Add support for label merge

There are two scenarios where handling a label configuration would require merging one label into another:

- The configured label exists in the current labels and its alias exists in the current labels
- Multiple aliases of a configured label exist in the current labels

Previously, "GitHub Label Sync" did not have support for label merge, and would return an error when the user's
configuration would have required a merge operation.

The merge capability is added here.

The GitHub labels API does not provide a merge capability, so merging labels requires the use of the issues API to
add the "expected" label to each issue/PR that has the "actual" label.

From the user's perspective, the outcome of this "merge" operation is no different from any simple "changed" operation:
the "actual" label has been replaced by the "expected" label in the repository labels and all issues/PRs the label was
applied to.

lib/action-label-diff.js

@@ -14,6 +14,28 @@ function actionLabelDiff(options) {
 		if (diffEntry.type === 'changed') {
 			return apiClient.updateLabel(repo, diffEntry.name, diffEntry.expected);
 		}
+		if (diffEntry.type === 'merge') {
+			return apiClient.getLabeledIssues(repo, diffEntry.name) // Get all issues and PRs that have the "actual" label
+				.then((issues) => {
+					const mergeIssues = issues.filter((issue) => {
+						// Per-issue label application is needed where the "expected" label is not already present
+						return !issue.labels.some((label) => {
+							return label.name === diffEntry.expected.name;
+						});
+					});
+
+					// Add the "expected" label to the issues
+					const issueActions = mergeIssues.map((issue) => {
+						return apiClient.labelIssue(repo, issue.number, diffEntry.expected.name);
+					});
+
+					return Promise.all(issueActions);
+				})
+				.then(() => {
+					// Now that all issues with "actual" label have "expected" label, "actual" label can be deleted
+					return apiClient.deleteLabel(repo, diffEntry.name);
+				});
+		}
 		if (diffEntry.type === 'added') {
 			return apiClient.deleteLabel(repo, diffEntry.name);
 		}

lib/github-label-api.js

@@ -87,6 +87,54 @@ class ApiClient {
 		});
 	}
 
+	getLabeledIssues (repo, labelName) {
+		return new Promise((resolve, reject) => {
+			const allIssues = [];
+			const endpoint = `/repos/${repo}/issues`;
+			const params = {
+				labels: labelName,
+				page: 1,
+				per_page: 100,
+				state: 'all'
+			};
+			const getCallback = (error, status, issues) => {
+				if (error) {
+					error.method = 'GET';
+					error.endpoint = endpoint;
+					return reject(error);
+				}
+				if (status !== 200) {
+					return reject(new Error(`API responded with ${status} status`));
+				}
+				allIssues.push.apply(allIssues, issues);
+				if (issues.length === params.per_page) {
+					params.page += 1;
+					this.apiClient.get(endpoint, params, getCallback);
+				} else {
+					resolve(allIssues);
+				}
+			};
+			this.apiClient.get(endpoint, params, getCallback);
+		});
+	}
+
+	labelIssue (repo, issueNumber, labelName) {
+		return new Promise((resolve, reject) => {
+			const endpoint = `/repos/${repo}/issues/${issueNumber}/labels`;
+			this.apiClient.post(endpoint, {labels: [labelName]}, (error, status) => {
+				if (error) {
+					error.method = 'POST';
+					error.endpoint = endpoint;
+					return reject(error);
+				}
+				if (status !== 200) {
+					return reject(new Error(`API responded with ${status} status`));
+				}
+				resolve();
+			});
+		});
+	}
+
 	deleteLabel (repo, labelName) {
 		labelName = encodeURIComponent(labelName);
 		return new Promise((resolve, reject) => {

lib/github-label-sync.js

@@ -74,15 +74,6 @@ function githubLabelSync(options) {
 			stringifyLabelDiff(labelDiff).forEach((diffLine) => {
 				log.info(format.diff(diffLine));
 			});
-
-			const labelDiffMerges = labelDiff.filter((diff) => {
-				return diff.type === 'merge';
-			});
-			if (labelDiffMerges.length) {
-				const messages = labelDiffMerges.map(diff => `Configuration of the "${diff.expected.name}" label requires unsupported merge operation`);
-				return Promise.reject({ message: messages.join('\n\n') });
-			}
-
 			return labelDiff;
 		})
 		.then((labelDiff) => {

test/unit/lib/action-label-diff.test.js

@@ -77,6 +77,121 @@ describe('lib/action-label-diff', () => {
 			assert.strictEqual(actions[0], updatePromise);
 		});
 
