status: API for status reporting.

The API formalizes how daemons should report their statuses back to
the main lightningd.  It's a simple write API, which includes tracing
support (currently it always sends traces, later it could send iff
there's a failure, for example).

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

status.h

@@ -0,0 +1,29 @@
+#ifndef LIGHTNING_STATUS_H
+#define LIGHTNING_STATUS_H
+#include "config.h"
+#include <ccan/compiler/compiler.h>
+#include <ccan/short_types/short_types.h>
+#include <stdlib.h>
+
+/* Simple status reporting API. */
+void status_setup(int fd);
+
+/* Convenient context, frees up after every status_update/failed */
+extern const void *trc;
+
+/* Special status code for tracing messages. */
+#define STATUS_TRACE 0x7FFF
+
+/* Failure codes always have high bit set. */
+#define STATUS_FAIL 0x8000
+
+/* Send a message (frees the message). */
+void status_send(const u8 *msg);
+/* Send a file descriptor (closes fd). */
+void status_send_fd(int fd);
+/* Send a printf-style debugging trace. */
+void status_trace(const char *fmt, ...) PRINTF_FMT(1,2);
+/* Send a failure status code with printf-style msg, and exit. */
+void status_failed(u16 code, const char *fmt, ...) PRINTF_FMT(2,3) NORETURN;
+
+#endif /* LIGHTNING_STATUS_H */