fix(client): make H.264 decoder selection startup-safe for previews

client/src/launcher/preview.rs

@@ -823,9 +823,22 @@ impl PreviewFeed {
                 session_active_flag,
                 active_bindings_flag,
                 running_flag,
-                shared_state,
-                log_sink,
+                Arc::clone(&shared_state),
+                Arc::clone(&log_sink),
             ) {
+                set_shared_status(
+                    &shared_state,
+                    &log_sink,
+                    monitor_id,
+                    "Preview pipeline setup failed. See session log.",
+                    true,
+                );
+                log_preview_issue(
+                    &shared_state,
+                    &log_sink,
+                    monitor_id,
+                    &format!("Preview feed startup failed: {err:#}"),
+                );
                 warn!(monitor_id, ?err, "launcher preview feed exited");
             }
         });
@@ -981,7 +994,33 @@ fn run_preview_feed(
     shared: Arc<Mutex<SharedPreviewState>>,
     log_sink: Arc<Mutex<Option<std::sync::mpsc::Sender<String>>>>,
 ) -> Result<()> {
-    let (pipeline, appsrc, appsink, decoder_name) = build_preview_pipeline(profile)?;
+    let mut startup_error = None;
+    let mut selected = None;
+    for decoder_name in preview_decoder_candidates() {
+        match build_preview_pipeline(profile, &decoder_name) {
+            Ok((pipeline, appsrc, appsink, decoder_label)) => {
+                match pipeline
+                    .set_state(gst::State::Playing)
+                    .context("starting launcher preview pipeline")
+                {
+                    Ok(_) => {
+                        selected = Some((pipeline, appsrc, appsink, decoder_label));
+                        break;
+                    }
+                    Err(err) => {
+                        let _ = pipeline.set_state(gst::State::Null);
+                        startup_error = Some(err);
+                    }
+                }
+            }
+            Err(err) => {
+                startup_error = Some(err);
+            }
+        }
+    }
+    let (pipeline, appsrc, appsink, decoder_name) = selected.ok_or_else(|| {
+        startup_error.unwrap_or_else(|| anyhow::anyhow!("no usable H.264 decoder"))
+    })?;
     let parser = pipeline.by_name("preview_parse");
     let decoder = pipeline.by_name("decoder");
     if let Ok(mut slot) = shared.lock() {
@@ -997,10 +1036,6 @@ fn run_preview_feed(
             }
         });
     }
-    pipeline
-        .set_state(gst::State::Playing)
-        .context("starting launcher preview pipeline")?;
-
     {
         let shared = Arc::clone(&shared);
         let appsink = appsink.clone();
@@ -1410,8 +1445,8 @@ fn looks_like_preview_problem(status: &str) -> bool {
 #[cfg(not(coverage))]
 fn build_preview_pipeline(
     profile: PreviewProfile,
+    decoder_name: &str,
 ) -> Result<(gst::Pipeline, gst_app::AppSrc, gst_app::AppSink, String)> {
-    let decoder_name = pick_h264_decoder();
     let source_mode = eye_source_mode_for_request(
         profile.requested_width.max(2) as u32,
         profile.requested_height.max(2) as u32,
@@ -1421,10 +1456,11 @@ fn build_preview_pipeline(
     let desc = format!(
         "appsrc name=src is-live=true format=time do-timestamp=true block=false ! \
          queue max-size-buffers=6 max-size-time=0 max-size-bytes=0 leaky=downstream ! \
-         h264parse name=preview_parse disable-passthrough=true ! {decoder_name} name=decoder ! videoconvert ! \
+         h264parse name=preview_parse disable-passthrough=true ! {} name=decoder ! videoconvert ! \
          videoscale add-borders=false ! \
          video/x-raw,format=RGBA,width=(int){render_width},height=(int){render_height},pixel-aspect-ratio=1/1 ! \
          appsink name=sink emit-signals=false sync=false max-buffers=1 drop=true",
+        decoder_name,
     );
     let pipeline = gst::parse::launch(&desc)?
         .downcast::<gst::Pipeline>()
@@ -1457,7 +1493,7 @@ fn build_preview_pipeline(
             .build(),
     ));
 
