Add a cookiePrefix option

[rclmenezes]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

Add an option called expressSessionCompatibilityMode that defaults to false. When enabled, @fastify/session and express-session can be interchangable.

Alternatively, we can add a "cookieSignaturePrefix" option that defaults to "". When set to s:, it will make things compatible with express-session.

Motivation

I'm migrating from an Express monolith to multiple Fastify services. I want them all to share the same sessions and I don't want to use fastify-express unless I have to.

My express-session:

export const expressSession = ExpressSession({
  cookie: {
    maxAge: COOKIE_EXPIRATION_IN_SECONDS * 1000, // in ms
  },
  genid: generateSessionId,
  resave: false,
  rolling: true, 
  saveUninitialized: true,
  secret: SESSION_SECRET,
  store: ...,
});

My fastify-session:

    .register(fastifySession, {
      cookie: {
        maxAge: COOKIE_EXPIRATION_IN_SECONDS * 1000,
      },
      cookieName: "connect.sid",  // Name of the cookie in express-session
      idGenerator: generateSessionId,
      rolling: true,
      secret: SESSION_SECRET,
      saveUninitialized: true,
      store: ...
    })

The only reason this doesn't work? express-session signs cookies with an s: prefix. So the cookie header will look something like s:aa457ab3-ef20-4cee-8cf8-5a915ce82971.nJR5x4qCb5sjfNBGElmb9AirUxkjgeatE1fLm2W1rME

Relevant code here:
https://github.com/expressjs/session/blob/master/index.js#L656

@fastify/session (reasonably) does not do this and breaks inside decryptSession. It clears the cookie.

One work-around is to use hooks around fastifySession which is... rather hacky.

    .addHook("onRequest", async (req, res) => {
      // Transform "connect.sid" to "sessionId"
      req.cookies.sessionId = req.cookies["connect.sid"]?.replace("s:", "");
    })
    .register(fastifySession, {
      cookieName: "sessionId"  // the default
      ...
    })
    .addHook("onResponse", async (req, res) => {
      // Transform "sessionId" back to "connect.sid"
      await res.header(
        "set-cookie",
        res.getHeader("set-cookie")?.replace("sessionId", "connect.sid"),
      );
    })

I would be happy to make this PR myself!

Example

No response

[mcollina]

Let's add an option called cookiePrefix to add it. There might be multiple reasons where this might be needed and not just express.