Crash in cmFileMonitor.cxx on directory change on Windows

on_directory_change is the callback passed to uv_fs_event_start, and on_directory_change will unconditionally construct a std::string from the filename arguments passed on from libuv. The documentation from libuv says that the filename argument may be NULL. Constructing a std::string from NULL is illegal. In debug mode, this results in a crash.

libuv doesn't seem to document why filename would be null, but I investigated further.

The reason libuv passes NULL is because the OVERLAPPED structure does not receive filesystem information in InternalHigh which should say information about why libuv was notified about the filesystem change. InternalHigh is only NULL in the case of some error, where the error code is stored in the Internal field of the OVERLAPPED structure. In my test case, the Internal value of OVERLAPPED is 268, which corresponds with the status STATUS_NOTIFY_ENUM_DIR. The appearance of this status message is not documented and seems to actually be a bug in Windows and it seems to correspond to ERROR_NOTIFY_ENUM_DIR, as discussed here. This happens when the buffer allocated to store the notification information is too small.

In this case, the notification is regarding the CMakeFiles subdirectory of the build directory. My best guess is that there was a notification on too many files in the directory simultaneously to be stored properly. In this case, the directory watcher is not notified regarding a single file but is notified that the directory, in general, has multiple files changing within.

I'm not sure what the correct behavior might be. Perhaps drop the notification? Perhaps forward a notification for each file in the directory?