-    Ok((pipeline, appsrc, appsink, decoder_name))
+    Ok((pipeline, appsrc, appsink, decoder_name.to_string()))
 }
 
 #[cfg(not(coverage))]
@@ -1481,6 +1517,40 @@ fn preview_render_size(
     (render_w.max(2), render_h.max(2))
 }
 
+#[cfg(not(coverage))]
+fn preview_decoder_candidates() -> Vec<String> {
+    let mut candidates = Vec::new();
+    let preferred = pick_h264_decoder();
+    if !preferred.trim().is_empty() {
+        candidates.push(preferred);
+    }
+    for name in [
+        "avdec_h264",
+        "openh264dec",
+        "vah264dec",
+        "vaapih264dec",
+        "v4l2h264dec",
+        "v4l2slh264dec",
+        "nvh264dec",
+        "nvh264sldec",
+        "decodebin",
+    ] {
+        if name == "decodebin" || gst::ElementFactory::find(name).is_some() {
+            candidates.push(name.to_string());
+        }
+    }
+    candidates.sort();
+    candidates.dedup();
+    if let Some(pos) = candidates
+        .iter()
+        .position(|name| name == &pick_h264_decoder())
+    {
+        let preferred = candidates.remove(pos);
+        candidates.insert(0, preferred);
+    }
+    candidates
+}
+
 #[cfg(not(coverage))]
 fn push_preview_packet(appsrc: &gst_app::AppSrc, pkt: VideoPacket) {
     let mut buf = gst::Buffer::from_slice(pkt.data);

client/src/output/video.rs

@@ -16,22 +16,22 @@ fn pick_h264_decoder() -> String {
         if name.eq_ignore_ascii_case("decodebin") {
             return "decodebin".to_string();
         }
-        if !name.is_empty() && gst::ElementFactory::find(name).is_some() {
+        if !name.is_empty() && buildable_decoder(name) {
             return name.to_string();
         }
     }
 
     for name in [
+        "avdec_h264",
+        "openh264dec",
         "nvh264dec",
         "nvh264sldec",
         "vah264dec",
         "vaapih264dec",
         "v4l2h264dec",
         "v4l2slh264dec",
-        "openh264dec",
-        "avdec_h264",
     ] {
-        if gst::ElementFactory::find(name).is_some() {
+        if buildable_decoder(name) {
             return name.to_string();
         }
     }
@@ -39,6 +39,10 @@ fn pick_h264_decoder() -> String {
     "decodebin".to_string()
 }
 
+fn buildable_decoder(name: &str) -> bool {
+    gst::ElementFactory::find(name).is_some() && gst::ElementFactory::make(name).build().is_ok()
+}
+
 pub struct MonitorWindow {
     _pipeline: gst::Pipeline,
     src: gst_app::AppSrc,

client/src/video_support.rs

@@ -17,25 +17,29 @@ pub fn pick_h264_decoder() -> String {
         if name.eq_ignore_ascii_case("decodebin") {
             return "decodebin".to_string();
         }
-        if !name.is_empty() && gst::ElementFactory::find(name).is_some() {
+        if !name.is_empty() && buildable_decoder(name) {
             return name.to_string();
         }
     }
 
     for name in [
+        "avdec_h264",
+        "openh264dec",
         "nvh264dec",
         "nvh264sldec",
         "vah264dec",
         "vaapih264dec",
         "v4l2h264dec",
         "v4l2slh264dec",
-        "openh264dec",
-        "avdec_h264",
     ] {
-        if gst::ElementFactory::find(name).is_some() {
+        if buildable_decoder(name) {
             return name.to_string();
         }
     }
 
     "decodebin".to_string()
 }
+
+fn buildable_decoder(name: &str) -> bool {
+    gst::ElementFactory::find(name).is_some() && gst::ElementFactory::make(name).build().is_ok()
+}