alsa_seq: no crash when the sequencer device cannot be opened

include/libremidi/backends/alsa_seq/helpers.hpp

@@ -76,6 +76,8 @@ inline void for_all_ports(
     const libasound& snd, snd_seq_t* seq,
     const std::function<void(snd_seq_client_info_t&, snd_seq_port_info_t&)>& func)
 {
+  if (!seq)
+    return;
   snd_seq_client_info_t* cinfo{};
   snd_seq_client_info_alloca(&cinfo);
   snd_seq_port_info_t* pinfo{};
@@ -102,6 +104,8 @@ inline void for_all_ports(
     const libasound& snd, snd_seq_t* seq, int client,
     const std::function<void(snd_seq_port_info_t&)>& func)
 {
+  if (!seq)
+    return;
   snd_seq_port_info_t* pinfo{};
   snd_seq_port_info_alloca(&pinfo);
 
@@ -119,6 +123,8 @@ inline unsigned int iterate_port_info(
     const libasound& snd, snd_seq_t* seq, snd_seq_port_info_t* pinfo, unsigned int type,
     int portNumber)
 {
+  if (!seq)
+    return 0;
   snd_seq_client_info_t* cinfo{};
   int count = 0;
   snd_seq_client_info_alloca(&cinfo);
@@ -182,7 +188,11 @@ struct alsa_data
       // Set up the ALSA sequencer client.
       int ret = snd.seq.open(&seq, "default", SND_SEQ_OPEN_DUPLEX, SND_SEQ_NONBLOCK);
       if (ret < 0)
+      {
+        // Every user of seq checks it against nullptr to know the client failed
+        seq = nullptr;
         return ret;
+      }
 
       // Set client name.
       if (!configuration.client_name.empty())

include/libremidi/backends/alsa_seq/midi_in.hpp

@@ -90,6 +90,9 @@ class midi_in_impl
 
   ~midi_in_impl() override
   {
+    if (!this->seq)
+      return;
+
     // Cleanup.
     if (this->vport >= 0)
       snd.seq.delete_port(this->seq, this->vport);
@@ -190,6 +193,8 @@ class midi_in_impl
 
   stdx::error close_port() override
   {
+    if (!this->seq)
+      return stdx::error{};
     unsubscribe();
     stop_queue();
     return stdx::error{};
@@ -348,6 +353,11 @@ class midi_in_alsa_threaded : public midi_in_impl<ConfigurationBase, Configurati
   midi_in_alsa_threaded(ConfigurationBase&& conf, ConfigurationImpl&& apiconf)
       : midi_in_impl<ConfigurationBase, ConfigurationImpl>{std::move(conf), std::move(apiconf)}
   {
+    // The base already reported the failed client. Do not claim to be open, or
+    // midi_in::open_*_port goes on to snd_seq_create_port(NULL, ...).
+    if (!this->seq)
+      return;
+
     if (this->m_termination_event < 0)
     {
       this->libremidi_handle_error(this->configuration, "error creating eventfd.");

include/libremidi/backends/alsa_seq/midi_out.hpp

@@ -42,6 +42,9 @@ class midi_out_impl final
 
   ~midi_out_impl() override
   {
+    if (!this->seq)
+      return;
+
     // Close a connection if it exists.
     midi_out_impl::close_port();
 

include/libremidi/backends/alsa_seq/observer.hpp

@@ -370,6 +370,11 @@ class observer_threaded : public observer_impl<ConfigurationImpl>
   observer_threaded(libremidi::observer_configuration&& conf, ConfigurationImpl&& apiconf)
       : observer_impl<ConfigurationImpl>{std::move(conf), std::move(apiconf)}
   {
+    // init_client failed (no /dev/snd/seq, e.g. snd-seq not loaded): nothing
+    // to poll, and snd_seq_poll_descriptors(NULL, ...) segfaults.
+    if (!this->seq)
+      return;
+
     // Create relevant descriptors
     auto& snd = alsa_data::snd;
 
@@ -474,14 +479,20 @@ class observer_manual : public observer_impl<ConfigurationImpl>
   observer_manual(libremidi::observer_configuration&& conf, ConfigurationImpl&& apiconf)
       : observer_impl<ConfigurationImpl>{std::move(conf), std::move(apiconf)}
   {
+    if (!this->seq)
+      return;
     this->configuration.manual_poll(
         poll_parameters{.addr = this->vaddr, .callback = [this](const auto& v) {
       this->handle_event_direct(v);
       return 0;
     }});
   }
 
-  ~observer_manual() { this->configuration.stop_poll(this->vaddr); }
+  ~observer_manual()
+  {
+    if (this->seq)
+      this->configuration.stop_poll(this->vaddr);
+  }
 };
 }