Improve handling of gnuplot executable.

(`which` at startup, check for executable)

scheme/httpd/surflets/web-server/root/surflets/admin-profiling.scm

@@ -9,6 +9,7 @@
 	define-record-types
 	locks
 	let-opt
+	receiving
 	)
   (begin
 
@@ -30,8 +31,8 @@
     (define (state-counter)
       (state:counter (get-servlet-data)))
     
-    ;; Leave this global. Server is running on a single system.
-    (define gnuplot "/usr/bin/gnuplot")
+    ;; Leave this global. Servers are running on a single system.
+    (define gnuplot #f)			;; Set in main.
     (define lock (make-lock))
 
     (define (profile req . maybe-update-text)
@@ -63,7 +64,11 @@
 			       ,new-url
 			       POST
 			       (p "This uses " (var "gnuplot") " that is searched at "
-				  ,input-field ,change-button)))
+				  ,input-field ,change-button (br)
+				  ,(if (not (gnuplot-executable? gnuplot))
+				   '(font (@ (color "red"))
+					  "Note: There is no executable.")
+				   #f))))
 			  (li (URL ,(reset-address new-url)
 				   "Delete files and reset profile state.")))
 			 (hr)
@@ -90,12 +95,15 @@
 	  (reset-and-return-to-main-page req))
 	 (else
 	  (let ((new-gnuplot-location (input-field-value input-field bindings)))
-	    (if (and new-gnuplot-location
-		     (file-executable? new-gnuplot-location))
+	    (if (gnuplot-executable? new-gnuplot-location)
 		(begin
 		  (set! gnuplot new-gnuplot-location)
 		  (profile req (format #f "Gnuplot is now searched at ~a." gnuplot)))
 		(profile req "Please enter a file name of an existing executable.")))))))
+    
+    (define (gnuplot-executable? gnuplot-file-name)
+      (and gnuplot-file-name
+	   (file-executable? gnuplot-file-name)))
 
     (define (new-profile req)
       (let ((state (get-servlet-data)))
@@ -223,8 +231,14 @@ plot '~a' title 'Servlet Profiling ~a' with lines"
 
     (define (main req)
       ;; We'll fill this out soon.
+      (set! gnuplot (search-gnuplot))
       (set-servlet-data! (make-state #f #f 0))
       (reset-profiling-state!)
       (profile req))
 
+    (define (search-gnuplot)
+      (receive (status ports) (run/collecting (1) (which gnuplot))
+	(if (zero? status)
+	    (read-line ports)
+	    "")))
     ))