Add a workflow step for adding comments to a MR

src/main/java/com/dabsquared/gitlabjenkins/gitlab/api/GitLabApi.java

@@ -100,6 +100,13 @@ public interface GitLabApi {
                             @QueryParam("merge_commit_message") String mergeCommitMessage,
                             @QueryParam("should_remove_source_branch") boolean shouldRemoveSourceBranch);
 
+    @POST
+    @Produces(MediaType.APPLICATION_JSON)
+    @Path("/projects/{projectId}/merge_requests/{mergeRequestId}/notes")
+    void createMergeRequestNote(@PathParam("projectId") Integer projectId,
+                                @PathParam("mergeRequestId") Integer mergeRequestId,
+                                @QueryParam("body") String body);
+
     @POST
     @Produces(MediaType.APPLICATION_JSON)
     @Path("/projects/{projectId}/merge_requests/{mergeRequestId}/notes")

src/main/java/com/dabsquared/gitlabjenkins/workflow/AddGitLabMergeRequestCommentStep.java

@@ -0,0 +1,123 @@
+package com.dabsquared.gitlabjenkins.workflow;
+
+import com.dabsquared.gitlabjenkins.cause.GitLabWebHookCause;
+import com.dabsquared.gitlabjenkins.gitlab.api.GitLabApi;
+import hudson.Extension;
+import hudson.model.Run;
+import hudson.model.TaskListener;
+import org.apache.commons.lang.StringUtils;
+import org.jenkinsci.plugins.workflow.steps.AbstractStepDescriptorImpl;
+import org.jenkinsci.plugins.workflow.steps.AbstractStepImpl;
+import org.jenkinsci.plugins.workflow.steps.AbstractSynchronousStepExecution;
+import org.jenkinsci.plugins.workflow.steps.StepContext;
+import org.jenkinsci.plugins.workflow.steps.StepContextParameter;
+import org.kohsuke.stapler.DataBoundConstructor;
+import org.kohsuke.stapler.export.ExportedBean;
+
+import javax.inject.Inject;
+import javax.ws.rs.ProcessingException;
+import javax.ws.rs.WebApplicationException;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+import static com.dabsquared.gitlabjenkins.connection.GitLabConnectionProperty.getClient;
+
+/**
+ * @author <a href="mailto:robin.mueller@1und1.de">Robin Müller</a>
+ */
+@ExportedBean
+public class AddGitLabMergeRequestCommentStep extends AbstractStepImpl {
+
+    private static final Logger LOGGER = Logger.getLogger(AddGitLabMergeRequestCommentStep.class.getName());
+
+    private String comment;
+
+    @DataBoundConstructor
+    public AddGitLabMergeRequestCommentStep(String comment) {
+        this.comment = StringUtils.isEmpty(comment) ? null : comment;
+    }
+
+    public String getComment() {
+        return comment;
+    }
+
+    public static class Execution extends AbstractSynchronousStepExecution<Void> {
+        private static final long serialVersionUID = 1;
+
+        @StepContextParameter
+        private transient Run<?, ?> run;
+
+        @Inject
+        private transient AddGitLabMergeRequestCommentStep step;
+
+        @Override
+        protected Void run() throws Exception {
+            GitLabWebHookCause cause = run.getCause(GitLabWebHookCause.class);
+            if (cause != null) {
+                Integer projectId = cause.getData().getTargetProjectId();
+                Integer mergeRequestId = cause.getData().getMergeRequestId();
+                if (projectId != null && mergeRequestId != null) {
+                    GitLabApi client = getClient(run);
+                    if (client == null) {
+                        println("No GitLab connection configured");
+                    } else {
+                        try {
+                            client.createMergeRequestNote(projectId, mergeRequestId, step.getComment());
+                        } catch (WebApplicationException | ProcessingException e) {
+                            printf("Failed to add comment on Merge Request for project '%s': %s%n", projectId, e.getMessage());
+                            LOGGER.log(Level.SEVERE, String.format("Failed to add comment on Merge Request for project '%s'", projectId), e);
+                        }
+                    }
+                }
+            }
+            return null;
+        }
+
+        private void println(String message) {
+            TaskListener listener = getTaskListener();
+            if (listener == null) {
+                LOGGER.log(Level.FINE, "failed to print message {0} due to null TaskListener", message);
+            } else {
+                listener.getLogger().println(message);
+            }
+        }
+
+        private void printf(String message, Object... args) {
+            TaskListener listener = getTaskListener();
+            if (listener == null) {
+                LOGGER.log(Level.FINE, "failed to print message {0} due to null TaskListener", String.format(message, args));
+            } else {
+                listener.getLogger().printf(message, args);
+            }
+        }
+
+        private TaskListener getTaskListener() {
+            StepContext context = getContext();
+            if (!context.isReady()) {
+                return null;
+            }
+            try {
+                return context.get(TaskListener.class);
+            } catch (Exception x) {
+                return null;
+            }
+        }
+    }
+
+    @Extension
+    public static final class DescriptorImpl extends AbstractStepDescriptorImpl {
+        public DescriptorImpl() {
+            super(Execution.class);
+        }
+
+        @Override
+        public String getDisplayName() {
+            return "Add comment on GitLab Merge Request";
+        }
+
+        @Override
+        public String getFunctionName() {
+            return "addGitLabMRComment";
+        }
+    }
+}

src/main/resources/com/dabsquared/gitlabjenkins/workflow/AddGitLabMergeRequestCommentStep/config.jelly

@@ -0,0 +1,6 @@
+<?jelly escape-by-default='true'?>
+<j:jelly xmlns:j="jelly:core" xmlns:f="/lib/form">
+  <f:entry title="${%Comment}" field="comment">
+    <f:textarea/>
+  </f:entry>
+</j:jelly>