trackrdrd: fixed a bug computing the SPMC queue length at UINT overflow

           (caught by slink)

configure.ac

 AC_PREREQ(2.59)
 AC_COPYRIGHT([Copyright (c) 2012 Otto Gmbh & Co KG])
-AC_INIT([trackrdrd], [1.0])
+AC_INIT([trackrdrd], [1.0.1_SNAPSHOT])
 AC_CONFIG_MACRO_DIR([m4])
 AC_CONFIG_SRCDIR(src/trackrdrd.c)
 AM_CONFIG_HEADER(config.h)

src/spmcq.c

@@ -43,10 +43,9 @@ static pthread_mutex_t spmcq_deq_lock;
 static inline unsigned
 spmcq_len(void)
 {
-    if (spmcq.tail < spmcq.head)
-        return UINT_MAX - spmcq.head - 1 - spmcq.tail;
-    else
+    if (spmcq.tail >= spmcq.head)
         return spmcq.tail - spmcq.head;
+    return UINT_MAX - spmcq.head + 1 + spmcq.tail;
 }
 
 static void
@@ -99,3 +98,30 @@ void
     AZ(pthread_mutex_unlock(&spmcq_deq_lock));
     return ptr;
 }
+
+#ifdef TEST_DRIVER
+int
+main(int argc, char * const *argv)
+{
+    (void) argc;
+
+    printf("\nTEST: %s\n", argv[0]);
+    printf("... test SMPCQ enqueue at UINT_MAX overflow\n");
+    
+    config.maxopen_scale = 0;
+    SPMCQ_Init();
+    spmcq.head = spmcq.tail = UINT_MAX - 2;
+
+    assert(SPMCQ_Enq(NULL));
+    assert(SPMCQ_Enq(NULL));
+    assert(SPMCQ_Enq(NULL));
+    assert(SPMCQ_Enq(NULL));
+    assert(SPMCQ_Enq(NULL));
+    assert(SPMCQ_Enq(NULL));
+    assert(SPMCQ_Enq(NULL));
+    assert(spmcq_len() == 7);
+
+    printf("%s: 1 test run\n", argv[0]);
+    exit(0);
+}
+#endif

src/test/Makefile.am

 INCLUDES = -I$(VARNISHSRC)/include -I$(VARNISHSRC) @AMQ_CFLAGS@
 
 TESTS = test_parse test_data test_mq test_spmcq test_spmcq_loop.sh \
-	test_worker regress.sh
+	test_spmcq_len test_worker regress.sh
 
-check_PROGRAMS = test_parse test_data test_mq test_spmcq test_worker
+check_PROGRAMS = test_parse test_data test_mq test_spmcq test_spmcq_len \
+	test_worker
 
 test_parse_SOURCES = \
 	minunit.h \
@@ -42,6 +43,16 @@ test_spmcq_LDADD = \
 	$(VARNISHSRC)/lib/libvarnish/libvarnish.la \
 	../spmcq.$(OBJEXT)
 
+test_spmcq_len_SOURCES = \
+	$(VARNISHSRC)/lib/libvarnish/libvarnish.la \
+	../spmcq.c \
+	../trackrdrd.h
+
+test_spmcq_len_LDADD = \
+	$(VARNISHSRC)/lib/libvarnish/libvarnish.la
+
+test_spmcq_len_CFLAGS = -DTEST_DRIVER
+
 test_worker_SOURCES = \
 	minunit.h \
 	test_worker.c \