change `hash-count' to take the lock; fix docs

collects/scribblings/reference/hashes.scrbl

@@ -366,10 +366,10 @@ within @scheme[proc]. @see-also-concurrency-caveat[]}
 @defproc[(hash-count [hash hash?])
          exact-nonnegative-integer?]{
 
-Returns the number of keys mapped by @scheme[hash]. If @scheme[hash]
-is not created with @scheme['weak], then the result is computed in
-constant time and atomically. If @scheme[hash] is created with
-@scheme['weak], see the @concurrency-caveat[] above.}
+Returns the number of keys mapped by @scheme[hash]. Unless @scheme[hash]
+retains keys weakly, the result is computed in
+constant time and atomically. If @scheme[hash] retains it keys weakly, a
+traversal is required to count the keys.}
 
 
 @defproc[(hash-iterate-first [hash hash?])

src/racket/src/list.c

@@ -1972,6 +1972,8 @@ static Scheme_Object *hash_table_count(int argc, Scheme_Object *argv[])
     Scheme_Bucket **buckets, *bucket;
     const char *key;
 
+    if (t->mutex) scheme_wait_sema(t->mutex,0);
+
     buckets = t->buckets;
     weak = t->weak;
 
@@ -1989,6 +1991,8 @@ static Scheme_Object *hash_table_count(int argc, Scheme_Object *argv[])
       SCHEME_USE_FUEL(1);
     }
 
+    if (t->mutex) scheme_post_sema(t->mutex);
+
     return scheme_make_integer(count);
   } else {
     scheme_wrong_type("hash-count", "hash", 0, argc, argv);