Add fallback file for single page applications

[cwen0]

In the case of SPA apps (Angular, React, Vue, ...) when you hit the refresh button and the route doesn't match the pattern you can specify a fallback file that is served if no other is found.

eg: in most cases, we can specify index.html as the fallback file.

[dmitshur]

Hi there, thanks for the PR.

This functionality makes sense, but it can and should be out of scope for vfsgen. The goal of vfsgen is to convert a dynamic http.FileSystem into a statically-encoded one. Everything else that is higher level should be done by other libraries or application.

The functionality to serve a fallback file can be implemented quite easily:

f, err := fs.Open(path)
if os.IsNotExist(err) {
    f, err = fs.Open("/index.html")
}
if err != nil {
    // handle error
}
// serve file f (e.g., with http.ServeContent)

[cwen0]

@dmitshur Thanks for your advice. I will close this pr.