🐛 fix(av1-transfer): silently fall back when preexec_fn exec fails in multithreaded context

GIL contention after fork in ThreadPoolExecutor can cause os.execvp to fail.
Catch the exception so subprocess proceeds with its own exec, keeping
transcoding functional even if process renaming fails.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

av1-transfer/main.py

@@ -132,15 +132,19 @@ def transcode_one_file(file_info, overall_bar, slot_bars, slot_queue, process_na
 
         str_args = [str(a) for a in ffmpeg_args]
         if process_name:
-            # fork 后、subprocess 执行 exec 之前，用 os.execvp 替换进程：
-            # - execvp 通过 PATH 查找 ffmpeg（规避 execve 直接调用 symlink 的问题）
-            # - argv[0] 设为别名，ps/top 显示别名而非 ffmpeg
-            # - subprocess 设好的 stdout/stderr pipe 已继承，无需额外处理
+            # 尝试在 fork 后、subprocess exec 之前用 os.execvp 替换进程（设 argv[0] 为别名）
+            # 多线程环境下 GIL 可能导致 execvp 失败；失败时静默返回，
+            # subprocess 会接着正常执行 ffmpeg，转码不受影响
             _pname = process_name
             _args = str_args
+            def _try_exec():
+                try:
+                    os.execvp("ffmpeg", [_pname] + _args)
+                except Exception:
+                    pass
             proc = subprocess.Popen(
                 ["ffmpeg"] + str_args,
-                preexec_fn=lambda: os.execvp("ffmpeg", [_pname] + _args),
+                preexec_fn=_try_exec,
                 stdout=subprocess.PIPE,
                 stderr=subprocess.PIPE,
             )