Add an early validation of repo names specified in the pom.

An invalid name will now cause an early build failure with a clear message in the build log, rather than the incomprehensible “broken pipe” error at docker build time.

plugin/src/main/java/com/spotify/plugin/dockerfile/BuildMojo.java

@@ -34,6 +34,7 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
+import java.util.regex.Pattern;
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import org.apache.maven.plugin.MojoExecutionException;
@@ -221,6 +222,7 @@ static String buildImage(@Nonnull DockerClient dockerClient,
     log.info(""); // Spacing around build progress
     try {
       if (repository != null) {
+        validateRepository(repository);
         final String name = formatImageName(repository, tag);
         log.info(MessageFormat.format("Image will be built as {0}", name));
         log.info(""); // Spacing around build progress
@@ -238,6 +240,20 @@ static String buildImage(@Nonnull DockerClient dockerClient,
     return progressHandler.builtImageId();
   }
 
+  private static String VALID_REPO_REGEX =
+          "^([a-z0-9_.-])+(\\/(?=[a-z0-9_.-])[a-z0-9_.-]+)*$";
+
+
+  static void validateRepository(@Nonnull String repository)
+      throws MojoFailureException {
+    Pattern pattern = Pattern.compile(VALID_REPO_REGEX);
+    if (!pattern.matcher(repository).matches()) {
+      throw new MojoFailureException(
+              "Repo name \"" + repository +
+                      "\" must contain only lowercase, numbers, '-', '_' or '.'.");
+    }
+  }
+
   private static void requireValidDockerFilePath(@Nonnull Log log,
                                                  @Nonnull Path contextDirectory,
                                                  @Nullable Path dockerfile)

plugin/src/test/java/com/spotify/plugin/dockerfile/TestRepoNameValidation.java

@@ -0,0 +1,72 @@
+/*-
+ * -\-\-
+ * Dockerfile Maven Plugin
+ * --
+ * Copyright (C) 2019 Simon Woodward
+ * --
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * 
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ * -/-/-
+ */
+
+package com.spotify.plugin.dockerfile;
+
+import static org.junit.Assert.assertTrue;
+
+import org.apache.maven.plugin.MojoFailureException;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+
+public class TestRepoNameValidation {
+  public ExpectedException thrown = ExpectedException.none();
+
+  @Test
+  public void testSuccess() throws MojoFailureException {
+    BuildMojo.validateRepository("alllowercase");
+    BuildMojo.validateRepository("with000numbers");
+    BuildMojo.validateRepository("00withnumbers");
+    BuildMojo.validateRepository("00757383");
+    BuildMojo.validateRepository("withnumbers34343");
+    BuildMojo.validateRepository("with-hyphens");
+    BuildMojo.validateRepository("with_underscores");
+    BuildMojo.validateRepository("with.dots");
+    BuildMojo.validateRepository("______");
+    BuildMojo.validateRepository("------");
+    BuildMojo.validateRepository("......");
+    BuildMojo.validateRepository("example.com/okay./.path");
+    BuildMojo.validateRepository(".start.and.end.");
+    BuildMojo.validateRepository("-start-and-end-");
+    BuildMojo.validateRepository("_start_and_end_");
+    // Forward slash delimits the repo user from the repo name; strictly speaking,
+    // you're allowed only one slash, somewhere in the middle.
+    BuildMojo.validateRepository("with/forwardslash");
+    BuildMojo.validateRepository("with/multiple/forwardslash");
+  }
+
+  private boolean throwsMojoFailure(String repoName) {
+    boolean threw = false;
+    try {
+      BuildMojo.validateRepository(repoName);
+    } catch (MojoFailureException e) {
+      threw = true;
+    }
+    return threw;
+  }
+
+  @Test
+  public void testFailCases() {
+    assertTrue("Mixed case didn't fail", throwsMojoFailure("ddddddDddddd"));
+    assertTrue("Symbols didn't fail", throwsMojoFailure("ddddddDd+dddd"));
+    assertTrue("Starting slash didn't fail", throwsMojoFailure("/atstart"));
+    assertTrue("Ending slash didn't fail", throwsMojoFailure("atend/"));
+  }
+}