Use base64url instead of base64 to avoid escaping

[fonsp]

PlutoSliderServer uses a lot of base64, for example, in a staterequest:

GET /staterequest/$notebook_hash/$bonds_data

We currently use the following as the Pluto hash:

• sha1
• then Base64 encoding
• then URI escaping (e.g. Hello world => Hello%20world)

And for the bonds data, the same, but without hashing.

The hope was that by URI escaping, we would get legal URLs for base64 data, and additionally, in #29 , legal file names for precomputed states.

But! It turns out that some proxies and some file servers perform URI unescaping before it reaches this package. List of problems:

• TypeError: ids_of_cells_that_ran is undefined / Request hash not found / interaction slider/CairoMakie/CCS? #42
• Escaped characters in URLs with hashes working for deno fileserver but not Python #45
• Request hash not found - Issue caused by escape character when running nginx reverse proxy #46
• More discussion in fix unescaped '/' in URI #50 (comment)

Solution

This PR uses the base64url algorithm instead of base64:

This function implements the base64url algorithm, which is like regular base64 but it produces legal URL/filenames: it uses - instead of +, _ instead of /, and it does not pad (with = originally).

See the wiki or the official spec (RFC 4648 §5).

TODO

TODO:

• Merge matching changes in Pluto: Use base64url instead of base64 to avoid escaping PlutoSliderServer requests fonsp/Pluto.jl#1969
• Register a breaking Pluto release and update the compat of PlutoSliderServer
• Test combination with Precompute all possible slider states #29 , done in this branch: https://github.com/JuliaPluto/PlutoSliderServer.jl/tree/static-export-1-base64url
• Test combination with Pluto changes