Hi all,
I have a possible fix for issue #37 (crash on startup). After closer
inspection (attached to the issue :) ), it looks like the reason
Hydrogen crashes on startup is because ALSA MIDI driver calls
Hydrogen::get_instance() while first Hydrogen::get_instance() (which
creates the singleton object) is still running. This causes two Hydrogen
objects to be created, which is responsible for the crash.
First fix (mine) is very simple - set the __instance variable before the
drivers are started. See: hydrogen-crash-patch.diff - the drawback is
that something (MIDI or audio driver) may still potentially call things
on Hydrogen object before it's fully constructed.
Second fix: Torben Hohn suggested on #lad IRC channel to use a mutex in
get_instance() to prevent situations like this. I've found a more
lightweight way - ensuring that MIDI thread doesn't proceed until the
drivers are fully set up. It's actually quite easy to do, as
startAudioDrivers locks the audio engine until it completes. Because
MIDI thread is started from that function (with audio engine mutex
locked), it's enough to lock and unlock the audio engine once from MIDI
thread (so it doesn't start doing anything on Hydrogen object until
driver initialization stuff completes and unlocks the mutex).
I did this in the second patch (hydrogen-crash-patch-improvement) for
ALSA MIDI only, but the same fix should probably be applied to other
MIDI drivers (and possibly ALSA audio driver as well, just to be safe -
there is no point in doing any processing until the driver
initialization is complete).
More details (much more than anyone is willing to digest, I guess) are
added as comments to the ticket.
Any comments/suggestions/mistakes in the analysis above? (I'm still
mostly unfamiliar with Hydrogen code, so might have missed something
important)
Krzysztof
Index: libs/hydrogen/src/hydrogen.cpp
===================================================================
--- libs/hydrogen/src/hydrogen.cpp (revision 767)
+++ libs/hydrogen/src/hydrogen.cpp (working copy)
@@ -1719,6 +1719,8 @@
hydrogenInstance = this;
// __instance = this;
audioEngine_init();
+ // Prevent double creation caused by calls from MIDI thread
+ __instance = this;
audioEngine_startAudioDrivers();
}
Index: libs/hydrogen/src/IO/alsa_midi_driver.cpp
===================================================================
--- libs/hydrogen/src/IO/alsa_midi_driver.cpp (revision 767)
+++ libs/hydrogen/src/IO/alsa_midi_driver.cpp (working copy)
@@ -29,6 +29,7 @@
#include <hydrogen/globals.h>
#include <hydrogen/event_queue.h>
+#include <hydrogen/audio_engine.h>
#include <pthread.h>
@@ -56,6 +57,10 @@
{
AlsaMidiDriver *pDriver = ( AlsaMidiDriver* )param;
_INFOLOG( "starting" );
+
+ // wait until audioEngine_startAudioDrivers sets Hydrogen::__instance (then it releases the lock)
+ AudioEngine::get_instance()->lock("alsaMidiDriver_thread");
+ AudioEngine::get_instance()->unlock();
if ( seq_handle != NULL ) {
_ERRORLOG( "seq_handle != NULL" );
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