don't write more than PIPE_BUF bytes in any one write() in io.c

this makes sure that the write never blocks.

io.c

@@ -361,8 +361,11 @@ static void writefd_unbuffered(int fd,char *buf,int len)
 		}
 
 		if (FD_ISSET(fd, &w_fds)) {
-			int n = len-total;
-			int ret = write(fd,buf+total,n?n:1);
+			int ret, n = len-total;
+			
+			if (n > PIPE_BUF) n = PIPE_BUF;
+
+			ret = write(fd,buf+total,n?n:1);
 
 			if (ret == -1 && errno == EINTR) {
 				continue;

rsync.h

@@ -285,6 +285,10 @@
 #define MAXPATHLEN 1024
 #endif
 
+#ifndef PIPE_BUF
+#define PIPE_BUF 512
+#endif
+
 #ifndef INADDR_NONE
 #define INADDR_NONE 0xffffffff
 #endif