[tech] Client history behavior with redirects

[Michael Lazar]

Hi all,

I have a question on a common design pattern in gemini. Say we have
a guestbook, message board, etc. that accepts user comments via an
input query. A normal session with a client might look like this:

```
C: /
S: 20 text/gemini (display the home page)

C: /guestbook
S: 20 text/gemini (display the guestbook page)

C: /guestbook/input
S: 10 enter your message

C: /guestbook/input?hello%20world
S: 30 /guestbook (submit the user's message and redirect)

C: /guestbook
S: 20 text/gemini (display the guestbook page)
```

Now, what should happen at this point when the user clicks the
"back" button in their client?

A. The client resubmits "/guestbook/input?hello%20world". This is not
   desired because it will result in double posting the message. This
   same problem is solved on the web using the Post/Redirect/Get
   pattern [1]. I suppose the gemini equivalent would be "don't store
   requests that return 3x in the client's history". With this rule,
   queries like GUS searches would still be cached because
   "gemini://geminispace.info/search?hello" returns a 20 response.

B. The client re-prompts the user with "enter your message". I've
   seen a few (most?) clients do this and it's a pretty jarring
   experience as a user, because I did not click on a link first
   to prime the input prompt. IMO we should also make a rule "don't
   store requests that return a 1x in the client's history". There's
   no web equivalent for this that I could find.

C. The client loads "/", which is the latest cached request before
   the current page. This seems like the desired behavior to me.

Thoughts?

- Michael

[1] https://en.wikipedia.org/wiki/Post/Redirect/Get