completion: add optional debugging name

doc/reference/misc.md

@@ -855,13 +855,17 @@ The following low level unsafe operations are also exported.
 ### make-completion
 ``` scheme
 (make-completion) -> completion
+(make-completion name) -> completion
 ```
 
 Creates a new asynchronous completion, a synchronization construct which blocks
 until a thread signals that its task either succeeded or failed via
 `completion-post!` or `completion-error!`, respectively, notifying all waiting
 threads about the result.
 
+An optional `name` may be provided for debugging purposes:
+if you deadlock, you'll be able to more easily identify which completion went wrong.
+
 ::: tip Examples:
 ``` scheme
 > (import :gerbil/gambit/threads :std/iter :std/sugar)

src/std/misc/completion-test.ss

@@ -14,6 +14,6 @@
       (def thr (spawn completion-post! c 'done))
       (check (completion-wait! c) => 'done))
     (test-case "test completion error"
-      (def c (make-completion))
+      (def c (make-completion 'error-test))
       (def thr (spawn completion-error! c 'fail))
       (check-exception (completion-wait! c) (cut eq? <> 'fail)))))

src/std/misc/completion.ss

@@ -18,10 +18,10 @@
   constructor: :init!)
 
 (defmethod {:init! completion}
-  (lambda (self)
+  (lambda (self (name 'completion)) ;; name is for debugging
     (struct-instance-init! self
-                           (make-mutex 'completion)
-                           (make-condition-variable 'completion))))
+                           (make-mutex name)
+                           (make-condition-variable name))))
 
 (def (completion-wait! compl)
   (with ((completion mx cv) compl)