Make stderr line-buffered w/--msgs2stderr.

options.c

@@ -1852,6 +1852,12 @@ int parse_arguments(int *argc_p, const char ***argv_p)
 		}
 		setvbuf(stdout, (char *)NULL, mode, 0);
 	}
+
+	if (msgs2stderr) {
+		/* Make stderr line buffered for better sharing of the stream. */
+		fflush(stderr); /* Just in case... */
+		setvbuf(stderr, (char *)NULL, _IOLBF, 0);
+	}
 #endif
 
 	set_output_verbosity(verbose, DEFAULT_PRIORITY);

rsync.yo

@@ -565,6 +565,9 @@ messages back to the client side, so if you are doing any daemon-transfer
 debugging using this option, you should start up a daemon using bf(--no-detach)
 so that you can see the stderr output on the daemon side.
 
+This option has the side-effect of making stderr output get line-buffered so
+that the merging of the output of 3 programs happens in a more readable manner.
+
 dit(bf(-q, --quiet)) This option decreases the amount of information you
 are given during the transfer, notably suppressing information messages
 from the remote server. This option is useful when invoking rsync from