buffer: identify messages with length over the buffer limit

Identify messages that are over the 4KiB buffer limit

Address issue reported downstream by Debian[1] and upstream by kaliko
(@mxjeff)[2]
[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=953110
[2] MusicPlayerDaemon#51

src/async.c

@@ -129,6 +129,20 @@ mpd_async_copy_error(const struct mpd_async *async,
 	return mpd_error_copy(dest, &async->error);
 }
 
+bool
+mpd_async_set_error(struct mpd_async *async, enum mpd_error error,
+		    const char *error_message)
+{
+	assert(async != NULL);
+
+	if (mpd_error_is_defined(&async->error))
+		return false;
+
+	mpd_error_code(&async->error, error);
+	mpd_error_message(&async->error, error_message);
+	return true;
+}
+
 int
 mpd_async_get_fd(const struct mpd_async *async)
 {

src/iasync.h

@@ -44,4 +44,15 @@ bool
 mpd_async_copy_error(const struct mpd_async *async,
 		     struct mpd_error_info *dest);
 
+/**
+ * Sets the object's error condition.
+ *
+ * @return true if there was no error in #async, false if an error
+ * condition is stored in #async; the #error is only set if there was no error
+ * previously present
+ */
+bool
+mpd_async_set_error(struct mpd_async *async, enum mpd_error error,
+		    const char *error_message);
+
 #endif

src/sync.c

@@ -29,6 +29,7 @@
 #include "sync.h"
 #include "socket.h"
 #include "binary.h"
+#include "iasync.h"
 
 #include <mpd/async.h>
 
@@ -115,6 +116,13 @@ mpd_sync_send_command_v(struct mpd_async *async, const struct timeval *tv0,
 		success = mpd_async_send_command_v(async, command, copy);
 		va_end(copy);
 
+		/* no characters were written to async buffer */
+		if ((mpd_async_events(async) & MPD_ASYNC_EVENT_WRITE) == 0) {
+			(void)mpd_async_set_error(async, MPD_ERROR_OOM,
+					"Not enough buffer space for message");
+			return false;
+		}
+
 		if (success)
 			return true;