Fix updating the thread-proc-vector in thread pools

knots/thread-pool.scm

@@ -269,7 +269,7 @@ from there, or #f if that would be an empty string."
             (sleep 1)
             (destructor/safe args)))))
 
-  (define (process channel args)
+  (define (process thread-index channel args)
     (let loop ()
       (match (get-message channel)
         ('destroy #f)
@@ -292,6 +292,9 @@ from there, or #f if that would be an empty string."
                                 internal-time-units-per-second)
                              exn))
                    (lambda ()
+                     (vector-set! thread-proc-vector
+                                  thread-index
+                                  proc)
                      (with-exception-handler
                          (lambda (exn)
                            (let ((stack
@@ -319,6 +322,10 @@ from there, or #f if that would be an empty string."
                                    vals))))))
                    #:unwind? #t)))
 
+           (vector-set! thread-proc-vector
+                        thread-index
+                        #f)
+
            (put-message reply
                         response)
 
@@ -358,7 +365,7 @@ from there, or #f if that would be an empty string."
                        "knots: thread-pool: internal exception: ~A\n" exn))
                   (lambda ()
                     (parameterize ((param args))
-                      (process channel args)))
+                      (process index channel args)))
                   #:unwind? #t)))
 
            (when thread-destructor