Can't install Fluentd `1.19.0` with bundler

[stevehipwell]

Describe the bug

I can't upgrade to Fluentd 1.19.0 using bundler due to an inability to reconcile the dependencies.

To Reproduce

Attempt to use Fluentd 1.19.0 with bundler.

Expected behavior

I'd expect to be able to install Fluentd 1.19.0 with bundler.

Your Environment

- Fluentd version: 1.19.0
- Package version: n/a
- Operating system: n/a
- Kernel version: n/a

Your Configuration

n/a

Your Error Log

Fetching gem metadata from https://rubygems.org/..........
Resolving dependencies...
Could not find compatible versions

Because every version of async-http depends on async >= 2.10.2
  and every version of async depends on io-event ~> 1.11,
  every version of async-http requires io-event ~> 1.11.
And because fluentd >= 1.19.0 depends on async-http ~> 0.86,
  fluentd >= 1.19.0 requires io-event ~> 1.11.
So, because fluentd >= 1.19.0 depends on io-event < 1.11.0
  and Gemfile depends on fluentd = 1.19.0,
  version solving has failed.

Additional context

No response

[stevehipwell]

It looks like this was introduced in #5042.

CC @daipom

[Watson1978]

Can you attach your Gemfile to reproduce?

[daipom]

@stevehipwell
Thanks for your report!
Please try bundle update.
If it does not work, then please remove Gemfile.lock and bundle again.

$ rm Gemfile.lock
$ bundle

[stevehipwell]

Thanks @daipom, deleting the lock file was the fix. I had async & async-http explicitly versioned in the Gemfile due to previous issues; this meant the version was already too high in the lockfile even though I was removing the direct dependencies as part of the Fluentd upgrade.

[daipom]

@stevehipwell
Thanks for your report!

It seems that this issue occurs when dependencies were updated to the latest versions.
In v1.19.0, we added version constraints for certain gems (#5042), which conflict with the recent versions of async.
Although the dependencies can be resolved by downgrading async as well, somehow bundle update does not do that.
So, it's necessary to delete the Gemfile.lock and rebuild the entire dependencies...