add max retry parameter to exponential backoff

[snowp]

Adds a parameter to the ExponentatialBackoff allowing for the maximum number of retires to be specified, allowing for both a max retry count as well as a max duration to be specified for the backoff.

Fixes #55

[yaozongyou]

same problem here, is there any will to merge this pull request?

[yaozongyou]

Currently without this commit, normal retry can be worked around this:

fn retry<F, B, T, E>(max_attempts: usize, backoff: B, mut op: F) -> Result<T, backoff::Error<E>>
where
    F: FnMut() -> Result<T, backoff::Error<E>>,
    B: backoff::backoff::Backoff,
{
    let mut attempt_count = 0;

    backoff::retry(backoff, || {
        let result = op();
        attempt_count += 1;
        if attempt_count >= max_attempts {
            result.map_err(|err| match err {
                backoff::Error::Permanent(err) => backoff::Error::permanent(err),
                backoff::Error::Transient { err, .. } => backoff::Error::permanent(err),
            })
        } else {
            result
        }
    })
}

but with future retry, it is more complicated, i have tried to using backoff::future::retry_notify and backoff::Notify, record each attempt to Notify, if the record in notify exceeds maximum, return some permanent error, but in the end, it does not work out.

[aawsome]

Another workaround is to use a custom struct implementing Backoff, e.g.

struct ExpBackoffWithMaxRetries {
   exp: ExponentialBackoff,
   retries: usize,
}

impl Backoff for ExpBackoffWithMaxRetries {
   fn reset(&mut self) {
      self.exp.reset();
      self.retries = 0;
   }

   fn next_backoff(&mut self) -> Option<Duration> {
      self.retries += 1;
      if self.retries >= MAX_RETRIES {
         self.exp.next_backoff()
      } else {
         None
      }
   }
}