官方软件包存储库中的某些软件包似乎有一个新的依赖项，但官方存储库中没有：
$ sudo pacman --sync --refresh --refresh --sysupgrade
:: Synchronising package databases...
core 122.3 KiB 284K/s 00:00 [###] 100%
extra 1773.8 KiB 2.45M/s 00:01 [###] 100%
community 3.4 MiB 2.64M/s 00:01 [###] 100%
multilib 163.1 KiB 426K/s 00:00 [###] 100%
:: Starting full system upgrade...
resolving dependencies...
warning: cannot resolve "ffmpeg2.8", a dependency of "firefox"
warning: cannot resolve "ffmpeg2.8", a dependency of "gst-libav"
warning: cannot resolve "ffmpeg2.8", a dependency of "vlc"
:: The following packages cannot be upgraded due to unresolvable dependencies:
firefox gst-libav vlc
我怀疑问题可能是镜像没有正确更新，那么我如何确定我是从哪个镜像下载的呢？
这不是关于镜像更新后如何解决问题。另一个刷新修复了这个问题。但这是第一次发生这种情况，我想避免仅部分更新的镜像。