Fix a bug in fibers-batch-map (and related procedures)

And add more tests.

knots/parallelism.scm

@@ -79,7 +79,10 @@
   (define result-vec
     (make-vector vecs-length))
 
-  (let loop ((next-to-process-index 0)
+  (let loop ((next-to-process-index
+              (if (= 0 vecs-length)
+                  #f
+                  0))
              (channel-indexes '()))
     (if (and (eq? #f next-to-process-index)
              (null? channel-indexes))

tests/parallelism.scm

@@ -24,6 +24,16 @@
               100
               (iota 34))))))
 
+;; Test fibers-map with an empty list
+(run-fibers-for-tests
+ (lambda ()
+   (fibers-map identity '())))
+
+;; Test fibers-map with an empty vector
+(run-fibers-for-tests
+ (lambda ()
+   (fibers-map identity #())))
+
 ;; Test fibers-map with vectors
 (run-fibers-for-tests
  (lambda ()
@@ -43,4 +53,11 @@
       (* 2 i))
     (iota 34))))
 
+;; Test fibers-map-with-progress with an empty list
+(run-fibers-for-tests
+ (lambda ()
+   (fibers-map-with-progress
+    identity
+    '(()))))
+
 (display "parallelism test finished successfully\n")