ld->dev_hsmd_warn_on_overgrind,
 		       "Warn if we create signatures that are not exactly 71 bytes.");
+	clnopt_witharg("--dev-save-plugin-io", OPT_DEV,
+		       opt_set_charp, opt_show_charp,
+		       &ld->plugins->dev_save_io,
+		       "Directory to place all plugin notifications/hooks JSON into.");
 	/* This is handled directly in daemon_developer_mode(), so we ignore it here */
 	clnopt_noarg("--dev-debug-self", OPT_DEV,
 		     opt_ignore,
diff --git a/lightningd/plugin.c b/lightningd/plugin.c
index ccbdf83a..1dba9a94 100644
--- a/lightningd/plugin.c
+++ b/lightningd/plugin.c
@@ -4,9 +4,11 @@
 #include <ccan/crc32c/crc32c.h>
 #include <ccan/io/io.h>
 #include <ccan/json_escape/json_escape.h>
+#include <ccan/json_out/json_out.h>
 #include <ccan/mem/mem.h>
 #include <ccan/opt/opt.h>
 #include <ccan/pipecmd/pipecmd.h>
+#include <ccan/read_write_all/read_write_all.h>
 #include <ccan/tal/path/path.h>
 #include <ccan/tal/str/str.h>
 #include <ccan/utf8/utf8.h>
@@ -23,6 +25,7 @@
 #include <db/exec.h>
 #include <dirent.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <lightningd/io_loop_with_timers.h>
 #include <lightningd/notification.h>
 #include <lightningd/plugin.h>
@@ -76,6 +79,7 @@ struct plugins *plugins_new(const tal_t *ctx, struct log_book *log_book,
 	p->plugin_idx = 0;
 	p->dev_builtin_plugins_unimportant = false;
 	p->want_db_transaction = true;
+	p->dev_save_io = NULL;
 	p->subscriptions = tal(p, struct plugin_subscription_htable);
 	plugin_subscription_htable_init(p->subscriptions);
 
@@ -2534,6 +2538,10 @@ void plugins_notify(struct plugins *plugins,
 	if (!plugins)
 		return;
 
+	dev_save_plugin_io_out(plugins,
+			       "notification_out",
+			       n->method, n->stream);
+
 	for (struct plugin_subscription *sub
 		     = plugin_subscription_htable_getfirst(plugins->subscriptions,
 							   n->method, &it);
@@ -2673,3 +2681,49 @@ void shutdown_plugins(struct lightningd *ld)
 		}
 	}
 }
+
+static void dev_save_plugin_io(struct plugins *plugins,
+ 			       const char *type,
+			       const char *name,
+			       const char *buf, size_t len)
+{
+	static size_t counter;
+	const char *file;
+	int fd;
+
+	if (!plugins->dev_save_io)
+		return;
+
+	file = path_join(tmpctx, plugins->dev_save_io,
+			 take(tal_fmt(NULL, "%s-%s-%u-%zu",
+				      type, name,
+				      (unsigned int)getpid(),
+				      counter++)));
+	fd = open(file, O_CREAT|O_EXCL|O_WRONLY, 0600);
+	if (fd < 0 || !write_all(fd, buf, len))
+		fatal("Writing --dev-save-plugin-io %s: %s",
+		      file, strerror(errno));
+	close(fd);
+}
+
+void dev_save_plugin_io_in(struct plugins *plugins,
+			   const char *type,
+			   const char *name,
+			   const char *buffer,
+			   const jsmntok_t *tok)
+{
+	dev_save_plugin_io(plugins, type, name,
+			   buffer + tok->start, tok->end - tok->start);
+}
+
+void dev_save_plugin_io_out(struct plugins *plugins,
+			    const char *type,
+			    const char *name,
+			    const struct json_stream *stream)
+{
+	size_t len;
+	const char *buf;
+
+	buf = json_out_contents(stream->jout, &len);
+	dev_save_plugin_io(plugins, type, name, buf, len);
+}
diff --git a/lightningd/plugin.h b/lightningd/plugin.h
index 14904e3f..621f3b8d 100644
--- a/lightningd/plugin.h
+++ b/lightningd/plugin.h
@@ -154,6 +154,9 @@ struct plugins {
 
 	/* Whether builtin plugins should be overridden as unimportant.  */
 	bool dev_builtin_plugins_unimportant;
+
+	/* Whether to save all IO to a file */
+	char *dev_save_io;
 };
 
 /**
@@ -415,4 +418,17 @@ struct command_result *plugin_set_dynamic_opt(struct command *cmd,
 					       const struct opt_table *,
 					       const char *,
 					       bool));
+
+/* --dev-plugin-save-io */
+void dev_save_plugin_io_in(struct plugins *plugins,
+			   const char *type,
+			   const char *name,
+			   const char *buffer,
+			   const jsmntok_t *tok);
+
+void dev_save_plugin_io_out(struct plugins *plugins,
+			    const char *type,
+			    const char *name,
+			    const struct json_stream *stream);
+
 #endif /* LIGHTNING_LIGHTNINGD_PLUGIN_H */
diff --git a/lightningd/plugin_hook.c b/lightningd/plugin_hook.c
index b22d1546..cd69b794 100644
--- a/lightningd/plugin_hook.c
+++ b/lightningd/plugin_hook.c
@@ -169,6 +169,9 @@ static void plugin_hook_callback(const char *buffer, const jsmntok_t *toks,
 			      ph_req->hook->name, toks->end - toks->start,
 			      buffer + toks->start);
 
+		dev_save_plugin_io_in(h->plugin->plugins, "hook_in",
+				      ph_req->hook->name,
+				      buffer, toks);
 		if (!ph_req->hook->deserialize_cb(ph_req->cb_arg,
 						  buffer, resulttok)) {
 			tal_free(ph_req->cb_arg);
@@ -209,6 +212,11 @@ static void plugin_hook_call_next(struct plugin_hook_request *ph_req)
 
 	hook->serialize_payload(ph_req->cb_arg, req->stream, plugin);
 	jsonrpc_request_end(req);
+
+	dev_save_plugin_io_out(plugin->plugins,
+			       "hook_out", hook->name,
+			       req->stream);
+
 	plugin_request_send(plugin, req);
 }