fix(mesh): stop probing one physical radio as two devices

/dev/mesh-radio is a udev symlink to a ttyUSB*/ttyACM* node that is also
in SERIAL_CANDIDATES, so one board was detected, probed and DTR/RTS-reset
twice per reconnect cycle (and shown twice in the UI):

- detect_serial_devices() dedupes candidates by canonical path, keeping
  the stable /dev/mesh-radio name
- auto-detect fallback skips the preferred path it just probed this cycle
  instead of immediately resetting the same board again
- probe_device's active-session guard compares canonical paths, so a
  probe via the alias can no longer open the tty the live session holds

Together with the 2s boot-settle and stable-session backoff gate, this
takes a plugged-in CP2102/ESP32 board from up to 9 reset events per
cycle at a permanent 5s retry floor down to one probe sequence per
backoff window — enough for the radio to actually finish booting.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/archipelago/src/mesh/listener/session.rs

@@ -269,10 +269,23 @@ async fn auto_detect_and_open(
     our_ed_pubkey_hex: &str,
     our_x25519_pubkey_hex: &str,
     device_kind: Option<DeviceType>,
+    skip_path: Option<&str>,
 ) -> Result<(String, MeshRadioDevice, DeviceInfo)> {
-    let paths = super::super::serial::detect_serial_devices().await;
+    let mut paths = super::super::serial::detect_serial_devices().await;
+    // When falling back from a just-failed preferred path, don't probe that
+    // same device again in the same cycle — every open() toggles DTR/RTS,
+    // which resets ESP32-family boards, and back-to-back re-probes are what
+    // keeps a mid-boot board from ever finishing its boot.
+    if let Some(skip) = skip_path {
+        let canon = |p: &str| std::fs::canonicalize(p).unwrap_or_else(|_| p.into());
+        let skip_canon = canon(skip);
+        paths.retain(|p| canon(p) != skip_canon);
+    }
     if paths.is_empty() {
-        anyhow::bail!("No serial devices found in /dev");
+        anyhow::bail!(match skip_path {
+            Some(skip) => format!("No serial devices found in /dev besides {skip}, which was already probed this cycle"),
+            None => "No serial devices found in /dev".to_string(),
+        });
     }
     for path in &paths {
         debug!(path = %path, "Probing for mesh radio device");
@@ -986,6 +999,7 @@ pub(super) async fn run_mesh_session(
                     our_ed_pubkey_hex,
                     our_x25519_pubkey_hex,
                     device_kind,
+                    Some(path),
                 )
                 .await?
             }
@@ -996,6 +1010,7 @@ pub(super) async fn run_mesh_session(
             our_ed_pubkey_hex,
             our_x25519_pubkey_hex,
             device_kind,
+            None,
         )
         .await?
     };

core/archipelago/src/mesh/mod.rs

@@ -1063,9 +1063,19 @@ impl MeshService {
     /// with the reconnect loop (whichever loses just retries).
     pub async fn probe_device(&self, path: &str) -> Result<listener::DeviceProbe> {
         let status = self.state.status.read().await;
-        if status.device_connected && status.device_path.as_deref() == Some(path) {
+        if status.device_connected {
+            if let Some(active) = status.device_path.as_deref() {
+                // Compare canonical paths: /dev/mesh-radio is a symlink to the
+                // ttyUSB*/ttyACM* node, and a probe through the alias would
+                // still open the very tty the live session is holding.
+                let canon = |p: &str| {
+                    std::fs::canonicalize(p).unwrap_or_else(|_| std::path::PathBuf::from(p))
+                };
+                if canon(active) == canon(path) {
                     anyhow::bail!("{path} is the active mesh radio — already connected");
                 }
+            }
+        }
         drop(status);
         listener::probe_device(path).await
     }

core/archipelago/src/mesh/serial.rs

@@ -556,14 +556,30 @@ fn likely_non_mesh_serial_device(path: &str) -> bool {
 
 /// Scan for serial devices that could be Meshcore radios.
 /// Returns paths to existing serial device files.
+///
+/// Candidates are deduplicated by canonical path: /dev/mesh-radio is a udev
+/// symlink to a ttyUSB*/ttyACM* node that is ALSO in the candidate list, so
+/// without this one physical board shows up (and gets probed, and gets its
+/// DTR/RTS reset toggled) twice per cycle. The first candidate wins, which
+/// keeps the stable /dev/mesh-radio name when the symlink exists.
 pub async fn detect_serial_devices() -> Vec<String> {
     let mut devices = Vec::new();
+    let mut seen_canonical: Vec<std::path::PathBuf> = Vec::new();
     for path in SERIAL_CANDIDATES {
         if tokio::fs::metadata(path).await.is_ok() {
             if likely_non_mesh_serial_device(path) {
                 debug!(path = %path, "Skipping known non-mesh serial device");
                 continue;
             }
+            let canonical = tokio::fs::canonicalize(path)
+                .await
+                .unwrap_or_else(|_| std::path::PathBuf::from(path));
+            if seen_canonical.contains(&canonical) {
+                debug!(path = %path, canonical = %canonical.display(),
+                    "Skipping alias of an already-detected serial device");
+                continue;
+            }
+            seen_canonical.push(canonical);
             devices.push(path.to_string());
         }
     }