hledger-web: Randomize port

[AstraLuma]

On a complex dev machine (hi), round-numbered ports are hard to come by. There's always multiple things on X000.

Allow randomizing the port. (Most software allows specifying port 0, and then they'll tell you what port they ended up on.)

[simonmichael]

You're right this is a problem; I'm not sure about the solution. We like predictable behaviour consistent across users and machines. Specifying a port of your choice with the --port option is that. But I imagine you are trying to automate this without any need for a static port number. How would hledger-web communicate the port number to you ?

[AstraLuma]

it's typical to for such local servers just just print a link. And many GUI terminal emulators recognize links and make them clickable.

I will also point out that --serve-browser is default--in which case, the user doesn't need to do anything, the browser just opens.

[AstraLuma]

Also, you might not need to implement special behavior--on unix-like socket systems, binding to port 0 will cause the OS to atomically pick a port for you. You can ask what port was picked with getsockname() (c & libc, i don't know haskell).

So instead of binding and using the arguments to the bind, you bind and then use the address from getsockname() as the link.

(I wonder if this also helps with IPv4 vs IPv6 oddities.)

[AstraLuma]

As for consistent behavior, it can be opt-in--again, extremely common in my experience.

As someone used to working with these ad-hoc HTTP servers, I was expecting hledger-web to have some default port, and asking for 0 for it to randomize.

[simonmichael]

I'm wondering how exactly a script would acquire the port number, since hledger-web usually keeps running and logging to stdio. (You're right that in --serve-browse mode, this isn't a problem.)

I'm not likely to work on this myself, but would welcome any PRs.