Golang. What to use? http.ServeFile(..) or http.FileServer(..)?

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The main difference is that http.FileServer does effectively almost 1:1 mapping of an HTTP prefix with a filesystem. In plain english, it serves up an entire directory path. and all its children.

Say you had a directory called /home/bob/static and you had this setup: 

fs := http.FileServer(http.Dir("/home/bob/static"))
http.Handle("/static/", http.StripPrefix("/static", fs))

Your server would take requests for e.g. /static/foo/bar and serve whatever is at /home/bob/static/foo/bar (or 404)

In contrast, the ServeFile is a lower level helper that can be used to implement something similar to FileServer, or implement your own path munging potentially, and any number of things. It simply takes the named local file and sends it over the HTTP connection. By itself, it won’t serve a whole directory prefix (unless you wrote a handler that did some lookup similar to FileServer)

NOTE Serving up a filesystem naively is a potentially dangerous thing (there are potentially ways to break out of the rooted tree) hence I recommend that unless you really know what you’re doing, use http.FileServer and http.Dir as they include checks to make sure people can’t break out of the FS, which ServeFile doesn’t.

Addendum
Your secondary question, how do you do a custom NotFound handler, unfortunately, is not easily answered. Because this is called from internal function serveFile as you noticed, there’s no super easy place to break into that. There are potentially some sneaky things like intercepting the response with your own ResponseWriter which intercepts the 404 response code, but I’ll leave that exercise to you.

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