From 22c002138d8b82a021564ddab447fafd90852e75 Mon Sep 17 00:00:00 2001
From: wm4 <wm4@nowhere>
Date: Sun, 6 May 2018 13:00:05 +0200
Subject: misc: add a synchronization helper

This is almost like rendezvous(), except it allows async wakeup, and
does not require global state. It will be used by a later commit.

struct mp_waiter is intended to be allocated on the stack, and uses an
initializer including PTHREAD_MUTEX_INITIALIZER. This is the first case
in mpv that it uses PTHREAD_MUTEX_INITIALIZER for stack-allocated
mutexes. It seems POSIX still does not allow this formally, but since
POSIX is worth less than used toilet paper, I don't really care. Modern
OSes use futexes, which means you can make _every_ memory location a
lock, and this code tries to make use of it, without using OS specific
code.

The name of the source file is rather generic, because I intend to dump
further small helpers there (or maybe move mp_rendezvous() to it).
---
 misc/thread_tools.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)
 create mode 100644 misc/thread_tools.c

(limited to 'misc/thread_tools.c')

diff --git a/misc/thread_tools.c b/misc/thread_tools.c
new file mode 100644
index 0000000000..4bcb952267
--- /dev/null
+++ b/misc/thread_tools.c
@@ -0,0 +1,62 @@
+/* Copyright (C) 2018 the mpv developers
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <assert.h>
+#include <string.h>
+
+#include "thread_tools.h"
+
+uintptr_t mp_waiter_wait(struct mp_waiter *waiter)
+{
+    pthread_mutex_lock(&waiter->lock);
+    while (!waiter->done)
+        pthread_cond_wait(&waiter->wakeup, &waiter->lock);
+    pthread_mutex_unlock(&waiter->lock);
+
+    uintptr_t ret = waiter->value;
+
+    // We document that after mp_waiter_wait() the waiter object becomes
+    // invalid. (It strictly returns only after mp_waiter_wakeup() has returned,
+    // and the object is "single-shot".) So destroy it here.
+
+    // Normally, we expect that the system uses futexes, in which case the
+    // following functions will do nearly nothing. This is true for Windows
+    // and Linux. But some lesser OSes still might allocate kernel objects
+    // when initializing mutexes, so destroy them here.
+    pthread_mutex_destroy(&waiter->lock);
+    pthread_cond_destroy(&waiter->wakeup);
+
+    memset(waiter, 0xCA, sizeof(*waiter)); // for debugging
+
+    return ret;
+}
+
+void mp_waiter_wakeup(struct mp_waiter *waiter, uintptr_t value)
+{
+    pthread_mutex_lock(&waiter->lock);
+    assert(!waiter->done);
+    waiter->done = true;
+    waiter->value = value;
+    pthread_cond_signal(&waiter->wakeup);
+    pthread_mutex_unlock(&waiter->lock);
+}
+
+bool mp_waiter_poll(struct mp_waiter *waiter)
+{
+    pthread_mutex_lock(&waiter->lock);
+    bool r = waiter->done;
+    pthread_mutex_unlock(&waiter->lock);
+    return r;
+}
-- 
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