libmyth: Prevent an access violation by the ALSA library

In audiooutputalsa.cpp, AudioOutputALSA::GetALSADevices calls the
ALSA API snd_device_name_hint with a card index of -1 to obtain a list
of hints for all cards.  Unfortunately, with libasound.so.2.0 this API
can cause an access violation when the card index is -1.

Running with valgrind shows that snd_device_name_hint makes reference
to previously freed memory when the card index is -1.

This patch enumerates the cards using other ALSA APIs, thus avoiding
the SEGV.

Signed-off-by: Jean-Yves Avenard <jyavenard@mythtv.org>

mythtv/libs/libmyth/audio/audiooutputalsa.cpp

@@ -1011,26 +1011,44 @@ bool AudioOutputALSA::OpenMixer(void)
 QMap<QString, QString> *AudioOutputALSA::GetDevices(const char *type)
 {
     QMap<QString, QString> *alsadevs = new QMap<QString, QString>();
-    void **hints, **n;
-    char *name, *desc;
 
-    if (snd_device_name_hint(-1, type, &hints) < 0)
-        return alsadevs;
-    n = hints;
-
-    while (*n != NULL)
+    // Loop through the sound cards to get Alsa device hints.
+    // NB Don't use snd_device_name_hint(-1,..) since there is a potential
+    // access violation using this ALSA API with libasound.so.2.0.0.
+    // See http://code.google.com/p/chromium/issues/detail?id=95797
+    int card = -1;
+    while (!snd_card_next(&card) && card >= 0)
     {
-          name = snd_device_name_get_hint(*n, "NAME");
-          desc = snd_device_name_get_hint(*n, "DESC");
-          if (name && desc && strcmp(name, "null"))
-              alsadevs->insert(name, desc);
-          if (name)
-              free(name);
-          if (desc)
-              free(desc);
-          n++;
+        void** hints = NULL;
+        int error = snd_device_name_hint(card, type, &hints);
+        if (error == 0)
+        {
+            void *hint;
+            for (int i = 0; (hint = hints[i]) != NULL; ++i)
+            {
+                char *name = snd_device_name_get_hint(hint, "NAME");
+                if (name)
+                {
+                    char *desc = snd_device_name_get_hint(hint, "DESC");
+                    if (desc)
+                    {
+                        if (strcmp(name, "null"))
+                            alsadevs->insert(name, desc);
+                        free(desc);
+                    }
+                    free(name);
+                }
+            }
+
+            snd_device_name_free_hint(hints);
+        }
+        else
+        {
+            VBERROR(QString("Failed to get device hints for card %1, error %2")
+                    .arg(card).arg(error));
+        }
     }
-    snd_device_name_free_hint(hints);
+
         // Work around ALSA bug < 1.0.22 ; where snd_device_name_hint can corrupt
         // global ALSA memory context
 #if SND_LIB_MAJOR == 1