Avoid invalid characters in file name (sanitise, or allow override)

I've been porting my manual calls to fetchurl and fetchgit with niv and it works really well, but I've encountered the following problem when fetching Firefox:

$ niv add firefox -v 78.0.2 -t 'https://ftp.mozilla.org/pub/firefox/releases/<version>/mac-EME-free/en-GB/Firefox%20<version>.dmg'
Adding package firefox
  Writing new sources file
Done: Adding package firefox

The resulting sources.json gets this added:

    "firefox": {
        "sha256": "1l1b6m1i9hya4awsp6pwkjlpbpk2mnlcpnmll45dxb8qcc2dn6gx",
        "type": "file",
        "url": "https://ftp.mozilla.org/pub/firefox/releases/78.0.2/mac-EME-free/en-GB/Firefox%2078.0.2.dmg",
        "url_template": "https://ftp.mozilla.org/pub/firefox/releases/<version>/mac-EME-free/en-GB/Firefox%20<version>.dmg",
        "version": "78.0.2"
    },

However, the filename Firefox%2078.0.2.dmg fails when we try to use it from Nix:

nix-repl> with import <darwin> {}; pkgs.sources.firefox
error: The path name 'Firefox%2078.0.2.dmg' is invalid: the '%' character is invalid. Path names are alphanumeric and can include the symbols +-._?= and must not begin with a period. Note: If 'Firefox%2078.0.2.dmg' is a source file and you cannot rename it on disk, builtins.path { name = ... } can be used to give it an alternative name.

Swapping the %20 for a space also fails for the same reason.

This could be avoided by allowing the resulting name to be overridden via sources.json, but the default sources.nix doesn't seem to allow this at the moment. Alternatively, the name could be sanitised like the tarball type does (switching to that type doesn't work in this case, since it's not a tarball, so tar dies).

nmattia / niv

Avoid invalid characters in file name (sanitise, or allow override) #262