semaphores: fix sem_trywait

This patch fixes a subtle but critical bug in sem_trywait()
underlying implementation (semaphore.cc) which
for example prevented new version of Java 12 and above
hang on startup in infinite loop.

It also adds new unit test around sem_* functions.

Signed-off-by: Waldemar Kozaczuk <[email protected]>
Message-Id: <[email protected]>

core/semaphore.cc

@@ -62,7 +62,7 @@ bool semaphore::trywait(unsigned units)
 {
     bool ok = false;
     WITH_LOCK(_mtx) {
-        if (_val > units) {
+        if (_val >= units) {
             _val -= units;
             ok = true;
         }

modules/tests/Makefile

@@ -128,7 +128,7 @@ tests := tst-pthread.so misc-ramdisk.so tst-vblk.so tst-bsd-evh.so \
 	tst-ttyname.so tst-pthread-barrier.so tst-feexcept.so tst-math.so \
 	tst-sigaltstack.so tst-fread.so tst-tcp-cork.so tst-tcp-v6.so \
 	tst-calloc.so tst-crypt.so tst-non-fpic.so tst-small-malloc.so \
-	tst-mmx-fpu.so tst-getopt.so tst-getopt-pie.so tst-non-pie.so
+	tst-mmx-fpu.so tst-getopt.so tst-getopt-pie.so tst-non-pie.so tst-semaphore.so
 #	libstatic-thread-variable.so tst-static-thread-variable.so \
 
 tests += testrunner.so

tests/tst-semaphore.c

@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2019 Waldemar Kozaczuk
+ *
+ * This work is open source software, licensed under the terms of the
+ * BSD license as described in the LICENSE file in the top-level directory.
+ */
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <unistd.h>
+#include <semaphore.h>
+#include <assert.h>
+
+#define THREAD_NUMBER 10
+
+static sem_t sem_sync, sem_done;
+static int counter = 0;
+
+static void* threadfunc(void* arg) {
+    int *thread_number = (int*)arg;
+    for (int i = 0; i < 10; i++) {
+        assert(sem_wait(&sem_sync) == 0);
+        counter++;
+        assert(sem_post(&sem_sync) == 0);
+        printf("Thread %d: Incremented %dth\n", *thread_number, i + 1);
+        usleep(1000 * (counter % 5)); // Pseudo-randomly sleep between 0-4ms
+    }
+    assert(sem_post(&sem_done) == 0);
+    return NULL;
+}
+
+int main(void) {
+    pthread_t threads[THREAD_NUMBER];
+    int threads_numbers[THREAD_NUMBER];
+
+    assert(sem_init(&sem_sync, 0, 1) == 0);
+    assert(sem_init(&sem_done, 0, 0) == 0);
+
+    for (int t = 0; t < THREAD_NUMBER; t++) {
+        threads_numbers[t] = t + 1;
+        pthread_create(threads + t, NULL, &threadfunc, threads_numbers + t);
+    }
+
+    for (int t = 0; t < THREAD_NUMBER; t++) {
+        while(sem_trywait(&sem_done));
+    }
+
+    assert(counter == 10 * THREAD_NUMBER);
+
+    for (int t = 0; t < THREAD_NUMBER; t++) {
+        pthread_join(threads[t], NULL);
+    }
+
+    assert(sem_destroy(&sem_sync) == 0);
+    assert(sem_destroy(&sem_done) == 0);
+
+    return 0;
+}