Pep Boys sells a glue to help stop this. IMO one has to move
quickly to stop the spread.

Also, it is easy to make the crack spread further by
applying too much pressure when attempting to get the glue
in the crack.

I base this on experience with one windshield crack repair.

Some repair shops will insist on replacing the whole
windshield. Furthermore, IIRC you might be in violation of
the law for driving with a driver's side crack--the glass's
integrity is compromised, and you are less safe driving like
this.

A nationwide company called "Safelite" does a mobile
windshield replacement for a couple hundred dollars or so.
At least, that's how much it was for my 91 Civic last Fall.
(Then a few days later I got the crack described above!)