Partially rewrite of read-string(!)/partial:

- immediately return on vacuous request

- try to avoid copying in read-string!/partial

- don't use read-string!/partial in read-string/partial to avoid
  duplicate copying

- if argument is fd, set port-buffer to bufpol/none as usual

scsh/rw.scm

@@ -21,28 +21,50 @@
 
     (cond ((integer? fd/port)
 	   (let ((port (fdes->inport fd/port)))
-	     (set-port-buffering port bufpol/block (- end start))
-	     (read-string!/partial port start end)))
+	     (set-port-buffering port bufpol/none)
+	     (read-string!/partial s port start end)))
 
-          (else ; no differnce between fd/ports and s48 ports
-	   (let* ((buffer (make-string (- end start)))
-		  (needed (if (> (byte-vector-length (port-buffer fd/port)) 1)
-			      'any 'immediate))   ;bufpol/none may return with 0
-		  (len (read-block buffer 0 needed fd/port)))
-	     (if (eof-object? len)
-		 #f
-		 (begin 
-		   (copy-bytes! buffer 0 s start len)
-		   len)))))))
+          ((open-input-port? fd/port)
+	   (if (= start end)
+	       0
+	       (let* ((buffer (if (= start 0)
+				  s
+				  (make-string (- end start))))
+		      (needed (if (> (byte-vector-length (port-buffer fd/port)) 1)
+				  'any 
+				  'immediate))   ;bufpol/none may return with 0
+		      (nread (read-block buffer 0 needed fd/port)))
+		 (if (eof-object? nread)
+		     #f
+		     (begin 
+		       (if (not (eq? s buffer))
+			   (copy-bytes! buffer 0 s start nread))
+		       nread)))))
+
+	  (else 
+	   (apply error "Not a fd/port in read-string!/partial" s args)))))
 
 
 (define (read-string/partial len . maybe-fd/port) 
-  (let* ((s (make-string len))
-	 (fd/port (:optional maybe-fd/port (current-input-port)))
-	 (nread (read-string!/partial s fd/port 0 len)))
-    (cond ((not nread) #f) ; EOF
-	  ((= nread len) s)
-	  (else (substring s 0 nread)))))
+  (let* ((fd/port (:optional maybe-fd/port (current-input-port))))
+    (cond ((integer? fd/port)
+	   (let ((port (fdes->inport fd/port)))
+	     (set-port-buffering port bufpol/none)
+	     (read-string/partial len port)))
+	  
+	  ((open-input-port? fd/port)
+	   (if (= len 0)
+	       0
+	       (let* ((buffer (make-string len))
+		      (needed (if (> (byte-vector-length (port-buffer fd/port)) 1)
+				  'any
+				  'immediate));; bufpol/none may return with 0
+		      (nread (read-block buffer 0 needed fd/port)))
+		 (cond ((eof-object? nread) #f)
+		       ((= nread len) buffer)
+		       (else (substring buffer 0 nread))))))
+	  (else 
+	   (error "Not a fd/port in read-string/partial" len fd/port)))))
 
 
 ;;; Persistent reading