bug#21887: 'monitor' form broken

[Taylan Ulrich Bayırlı/Kammer]

Here's a patch, tested minimally by running

    (par-for-each (lambda (x)
                    (monitor
                      (sleep 1)
                      (display "foo\n")))
                  (iota 10))

on a quad-core.  Previously it would print the "foo"s in groups of four
with a second between each group; now it prints them one by one with a
second between each, as should be.

>From 08c7f4cd98c86fbb6551c7c0b6f17262c67e7b23 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Taylan=20Ulrich=20Bay=C4=B1rl=C4=B1/Kammer?=
 <address@hidden>
Date: Sat, 25 Jun 2016 16:43:36 +0200
Subject: [PATCH] Fix 'monitor' macro.

* module/ice-9/threads.scm (monitor-mutex-table)
(monitor-mutex-table-mutex, monitor-mutex-with-id): New variables.
(monitor): Fix it.
---
 module/ice-9/threads.scm | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/module/ice-9/threads.scm b/module/ice-9/threads.scm
index 9f9e1bf..14da113 100644
--- a/module/ice-9/threads.scm
+++ b/module/ice-9/threads.scm
@@ -85,9 +85,24 @@
       (lambda () (begin e0 e1 ...))
       (lambda () (unlock-mutex x)))))
 
-(define-syntax-rule (monitor first rest ...)
-  (with-mutex (make-mutex)
-    first rest ...))
+(define monitor-mutex-table (make-hash-table))
+
+(define monitor-mutex-table-mutex (make-mutex))
+
+(define (monitor-mutex-with-id id)
+  (with-mutex monitor-mutex-table-mutex
+    (or (hashq-ref monitor-mutex-table id)
+        (let ((mutex (make-mutex)))
+          (hashq-set! monitor-mutex-table id mutex)
+          mutex))))
+
+(define-syntax monitor
+  (lambda (stx)
+    (syntax-case stx ()
+      ((_ body body* ...)
+       (let ((id (datum->syntax #'body (gensym))))
+         #`(with-mutex (monitor-mutex-with-id '#,id)
+             body body* ...))))))
 
 (define (par-mapper mapper cons)
   (lambda (proc . lists)
-- 
2.8.4