Don't pause between long polls (payjoin#436)

For 1. simplicity of implementation, 2. consistency, and 3. UX responsiveness don't
pause between long polls.

This actually has negligible effect on privacy compared to what the change commit
suggests as Yuval Kogman <[email protected]> comments:

> For a given subdirectory ID, the amount of time in between requests that
> the directory sees before this change was 5 seconds + R_i, where R is the
> round trip latency to the relay and i indexes { Alice, Bob or Carol }.

> The directory will is able to measure R_i indirectly before this change, and
> almost directly after it, thereby potentially identifying who is the receiver and
> who is the sender, since those roles are distinguishable based on the latency
> patterns of a pair of potentially related (i.e. temporally close but satisfying
> the ordering constraints of the BIP 77 messaging patterns). In other words
> R_i is potentially identifying metadata that leaks for each subdirectory, and
> by correlating different subdirectories (for which differences in this noisy
> measurement are helpful), the directory could still heuristically identify the
> parties involved, or at least learn some information or constraints about them
> with the same difficulty both before and after this change.  In other words
> R_i is potentially identifying metadata that leaks for each subdirectory, and
> by correlating different subdirectories (for which differences in this noisy
> measurement are helpful), the directory could still heuristically identify the
> parties involved, or at least learn some information or constraints about them
> with the same difficulty both before and after this change.

Author: DanGould

payjoin-cli/src/app/v2.rs

@@ -206,7 +206,6 @@ impl App {
                         Ok(Some(psbt)) => return Ok(psbt),
                         Ok(None) => {
                             println!("No response yet.");
-                            tokio::time::sleep(std::time::Duration::from_secs(5)).await;
                         }
                         Err(re) => {
                             println!("{}", re);
@@ -245,9 +244,8 @@ impl App {
                 .process_res(ohttp_response.bytes().await?.to_vec().as_slice(), context)
                 .map_err(|_| anyhow!("GET fallback failed"))?;
             log::debug!("got response");
-            match proposal {
-                Some(proposal) => break Ok(proposal),
-                None => std::thread::sleep(std::time::Duration::from_secs(5)),
+            if let Some(proposal) = proposal {
+                break Ok(proposal);
             }
         }
     }