fix(debug): handle mapping edges, stray traps and dying debuggees

Assisted-by: GLM 5.3

debug/ptrace_freebsd.go

@@ -97,6 +97,16 @@ func LaunchWithBuffers(gasmBin, asmPath, funcName string, args []byte, bufSpec s
 		if _, err := os.Stat(readyFile); err == nil {
 			break
 		}
+		// A debuggee that died before signalling readiness (unknown
+		// function, unparseable source) writes its failure notice to the
+		// handshake directory; read it and fail fast.  The poll never
+		// waits on the child: a wait here could consume the SIGSTOP park
+		// that waitStopped below must receive, hanging the launch.
+		if err := s.deadReason(); err != nil {
+			cmd.Wait()
+			os.RemoveAll(tmpDir)
+			return nil, nil, err
+		}
 		time.Sleep(5 * time.Millisecond)
 	}
 
@@ -134,6 +144,18 @@ func LaunchWithBuffers(gasmBin, asmPath, funcName string, args []byte, bufSpec s
 	return s, bufAddrs, nil
 }
 
+// deadReason reports the debuggee's own failure notice, the file its
+// failure paths write before exiting.  A debuggee killed without a notice
+// (a crash, SIGKILL) surfaces through waitStopped after the poll instead,
+// which is why the poll's budget stays finite.
+func (s *Session) deadReason() error {
+	data, err := os.ReadFile(filepath.Join(s.tmpDir, "dead"))
+	if err != nil {
+		return nil
+	}
+	return fmt.Errorf("debug: debuggee failed before signalling readiness: %s", strings.TrimSpace(string(data)))
+}
+
 // waitStopped consumes ptrace-stop events until one the debugger cares
 // about arrives: SIGTRAP (a breakpoint or a completed single-step), the
 // debuggee's own SIGSTOP, or a genuine signal-delivery-stop.  A Go tracee's