supervisor: PID file to prevent duplicate daemon spawns

Multiple supervisor instances (Mind init + channel polling) could
both see no socket and start the same daemon. The socket hasn't
bound yet by the time the second check runs.

Write a PID file on spawn, check it in is_alive(). kill(pid, 0)
verifies the process is still running. Stale PID files are cleaned
up automatically.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/thalamus/supervisor.rs

@@ -111,20 +111,30 @@ impl Supervisor {
         }
     }
 
-    /// Check if a daemon is alive by testing its socket.
+    /// Check if a daemon is alive by testing its socket or PID file.
     fn is_alive(name: &str) -> bool {
+        // Check socket first
         let sock = channels_dir().join(format!("{}.sock", name));
-        if !sock.exists() {
-            return false;
-        }
+        if sock.exists() {
             match std::os::unix::net::UnixStream::connect(&sock) {
-            Ok(_) => true,
-            Err(_) => {
-                let _ = std::fs::remove_file(&sock);
+                Ok(_) => return true,
+                Err(_) => { let _ = std::fs::remove_file(&sock); }
+            }
+        }
+        // Check PID file — daemon may still be starting up
+        let pid_file = channels_dir().join(format!("{}.pid", name));
+        if let Ok(pid_str) = std::fs::read_to_string(&pid_file) {
+            if let Ok(pid) = pid_str.trim().parse::<i32>() {
+                // kill(pid, 0) checks if process exists
+                if unsafe { libc::kill(pid, 0) } == 0 {
+                    return true;
+                }
+                // Process dead, clean up stale PID file
+                let _ = std::fs::remove_file(&pid_file);
+            }
+        }
         false
     }
-        }
-    }
 
     /// Ensure all configured autostart daemons are running.
     /// Acquires file lock to prevent races with other instances.
@@ -173,6 +183,8 @@ impl Supervisor {
         {
             Ok(child) => {
                 info!("channel {} started (pid {})", name, child.id());
+                let pid_file = channels_dir().join(format!("{}.pid", name));
+                let _ = std::fs::write(&pid_file, child.id().to_string());
                 self.children.insert(name.to_string(), child);
             }
             Err(e) => error!("failed to start channel {}: {}", name, e),