Keep single global pthread_cond_t per thread.

This will allow thread/select to be implemented.
Also add thread/close and close method to threads.

src/core/thread.c

@@ -29,9 +29,18 @@
 
 #ifdef JANET_THREADS
 
-#include <pthread.h>
 #include <setjmp.h>
 
+JANET_THREAD_LOCAL pthread_cond_t janet_vm_thread_cond;
+
+void janet_threads_init(void) {
+    pthread_cond_init(&janet_vm_thread_cond, NULL);
+}
+
+void janet_threads_deinit(void) {
+    pthread_cond_destroy(&janet_vm_thread_cond);
+}
+
 static JanetTable *janet_get_core_table(const char *name) {
     JanetTable *env = janet_core_env(NULL);
     Janet out = janet_wrap_nil();
@@ -44,20 +53,26 @@ static JanetTable *janet_get_core_table(const char *name) {
 static void janet_channel_init(JanetChannel *channel) {
     janet_buffer_init(&channel->buf, 0);
     pthread_mutex_init(&channel->lock, NULL);
-    pthread_cond_init(&channel->cond, NULL);
+    channel->rx_cond = NULL;
     channel->refCount = 2;
+    channel->mailboxFlag = 0;
 }
 
-/* Return 1 if channel memory should be freed, otherwise false */
+/* Return 1 if channel memory should be freed, otherwise 0 */
 static int janet_channel_deref(JanetChannel *channel) {
     pthread_mutex_lock(&channel->lock);
-    if (0 == --channel->refCount) {
+    if (1 == channel->refCount) {
         janet_buffer_deinit(&channel->buf);
         pthread_mutex_destroy(&channel->lock);
-        pthread_cond_destroy(&channel->cond);
         return 1;
     } else {
+        channel->refCount--;
         pthread_mutex_unlock(&channel->lock);
+        /* Wake up other side if they are blocked, otherwise
+         * they will block forever. */
+        if (NULL != channel->rx_cond) {
+            pthread_cond_signal(channel->rx_cond);
+        }
         return 0;
     }
 }
@@ -85,8 +100,8 @@ static int janet_channel_send(JanetChannel *channel, Janet msg, JanetTable *dict
         janet_marshal(&channel->buf, msg, dict, 0);
 
         /* Was empty, signal to cond */
-        if (oldcount == 0) {
-            pthread_cond_signal(&channel->cond);
+        if (oldcount == 0 && (NULL != channel->rx_cond)) {
+            pthread_cond_signal(channel->rx_cond);
         }
     }
 
@@ -106,7 +121,8 @@ static int janet_channel_receive(JanetChannel *channel, Janet *msg_out, JanetTab
     while (channel->buf.count == 0) {
         /* Check for closed channel (1 ref left means other side quit) */
         if (channel->refCount <= 1) return 1;
-        pthread_cond_wait(&channel->cond, &channel->lock);
+        /* Since each thread sets its own rx_cond, we know it's not NULL */
+        pthread_cond_wait(channel->rx_cond, &channel->lock);
     }
 
     /* Hack to capture all panics from marshalling. This works because
@@ -211,6 +227,7 @@ static int thread_worker(JanetChannel *tx) {
 
     /* Create self thread */
     JanetChannel *rx = tx + 1;
+    rx->rx_cond = &janet_vm_thread_cond;
     JanetThread *thread = janet_make_thread(rx, tx, encode, decode);
     Janet threadv = janet_wrap_abstract(thread);
 
@@ -281,6 +298,7 @@ static Janet cfun_thread_new(int32_t argc, Janet *argv) {
     JanetChannel *tx = rx + 1;
     janet_channel_init(rx);
     janet_channel_init(tx);
+    rx->rx_cond = &janet_vm_thread_cond;
     JanetThread *thread = janet_make_thread(rx, tx, encode, decode);
     if (janet_thread_start_child(thread))
         janet_panic("could not start thread");
@@ -310,9 +328,17 @@ static Janet cfun_thread_receive(int32_t argc, Janet *argv) {
     return out;
 }
 
+static Janet cfun_thread_close(int32_t argc, Janet *argv) {
+    janet_fixarity(argc, 1);
+    JanetThread *thread = janet_getthread(argv, 0);
+    janet_close_thread(thread);
+    return janet_wrap_nil();
+}
+
 static const JanetMethod janet_thread_methods[] = {
     {"send", cfun_thread_send},
     {"receive", cfun_thread_receive},
+    {"close", cfun_thread_close},
     {NULL, NULL}
 };
 
@@ -340,6 +366,12 @@ static const JanetReg threadlib_cfuns[] = {
         JDOC("(thread/receive thread)\n\n"
              "Get a value sent to thread. Will block if there is no value was sent to this thread yet. Returns the message sent to the thread.")
     },
+    {
+        "thread/close", cfun_thread_close,
+        JDOC("(thread/close thread)\n\n"
+             "Close a thread, unblocking it and ending communication with it. Note that closing "
+             "a thread is idempotent and does not cancel the thread's operation. Returns nil.")
+    },
     {NULL, NULL, NULL}
 };