Uncatchable exception when trying to serve a nonexistent file.

[heegaiximephoomeeghahyaiseekh]

(woo:run
    (lambda (env)
       `(200 (:content-type "text/html")
           #P"/file/that/does/not-exist.html")))

One would hope that this would result in a 404 error being returned to the Web client, but no. Instead, Woo attempts to open this using POSIX, and it doesn't even bother to check the return value (which is -1, with errno = 2 (ENOENT/"No such file or directory")). Instead, the error doesn't get caught until Woo calls (fd-file-size fd) with fd = -1. That invokes SBCL's binding to fstat, which properly checks for fstat's error code and signals a condition. The error comes up not in the browser, but in the Lisp debugger.

I guess I also wouldn't get a 403 error if I tried to serve a file that Woo didn't have permission to read.

The icing on the cake is that this error happens outside of the user-provided request handler, so it goes straight to the debugger and there is no way to catch it except by modifying Woo's source files.