common: move json_stream from lightningd/ to common/

It's not lightningd-specific and we are going to need it for libplugin. The only
drawback is the log_io removal in json_stream_output_write()..

common/json_stream.h

@@ -0,0 +1,134 @@
+/* lightningd/json_stream.h
+ * Helpers for outputting JSON results into a membuf.
+ */
+#ifndef LIGHTNING_COMMON_JSON_STREAM_H
+#define LIGHTNING_COMMON_JSON_STREAM_H
+#include "config.h"
+#include <ccan/membuf/membuf.h>
+#include <ccan/short_types/short_types.h>
+#include <ccan/tal/tal.h>
+#include <ccan/typesafe_cb/typesafe_cb.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+struct command;
+struct io_conn;
+struct json_stream;
+struct log;
+
+/**
+ * new_json_stream - create a new JSON stream.
+ * @ctx: tal context for allocation.
+ * @writer: object responsible for writing to this stream.
+ * @log: where to log the IO
+ */
+struct json_stream *new_json_stream(const tal_t *ctx, struct command *writer,
+				    struct log *log);
+
+/**
+ * Duplicate an existing stream.
+ *
+ * Mostly useful when we want to send copies of a given stream to
+ * multiple recipients, that might read at different speeds from the
+ * stream. For example this is used when construcing a single
+ * notification and then duplicating it for the fanout.
+ *
+ * @ctx: tal context for allocation.
+ * @original: the stream to duplicate.
+ * @log: log for new stream.
+ */
+struct json_stream *json_stream_dup(const tal_t *ctx,
+				    struct json_stream *original,
+				    struct log *log);
+
+/**
+ * json_stream_close - finished writing to a JSON stream.
+ * @js: the json_stream.
+ * @writer: object responsible for writing to this stream.
+ */
+void json_stream_close(struct json_stream *js, struct command *writer);
+
+/* For low-level JSON stream access: */
+void json_stream_log_suppress(struct json_stream *js, const char *cmd_name);
+
+/**
+ * json_stream_still_writing - is someone currently writing to this stream?
+ * @js: the json_stream.
+ *
+ * Has this json_stream not been closed yet?
+ */
+bool json_stream_still_writing(const struct json_stream *js);
+
+
+/* '"fieldname" : [ ' or '[ ' if fieldname is NULL */
+void json_array_start(struct json_stream *js, const char *fieldname);
+/* '"fieldname" : { ' or '{ ' if fieldname is NULL */
+void json_object_start(struct json_stream *ks, const char *fieldname);
+/* '],' */
+void json_array_end(struct json_stream *js);
+/* '},' */
+void json_object_end(struct json_stream *js);
+/* ' },' */
+void json_object_compat_end(struct json_stream *js);
+
+/**
+ * json_stream_append - literally insert this string into the json_stream.
+ * @js: the json_stream.
+ * @str: the string.
+ * @len: the length to append (<= strlen(str)).
+ */
+void json_stream_append(struct json_stream *js, const char *str, size_t len);
+
+/**
+ * json_add_member - add a generic member.
+ * @js: the json_stream.
+ * @fieldname: fieldname (if in object), otherwise must be NULL.
+ * @quote: true if should be escaped and wrapped in "".
+ * @fmt...: the printf-style format
+ *
+ * The resulting string from @fmt is escaped if quote is true:
+ * see json_member_direct to avoid quoting.
+ */
+void json_add_member(struct json_stream *js,
+		     const char *fieldname,
+		     bool quote,
+		     const char *fmt, ...) PRINTF_FMT(4,5);
+
+/**
+ * json_member_direct - start a generic member.
+ * @js: the json_stream.
+ * @fieldname: fieldname (if in object), otherwise must be NULL.
+ * @extra: the space to reserve.
+ *
+ * Returns NULL if oom, otherwise returns a ptr to @extra bytes.
+ */
+char *json_member_direct(struct json_stream *js,
+			 const char *fieldname, size_t extra);
+
+/**
+ * json_stream_output - start writing out a json_stream to this conn.
+ * @js: the json_stream
+ * @conn: the io_conn to write out to.
+ * @cb: the callback to call once it's all written.
+ * @arg: the argument to @cb
+ */
+#define json_stream_output(js, conn, cb, arg)				\
+	json_stream_output_((js), (conn),				\
+			    typesafe_cb_preargs(struct io_plan *,	\
+						void *,			\
+						(cb), (arg),		\
+						struct io_conn *,	\
+						struct json_stream *), \
+			    (arg))
+
+struct io_plan *json_stream_output_(struct json_stream *js,
+				    struct io_conn *conn,
+				    struct io_plan *(*cb)(struct io_conn *conn,
+							  struct json_stream *js,
+							  void *arg),
+				    void *arg);
+
+void json_stream_flush(struct json_stream *js);
+
+#endif /* LIGHTNING_COMMON_JSON_STREAM_H */