+		it('should convert "merge" diff entries to label API delete promises', () => {
+			const issues = [
+				{
+					number: 11,
+					labels: [
+						{
+							name: 'bar'
+						}
+					]
+				},
+				{
+					number: 42,
+					labels: [
+						{
+							name: 'bar'
+						},
+						{
+							name: 'baz'
+						}
+					]
+				}
+			];
+			apiClient.getLabeledIssues.returns(Promise.resolve(issues));
+			apiClient.labelIssue.returns(Promise.resolve());
+			const deletePromiseValue = 'asdf';
+			apiClient.deleteLabel.returns(Promise.resolve(deletePromiseValue));
+			options.diff.push({
+				name: 'foo',
+				type: 'merge',
+				actual: {
+					name: 'bar',
+					color: 'ff0000',
+					description: 'baz'
+				},
+				expected: {
+					name: 'foo',
+					color: '00ff00',
+					description: 'baz'
+				}
+			});
+			actions = actionLabelDiff(options);
+			assert.lengthEquals(actions, 1);
+			return actions[0]
+				.then((value) => {
+					assert.calledOnce(apiClient.getLabeledIssues);
+					assert.calledWithExactly(apiClient.getLabeledIssues, options.repo, options.diff[0].name);
+					assert.calledTwice(apiClient.labelIssue);
+					assert.calledWithExactly(apiClient.labelIssue.getCall(0), options.repo, issues[0].number, options.diff[0].expected.name);
+					assert.calledWithExactly(apiClient.labelIssue.getCall(1), options.repo, issues[1].number, options.diff[0].expected.name);
+					assert.calledOnce(apiClient.deleteLabel);
+					assert.calledWithExactly(apiClient.deleteLabel, options.repo, options.diff[0].name);
+					assert.strictEqual(value, deletePromiseValue);
+				});
+		});
+
+		it('should not attempt redundant issue labeling for "merge" diff entries', () => {
+			const issues = [
+				{
+					number: 11,
+					labels: [
+						{
+							name: 'bar'
+						}
+					]
+				},
+				{
+					number: 42,
+					labels: [
+						{
+							name: 'bar'
+						},
+						{
+							name: 'foo'
+						}
+					]
+				},
+				{
+					number: 123,
+					labels: [
+						{
+							name: 'bar'
+						},
+						{
+							name: 'baz'
+						}
+					]
+				}
+			];
+			apiClient.getLabeledIssues.returns(Promise.resolve(issues));
+			apiClient.labelIssue.returns(Promise.resolve());
+			apiClient.deleteLabel.returns(Promise.resolve());
+			options.diff.push({
+				name: 'foo',
+				type: 'merge',
+				actual: {
+					name: 'bar',
+					color: 'ff0000',
+					description: 'baz'
+				},
+				expected: {
+					name: 'foo',
+					color: '00ff00',
+					description: 'baz'
+				}
+			});
+			actions = actionLabelDiff(options);
+			assert.lengthEquals(actions, 1);
+			return actions[0]
+				.then(() => {
+					assert.calledTwice(apiClient.labelIssue);
+					assert.calledWithExactly(apiClient.labelIssue.getCall(0), options.repo, issues[0].number, options.diff[0].expected.name);
+					assert.calledWithExactly(apiClient.labelIssue.getCall(1), options.repo, issues[2].number, options.diff[0].expected.name);
+				});
+		});
+
 		it('should convert "added" diff entries to label API delete promises', () => {
 			const deletePromise = Promise.resolve();
 			apiClient.deleteLabel.returns(deletePromise);