feat: Add validateSingleCommit flag to validate the commit message if there's only a single commit in the PR (opt-in). This is intended to be used as a workaround for an issue with Github as in this case, the PR title won't be used as the default commit message when merging a PR. (#87)

* Validate commit message for 1 commit PRs

* Use catch-all error message for single-commit errors

* Add input description to action.yml

Author: simlrh

README.md

@@ -71,6 +71,11 @@ The action works without configuration, however you can provide options for cust
           # validation of the PR title and the pull request checks remain pending.
           # Note that a second check will be reported if this is enabled.
           wip: true
+          # When using "Squash and merge" on a PR with only one commit, GitHub
+          # will suggest using that commit message instead of the PR title for the
+          # merge commit, and it's easy to commit this by mistake. Enable this option
+          # to also validate the commit message for one commit PRs.
+          validateSingleCommit: true
 ```
 
 ## Event triggers

action.yml

@@ -26,3 +26,6 @@ inputs:
   wip:
     description: "For work-in-progress PRs you can typically use draft pull requests from Github. However, private repositories on the free plan don't have this option and therefore this action allows you to opt-in to using the special '[WIP]' prefix to indicate this state. This will avoid the validation of the PR title and the pull request checks remain pending. Note that a second check will be reported if this is enabled."
     required: false
+  validateSingleCommit:
+    description: "When using \"Squash and merge\" on a PR with only one commit, GitHub will suggest using that commit message instead of the PR title for the merge commit, and it's easy to commit this by mistake. Enable this option to also validate the commit message for one commit PRs." 
+    required: false

src/index.js

@@ -12,7 +12,8 @@ module.exports = async function run() {
       requireScope,
       wip,
       subjectPattern,
-      subjectPatternError
+      subjectPatternError,
+      validateSingleCommit
     } = parseConfig();
 
     const contextPullRequest = github.context.payload.pull_request;
@@ -48,6 +49,31 @@ module.exports = async function run() {
           subjectPattern,
           subjectPatternError
         });
+
+        if (validateSingleCommit) {
+          const {data: commits} = await client.pulls.listCommits({
+            owner,
+            repo,
+            pull_number: contextPullRequest.number,
+            per_page: 2
+          });
+
+          if (commits.length === 1) {
+            try {
+              await validatePrTitle(commits[0].commit.message, {
+                types,
+                scopes,
+                requireScope,
+                subjectPattern,
+                subjectPatternError
+              });
+            } catch (error) {
+              throw new Error(
+                `Pull request has only one commit and it's not semantic; this may lead to a non-semantic commit in the base branch (see https://github.community/t/how-to-change-the-default-squash-merge-commit-message/1155). Amend the commit message to match the pull request title, or add another commit.`
+              );
+            }
+          }
+        }
       } catch (error) {
         validationError = error;
       }

src/parseConfig.js

@@ -33,12 +33,20 @@ module.exports = function parseConfig() {
     wip = ConfigParser.parseBoolean(process.env.INPUT_WIP);
   }
 
+  let validateSingleCommit;
+  if (process.env.INPUT_VALIDATESINGLECOMMIT) {
+    validateSingleCommit = ConfigParser.parseBoolean(
+      process.env.INPUT_VALIDATESINGLECOMMIT
+    );
+  }
+
   return {
     types,
     scopes,
     requireScope,
     wip,
     subjectPattern,
-    subjectPatternError
+    subjectPatternError,
+    validateSingleCommit
   };
 };