[vlc-devel] [PATCH 1/2] win32: Add a vlc_mta_holder

[Hugo Beauzée-Luyssen]

This small helper will hold on to the MTA to ensure it won't be
destroyed between two calls to CoInitializeEx.
This will help us not leaking our TA configurations to libvlc/libvlccore
threads

--
Changes since previous patch #3:
- Fix a race condition causing the MTA to potentially not be acquired by
  the time vlc_mta_acquire releases.

Changes since previous patch #2:
- Don't use a VLC_OBJECT
- Use static inline functions instead of exported ones

Changes since previous patch:
- Fix potential race condition
- Do not export the functions on !_WIN32
---
 src/Makefile.am        |   2 +-
 src/win32/mta_holder.h | 128 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 129 insertions(+), 1 deletion(-)
 create mode 100644 src/win32/mta_holder.h

diff --git a/src/Makefile.am b/src/Makefile.am
index 5f3726c3e6..51062532ca 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -467,7 +467,7 @@ libvlccore_la_LDFLAGS = \
 libvlccore_la_DEPENDENCIES = libvlccore.sym
 if HAVE_WIN32
 libvlccore_la_DEPENDENCIES += libvlc_win32_rc.$(OBJEXT)
-libvlccore_la_LDFLAGS += -Wl,libvlc_win32_rc.$(OBJEXT) -avoid-version -Wc,-static
+libvlccore_la_LDFLAGS += -Wl,libvlc_win32_rc.$(OBJEXT) -avoid-version -Wc,-static $(LIBCOM)
 endif
 if HAVE_OS2
 libvlccore_la_LDFLAGS += -avoid-version
diff --git a/src/win32/mta_holder.h b/src/win32/mta_holder.h
new file mode 100644
index 0000000000..e1a53c6632
--- /dev/null
+++ b/src/win32/mta_holder.h
@@ -0,0 +1,128 @@
+/*****************************************************************************
+ * mta_holder.c: Hold a MTA from another thread
+ *****************************************************************************
+ * Copyright (C) 2002-2017 the VideoLAN and AUTHORS
+ *
+ * Author: Hugo Beauzée-Luyssen <hugo at beauzee.fr>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
+ *****************************************************************************/
+
+#ifndef MTA_HOLDER_H
+# define MTA_HOLDER_H
+
+#include <vlc_common.h>
+
+#include <assert.h>
+#include <windows.h>
+#include <objbase.h>
+
+typedef struct vlc_mta_holder
+{
+    vlc_thread_t thread;
+    vlc_cond_t   cond;
+    int          i_refcount;
+    bool         b_ready;
+} vlc_mta_holder;
+
+static vlc_mutex_t s_mutex = VLC_STATIC_MUTEX;
+
+static inline void* MtaMainLoop( void* opaque )
+{
+    vlc_mta_holder* p_mta = (vlc_mta_holder*)opaque;
+    CoInitializeEx( NULL, COINIT_MULTITHREADED );
+    vlc_mutex_lock( &s_mutex );
+
+    // Signal we now hold a reference to the MTA
+    p_mta->b_ready = true;
+    vlc_cond_signal( &p_mta->cond );
+
+    while( p_mta->i_refcount != 0 )
+    {
+        // And wait for all references to be released
+        vlc_cond_wait( &p_mta->cond, &s_mutex );
+    }
+    vlc_mutex_unlock( &s_mutex );
+    CoUninitialize();
+    return NULL;
+}
+
+/**
+ * Ensure an MTA context will be available until vlc_mta_release gets called.
+ *
+ * In the background, this will create a thread that does nothing but to keep the MTA
+ * refcount greater than 0.
+ *
+ * This is usefull in order not to commit a thread to a specific concurrency model.
+ * This function is win32 specific.
+ */
+static inline vlc_mta_holder* vlc_mta_acquire( vlc_object_t *p_parent )
+{
+    vlc_mutex_lock( &s_mutex );
+    vlc_mta_holder* p_mta = (vlc_mta_holder*)var_CreateGetAddress( p_parent->obj.libvlc, "mta-holder" );
+    if ( p_mta == NULL )
+    {
+        p_mta = (vlc_mta_holder*)malloc( sizeof( *p_mta ) );
+        if ( unlikely( p_mta == NULL ) )
+        {
+            vlc_mutex_unlock( &s_mutex );
+            return NULL;
+        }
+        vlc_cond_init( &p_mta->cond );
+        p_mta->i_refcount = 1;
+        p_mta->b_ready = false;
+        if ( vlc_clone( &p_mta->thread, MtaMainLoop, p_mta, VLC_THREAD_PRIORITY_LOW ) )
+        {
+            vlc_cond_destroy( &p_mta->cond );
+            free( p_mta );
+            p_mta = NULL;
+            vlc_mutex_unlock( &s_mutex );
+            return NULL;
+        }
+        var_SetAddress( p_parent->obj.libvlc, "mta-holder", p_mta );
+        while ( p_mta->b_ready == false )
+            vlc_cond_wait( &p_mta->cond, &s_mutex );
+    }
+    else
+        ++p_mta->i_refcount;
+    vlc_mutex_unlock( &s_mutex );
+    return p_mta;
+}
+
+/**
+ * Releases a reference to the MTA holder.
+ *
+ * When its refcount reaches 0, the thread created by
+ */
+static inline void vlc_mta_release( vlc_object_t* p_parent )
+{
+    vlc_mutex_lock( &s_mutex );
+    vlc_mta_holder *p_mta = (vlc_mta_holder*)var_InheritAddress( p_parent->obj.libvlc, "mta-holder" );
+    assert( p_mta != NULL );
+    int i_refcount = --p_mta->i_refcount;
+    if ( i_refcount == 0 )
+        var_SetAddress( p_parent->obj.libvlc, "mta-holder", NULL );
+    vlc_mutex_unlock( &s_mutex );
+    if ( i_refcount == 0 )
+    {
+        vlc_cond_signal( &p_mta->cond );
+
+        vlc_join( p_mta->thread, NULL );
+
+        free( p_mta );
+    }
+}
+
+#endif
-- 
2.11.0