improve summary: promote diffoscope in .buildcompare (#187)

fixes #175

Author: hboutemy

src/main/java/org/apache/maven/plugins/artifact/buildinfo/AbstractBuildinfoMojo.java

@@ -305,10 +305,10 @@ protected void skip(MavenProject last) throws MojoExecutionException {
         getLog().info("Skipping intermediate goal run, aggregate will be " + last.getArtifactId());
     }
 
-    protected void copyAggregateToRoot(File aggregate) throws MojoExecutionException {
+    protected File copyAggregateToRoot(File aggregate) throws MojoExecutionException {
         if (session.getProjects().size() == 1) {
             // mono-module, no aggregate file to deal with
-            return;
+            return aggregate;
         }
 
         // copy aggregate file to root target directory
@@ -323,10 +323,11 @@ protected void copyAggregateToRoot(File aggregate) throws MojoExecutionException
                     rootCopy.toPath(),
                     LinkOption.NOFOLLOW_LINKS,
                     StandardCopyOption.REPLACE_EXISTING);
-            getLog().info("Aggregate " + extension.substring(1) + " copied to " + rootCopy);
+            getLog().info("Aggregate " + extension.substring(1) + " copied to " + relative(rootCopy));
         } catch (IOException ioe) {
             throw new MojoExecutionException("Could not copy " + aggregate + " to " + rootCopy, ioe);
         }
+        return rootCopy;
     }
 
     protected BuildInfoWriter newBuildInfoWriter(PrintWriter p, boolean mono) {
@@ -413,4 +414,11 @@ private Toolchain getToolchain() {
 
         return tc;
     }
+
+    protected String relative(File file) {
+        File basedir = session.getTopLevelProject().getBasedir();
+        int length = basedir.getPath().length();
+        String path = file.getPath();
+        return path.substring(length + 1);
+    }
 }

src/main/java/org/apache/maven/plugins/artifact/buildinfo/CompareMojo.java

@@ -195,19 +195,14 @@ private void compareWithReference(Map<Artifact, String> artifacts, File referenc
         int missing = reference.size() / 3 /* 3 property keys par file: filename, length and checksums.sha512 */;
 
         if (ko + missing > 0) {
-            getLog().error("Reproducible Build output summary: "
-                    + MessageUtils.buffer().success(ok + " files ok")
-                    + ", " + MessageUtils.buffer().failure(ko + " different")
+            getLog().error("[Reproducible Builds] rebuild comparison result: "
+                    + MessageUtils.buffer().success(ok + " files match")
+                    + ", " + MessageUtils.buffer().failure(ko + " differ")
                     + ((missing == 0) ? "" : (", " + MessageUtils.buffer().failure(missing + " missing")))
                     + ((ignored.isEmpty()) ? "" : (", " + MessageUtils.buffer().warning(ignored.size() + " ignored"))));
-            getLog().error("see "
-                    + MessageUtils.buffer()
-                            .project("diff " + relative(referenceBuildinfo) + " " + relative(buildinfoFile))
-                            .build());
-            getLog().error("see also https://maven.apache.org/guides/mini/guide-reproducible-builds.html");
         } else {
-            getLog().info("Reproducible Build output summary: "
-                    + MessageUtils.buffer().success(ok + " files ok")
+            getLog().info("[Reproducible Builds] rebuild comparison result: "
+                    + MessageUtils.buffer().success(ok + " files match")
                     + ((ignored.isEmpty()) ? "" : (", " + MessageUtils.buffer().warning(ignored.size() + " ignored"))));
         }
 
@@ -243,15 +238,27 @@ private void compareWithReference(Map<Artifact, String> artifacts, File referenc
                 p.print("# ");
                 p.println(diffoscope);
             }
-            getLog().info("Reproducible Build output comparison saved to " + buildcompare);
         } catch (IOException e) {
             throw new MojoExecutionException("Error creating file " + buildcompare, e);
         }
 
-        copyAggregateToRoot(buildcompare);
+        String saved = "                                                 saved to " + relative(buildcompare);
+        if (ko + missing > 0) {
+            getLog().error(saved);
+        } else {
+            getLog().info(saved);
+        }
+        buildcompare = copyAggregateToRoot(buildcompare);
+
+        if (ko + missing > 0) {
+            getLog().error("[Reproducible Builds] to analyze the differences, see diffoscope instructions in "
+                    + relative(buildcompare));
+            getLog().error(
+                            "                      see also https://maven.apache.org/guides/mini/guide-reproducible-builds.html");
 
-        if (fail && (ko + missing > 0)) {
-            throw new MojoExecutionException("Build artifacts are different from reference");
+            if (fail) {
+                throw new MojoExecutionException("Rebuilt artifacts are different from reference");
+            }
         }
     }
 
@@ -300,13 +307,6 @@ private String getRepositoryFilename(Artifact a) {
         return path.substring(path.lastIndexOf('/'));
     }
 
-    private String relative(File file) {
-        File basedir = session.getTopLevelProject().getBasedir();
-        int length = basedir.getPath().length();
-        String path = file.getPath();
-        return path.substring(length + 1);
-    }
-
     private static String findPrefix(Properties reference, String actualGroupId, String actualFilename) {
         for (String name : reference.stringPropertyNames()) {
             if (name.endsWith(".filename") && actualFilename.equals(reference.getProperty(name))) {