Adopts [`go-changelog`](https://github.com/hashicorp/go-changelog) for managing Nomad's changelog. `go-changelog` is becoming the HashiCorp defacto standard tool for managing changelog, e.g. [Consul](https://github.com/hashicorp/consul/pull/8387), [Vault](https://github.com/hashicorp/vault/pull/10363), [Waypoint](https://github.com/hashicorp/waypoint/pull/1179). [Consul](https://github.com/hashicorp/consul/pull/8387) seems to be the first product to adopt it, and its PR has the most context - though I've updated `.changelog/README.md` with the relevant info here.
## Changes to developers workflow
When opening PRs, developers should add a changelog entry in `.changelog/<PR#>.txt`. Check [`.changelog/README.md`](https://github.com/hashicorp/nomad/blob/docs-adopt-gochangelog/.changelog/README.md#developer-guide).
For the WIP release, entries can be amended even after the PR merged, and new files may be added post-hoc (e.g. during transition period, missed accidentally, community PRs, etc).
### Transitioning
Pending PRs can start including the changelog entry files immediately.
For 1.1.3/1.0.9 cycle, the release coordinator should create the entries for any PR that gets merged without a changelog entry file. They should also move any 1.1.3 entry in CHANGELOG.md to a changelog entry file, as this PR done for GH-10818.
## Changes to release process
Before cutting a release, release coordinator should update the changelog by inserting the output of `make changelog` to CHANGELOG.md with appropriate headers. See [`.changelog/README.md`](https://github.com/hashicorp/nomad/blob/docs-adopt-gochangelog/.changelog/README.md#how-to-generate-changelog-entries-for-release) for more details.
## Details
go-changelog is a basic templating engine for maintaining changelog in HashiCorp environment.
It expects the changelog entries as files indexed by their PR number. The CLI generates the changelog section for a release by comparing two git references (e.g. `HEAD` and the latest release, e.g. `v1.1.2`), and still requires manual process for updating CHANGELOG.md and final formatting.
The approach has many nice advantages:
* Avoids changelog related merge conflicts: Each PR touches different file!
* Copes with amendments and post-PR updates: Just add or update a changelog entry file using the original PR numbers.
* Addresses the release backporting scenario: Cherry-picking PRs will cherry-pick the relevant changelog entry automatically!
* Only relies on data available through `git` - no reliance on GitHub metadata or require GitHub credentials
The approach has few downsides though:
* CHANGELOG.md going stale during development and must be updated manually before cutting the release
* Repository watchers can no longer glance at the CHANGELOG.md to see upcoming changes
* We can periodically update the file, but `go-changelog` tool does not aid with that
* `go-changelog` tool does not offer good error reporting. If an entry is has an invalid tag (e.g. uses `release-note:bugfix` instead of `release-note:bug`), the entry will be dropped silently
* We should update go-changelog to warn against unexpected entry tags
* TODO: Meanwhile, PR reviewers and release coordinators should watch out
## Potential follow ups
We should follow up with CI checks to ensure PR changes include a warning. I've opted not to include that now. We still make many non-changelog-worth PRs for website/docs, for large features that get merged in multiple small PRs. I did not want to include a check that fails often.
Also, we should follow up to have `go-changelog` emit better warnings on unexpected tag.
In Nomad 1.1.1 we generate a hosts file based on the Nomad-owned network
namespace, rather than using the default hosts file from the pause
container. This hosts file should be shared between tasks in the same
allocation so that tasks can update the file and have the results propagated
between tasks.
