Do not access unloading domains in debugger (#34013)

There was a race where a domain was being unloaded while debugger would access it. A domain was only removed from the 'appdomains_list' as a very last step. The domain was already invalid to access at this point (locks freed for example).

Worse, images would be unloaded if only referenced by that domain. All places the debugger iterates domains hold the loader lock. The loader lock is acquired by the domain unloading process, so as long as a domain is not unloading when we access it inside of the loader lock we are safe.

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Co-authored-by: UnityAlex <UnityAlex@users.noreply.github.com>

src/mono/mono/mini/debugger-agent.c

@@ -4359,6 +4359,9 @@ send_types_for_domain (MonoDomain *domain, void *user_data)
 	MonoDomain* old_domain;
 	AgentDomainInfo *info = NULL;
 
+	if (mono_domain_is_unloading (domain))
+		return;
+
 	info = get_agent_domain_info (domain);
 	g_assert (info);
 
@@ -4379,6 +4382,9 @@ send_assemblies_for_domain (MonoDomain *domain, void *user_data)
 	GSList *tmp;
 	MonoDomain* old_domain;
 
+	if (mono_domain_is_unloading (domain))
+		return;
+
 	old_domain = mono_domain_get ();
 
 	mono_domain_set_fast (domain, TRUE);
@@ -6864,6 +6870,10 @@ get_types (gpointer key, gpointer value, gpointer user_data)
 	MonoType *t;
 	GSList *tmp;
 	MonoDomain *domain = (MonoDomain*)key;
+
+	if (mono_domain_is_unloading (domain))
+		return;
+
 	MonoAssemblyLoadContext *alc = mono_domain_default_alc (domain);
 	GetTypesArgs *ud = (GetTypesArgs*)user_data;
 
@@ -6903,6 +6913,9 @@ get_types_for_source_file (gpointer key, gpointer value, gpointer user_data)
 	GetTypesForSourceFileArgs *ud = (GetTypesForSourceFileArgs*)user_data;
 	MonoDomain *domain = (MonoDomain*)key;
 
+	if (mono_domain_is_unloading (domain))
+		return;
+
 	AgentDomainInfo *info = (AgentDomainInfo *)domain_jit_info (domain)->agent_info;
 
 	/* Update 'source_file_to_class' cache */

src/mono/mono/mini/debugger-engine.c

@@ -378,6 +378,9 @@ collect_domain_bp (gpointer key, gpointer value, gpointer user_data)
 	CollectDomainData *ud = (CollectDomainData*)user_data;
 	MonoMethod *m;
 
+	if (mono_domain_is_unloading (domain))
+		return;
+
 	mono_domain_lock (domain);
 	g_hash_table_iter_init (&iter, domain_jit_info (domain)->seq_points);
 	while (g_hash_table_iter_next (&iter, (void**)&m, (void**)&seq_points)) {