I HAVE WASTED SO MUCH TIME ON FONTS.
Sometimes, your favorate pdf reader may tell you that this freshly xelatex-generated pdf with ctex has problem with its Chinese fonts. All Chinese characters are gone, leaving blankness behind.
And instantly, you panic. OH MY GOD THERE IS SOMETHING WRONG WITH MY CHINESE FONT SETUP AGAIN. AAAGGGGAAAAAAINNNNNNN.
But no, don’t panic. Just install xpdf. Those you trusted most would betray you at unexpected moments.
You found out that xpdf displays these Chinese characters just fine.
And it turns out what you should do is: (which at least solves some of the problems)
sudo pacman -S poppler-data
And that sounds so familiar that you suddenly recalls another night of desperate Internet searching, three or four years ago, for exactly the same reason, wasted almost the same hours, followed exactly the same steps, found exactly the same bug report, and took exactly the same measures. Only this time your back and neck and butts aches much more, because during these years I did not grow up, I grow old, physically.
I should have written it down somewhere back then. Now, I have this wiki. I should be proud of myself.