GitHub APIのreleaseオブジェクトから、tagを取り出してtagに紐づくコミットハッシュを取得したい、という場面がありました。

releaseオブジェクトの中身は以下のような具合でコミットハッシュを持っていません（APIはこちらを参照）。

[
  {
    "url": "https://api.github.com/repos/octocat/Hello-World/releases/1",
    "html_url": "https://github.com/octocat/Hello-World/releases/v1.0.0",
    "assets_url": "https://api.github.com/repos/octocat/Hello-World/releases/1/assets",
    "upload_url": "https://uploads.github.com/repos/octocat/Hello-World/releases/1/assets{?name,label}",
    "tarball_url": "https://api.github.com/repos/octocat/Hello-World/tarball/v1.0.0",
    "zipball_url": "https://api.github.com/repos/octocat/Hello-World/zipball/v1.0.0",
    "id": 1,
    "node_id": "MDc6UmVsZWFzZTE=",
    "tag_name": "v1.0.0",
    "target_commitish": "master",
    "name": "v1.0.0",
    "body": "Description of the release",
    "draft": false,
    "prerelease": false,
    "created_at": "2013-02-27T19:35:32Z",
    "published_at": "2013-02-27T19:35:32Z",
    "author": {
      "login": "octocat",
      "id": 1,
      "node_id": "MDQ6VXNlcjE=",
      "avatar_url": "https://github.com/images/error/octocat_happy.gif",
      "gravatar_id": "",
      "url": "https://api.github.com/users/octocat",
      "html_url": "https://github.com/octocat",
      "followers_url": "https://api.github.com/users/octocat/followers",
      "following_url": "https://api.github.com/users/octocat/following{/other_user}",
      "gists_url": "https://api.github.com/users/octocat/gists{/gist_id}",
      "starred_url": "https://api.github.com/users/octocat/starred{/owner}{/repo}",
      "subscriptions_url": "https://api.github.com/users/octocat/subscriptions",
      "organizations_url": "https://api.github.com/users/octocat/orgs",
      "repos_url": "https://api.github.com/users/octocat/repos",
      "events_url": "https://api.github.com/users/octocat/events{/privacy}",
      "received_events_url": "https://api.github.com/users/octocat/received_events",
      "type": "User",
      "site_admin": false
    },
    "assets": [
      {
        "url": "https://api.github.com/repos/octocat/Hello-World/releases/assets/1",
        "browser_download_url": "https://github.com/octocat/Hello-World/releases/download/v1.0.0/example.zip",
        "id": 1,
        "node_id": "MDEyOlJlbGVhc2VBc3NldDE=",
        "name": "example.zip",
        "label": "short description",
        "state": "uploaded",
        "content_type": "application/zip",
        "size": 1024,
        "download_count": 42,
        "created_at": "2013-02-27T19:35:32Z",
        "updated_at": "2013-02-27T19:35:32Z",
        "uploader": {
          "login": "octocat",
          "id": 1,
          "node_id": "MDQ6VXNlcjE=",
          "avatar_url": "https://github.com/images/error/octocat_happy.gif",
          "gravatar_id": "",
          "url": "https://api.github.com/users/octocat",
          "html_url": "https://github.com/octocat",
          "followers_url": "https://api.github.com/users/octocat/followers",
          "following_url": "https://api.github.com/users/octocat/following{/other_user}",
          "gists_url": "https://api.github.com/users/octocat/gists{/gist_id}",
          "starred_url": "https://api.github.com/users/octocat/starred{/owner}{/repo}",
          "subscriptions_url": "https://api.github.com/users/octocat/subscriptions",
          "organizations_url": "https://api.github.com/users/octocat/orgs",
          "repos_url": "https://api.github.com/users/octocat/repos",
          "events_url": "https://api.github.com/users/octocat/events{/privacy}",
          "received_events_url": "https://api.github.com/users/octocat/received_events",
          "type": "User",
          "site_admin": false
        }
      }
    ]
  }
]

refを渡すとrefが指すコミットハッシュを解決して渡してくれるようなエンドポイントは探しましたが見つからず。しばらくGitHub APIを眺めて解決策を2つ見つけました。

list tagsのエンドポイントを叩いてtagの一覧からtag_nameで頑張って検索する
get a commitのエンドポイントを叩いてtagからコミットを取得する

tagは作成日などでソートされないので1の手法ではタグが増えると検索に時間がかかってしまいます。paginationを使ってループ作るなどの手間を考えると微妙です。ということで2番を試したところうまくできました。

Rubyだとoctokitを使って以下のようにしてコミットハッシュを取得することができます。

require 'octokit'
access_token = 'hoge'
client = Octokit::Client.new(access_token: access_token)

target_repository = 'YourOrg/yourrepo'
ref = "r20210105.1"
commit = client.commit(target_repository, ref)
p commit.sha

参考
https://github.com/octokit/octokit.rb/blob/d56af021d2/lib/octokit/client/commits.rb#L151