Set compromise XDG_RUNTIME_DIR if necessary

It's not ideal to set XDG_RUNTIME_DIR in your shell environment,
rather than from your session manager (or systemd, or whatever), since
then you can't reliably provide the same lifetime guarantees that
XDG_RUNTIME_DIR is supposed to have? But this is still preferable to not
having an XDG_RUNTIME_DIR at all, so I'm gonna go with it.

zshenv

@@ -6,6 +6,26 @@
 
 export XDG_CONFIG_HOME XDG_CACHE_HOME XDG_DATA_HOME XDG_STATE_HOME
 
+# The other four XDG directories are very easy, but XDG_RUNTIME_DIR is
+# complicated. It kinda needs to be managed by a systemwide supervisor, like
+# systemd, in order to meet all of the XDG Base Directory specification's
+# requirements? In particular, the directory is supposed to come into being
+# when you log in, live as long as you have any active sessions, and be
+# destroyed when you log out - none of this can easily be done from the shell
+# environment alone, especially on an operating system like macOS where your
+# graphical session wasn't spawned from a shell using something like .xinitrc.
+#
+# We're going to assume XDG_RUNTIME_DIR will have already be set by a
+# systemwide supervisor if it can possibly have been. If it hasn't - say,
+# because we're on macOS - we'll compromise by using TMPDIR. Since TMPDIR has
+# a different lifetime than XDG_RUNTIME_DIR is supposed to, this is an
+# imperfect solution, but it's the best we can do.
+if [[ ! -d $XDG_RUNTIME_DIR ]] && [[ -d $TMPDIR ]]; then
+	export XDG_RUNTIME_DIR=${TMPDIR}xdg-$UID
+	mkdir -p $XDG_RUNTIME_DIR
+	chmod 0700 $XDG_RUNTIME_DIR
+fi
+
 # The real zsh config lives in XDG_CONFIG_HOME! ;)
 : ${ZDOTDIR:=$XDG_CONFIG_HOME/zsh}
 source $ZDOTDIR/zshenv