Add "zeroEntryTimestamps()" option

See javadoc on the method for details on why and how. This
is basically the half-assed deterministic build from #229,
which is to say still extremely helpful when dealing with rsync.

src/main/groovy/com/github/jengelman/gradle/plugins/shadow/tasks/ShadowCopyAction.groovy

@@ -39,6 +39,10 @@ import java.util.zip.ZipException
 
 @Slf4j
 public class ShadowCopyAction implements CopyAction {
+    /**
+     * Jan 1st, 1980. The day the zip timestamps stood still.
+     */
+    public static final int DOS_TIMESTAMP_ZERO = 315558000000L
 
     private final File zipFile
     private final ZipCompressor compressor
@@ -47,18 +51,19 @@ public class ShadowCopyAction implements CopyAction {
     private final List<Relocator> relocators
     private final PatternSet patternSet
     private final ShadowStats stats
+    private final boolean zeroEntryTimestamps
 
     public ShadowCopyAction(File zipFile, ZipCompressor compressor, DocumentationRegistry documentationRegistry,
                             List<Transformer> transformers, List<Relocator> relocators, PatternSet patternSet,
-                            ShadowStats stats) {
-
+                            ShadowStats stats, boolean zeroEntryTimestamps) {
         this.zipFile = zipFile
         this.compressor = compressor
         this.documentationRegistry = documentationRegistry
         this.transformers = transformers
         this.relocators = relocators
         this.patternSet = patternSet
         this.stats = stats
+        this.zeroEntryTimestamps = zeroEntryTimestamps
     }
 
     @Override
@@ -170,7 +175,7 @@ public class ShadowCopyAction implements CopyAction {
                             ZipEntry archiveEntry = new ZipEntry(mappedPath)
                             archiveEntry.setTime(fileDetails.lastModified)
                             archiveEntry.unixMode = (UnixStat.FILE_FLAG | fileDetails.mode)
-                            zipOutStr.putNextEntry(archiveEntry)
+                            putNextEntry(archiveEntry)
                             fileDetails.copyTo(zipOutStr)
                             zipOutStr.closeEntry()
                         } else {
@@ -209,7 +214,7 @@ public class ShadowCopyAction implements CopyAction {
 
         private void visitArchiveDirectory(RelativeArchivePath archiveDir) {
             if (recordVisit(archiveDir)) {
-                zipOutStr.putNextEntry(archiveDir.entry)
+                putNextEntry(archiveDir.entry)
                 zipOutStr.closeEntry()
             }
         }
@@ -277,7 +282,7 @@ public class ShadowCopyAction implements CopyAction {
 
             try {
                 // Now we put it back on so the class file is written out with the right extension.
-                zipOutStr.putNextEntry(new ZipEntry(mappedName + ".class"))
+                putNextEntry(new ZipEntry(mappedName + ".class"))
                 IOUtils.copyLarge(new ByteArrayInputStream(renamedClass), zipOutStr)
                 zipOutStr.closeEntry()
             } catch (ZipException e) {
@@ -289,7 +294,7 @@ public class ShadowCopyAction implements CopyAction {
             String mappedPath = remapper.map(archiveFile.entry.name)
             RelativeArchivePath mappedFile = new RelativeArchivePath(new ZipEntry(mappedPath), archiveFile.details)
             addParentDirectories(mappedFile)
-            zipOutStr.putNextEntry(mappedFile.entry)
+            putNextEntry(mappedFile.entry)
             IOUtils.copyLarge(archive.getInputStream(archiveFile.entry), zipOutStr)
             zipOutStr.closeEntry()
         }
@@ -301,7 +306,7 @@ public class ShadowCopyAction implements CopyAction {
                 ZipEntry archiveEntry = new ZipEntry(path)
                 archiveEntry.setTime(dirDetails.lastModified)
                 archiveEntry.unixMode = (UnixStat.DIR_FLAG | dirDetails.mode)
-                zipOutStr.putNextEntry(archiveEntry)
+                putNextEntry(archiveEntry)
                 zipOutStr.closeEntry()
                 recordVisit(dirDetails.relativePath)
             } catch (Exception e) {
@@ -326,6 +331,13 @@ public class ShadowCopyAction implements CopyAction {
             return transformers.any { it.canTransformResource(element) }
         }
 
+        // manually intercept `putNextEntry` to zero out timestamps if requested.
+        private void putNextEntry(ZipEntry entry) {
+            if (zeroEntryTimestamps)
+                entry.setTime(DOS_TIMESTAMP_ZERO)
+            zipOutStr.putNextEntry(entry)
+        }
+
     }
 
     class RelativeArchivePath extends RelativePath {

src/main/groovy/com/github/jengelman/gradle/plugins/shadow/tasks/ShadowJar.java

@@ -16,7 +16,6 @@
 import org.gradle.api.internal.DocumentationRegistry;
 import org.gradle.api.internal.file.FileResolver;
 import org.gradle.api.internal.file.copy.CopyAction;
-import org.gradle.api.tasks.Input;
 import org.gradle.api.tasks.InputFiles;
 import org.gradle.api.tasks.Optional;
 import org.gradle.api.tasks.TaskAction;
@@ -36,6 +35,8 @@ public class ShadowJar extends Jar implements ShadowSpec {
     private final ShadowStats shadowStats = new ShadowStats();
     private final GradleVersionUtil versionUtil;
 
+    private boolean zeroEntryTimestamps = false;
+
     public ShadowJar() {
         versionUtil = new GradleVersionUtil(getProject().getGradle().getGradleVersion());
         dependencyFilter = new DefaultDependencyFilter(getProject());
@@ -54,7 +55,27 @@ public InheritManifest getManifest() {
     protected CopyAction createCopyAction() {
         DocumentationRegistry documentationRegistry = getServices().get(DocumentationRegistry.class);
         return new ShadowCopyAction(getArchivePath(), getInternalCompressor(), documentationRegistry,
-                transformers, relocators, getRootPatternSet(), shadowStats);
+                transformers, relocators, getRootPatternSet(), shadowStats, zeroEntryTimestamps);
+    }
+
+    /**
+     * Configure the shadow jar such that all timestamps on entries
+     * inside are fixed to Jan 1st 1980.
+     *
+     * Why would you want to do this? The timestamps usually are just set to the
+     * time the jar was built. However, those timestamps are repeated across every
+     * file inside the jar, which scatters byte-level changes throughout the
+     * shadow jar, which breaks any efforts to calculate efficient deltas between
+     * shadow jar builds (e.g. rsync). With static timestamps though, rsync just works.
+     *
+     * This is generally safe to turn on because nothing cares about
+     * internal timestamps, but it's off by default.
+     *
+     * @return this
+     */
+    public ShadowJar zeroEntryTimestamps() {
+        this.zeroEntryTimestamps = true;
+        return this;
     }
 
     protected ZipCompressor getInternalCompressor() {
@@ -278,6 +299,14 @@ public <R extends Relocator> ShadowJar relocate(Class<R> relocatorClass, Action<
         return this;
     }
 
+    public boolean isZeroEntryTimestamps() {
+        return zeroEntryTimestamps;
+    }
+
+    public void setZeroEntryTimestamps(boolean zeroEntryTimestamps) {
+        this.zeroEntryTimestamps = zeroEntryTimestamps;
+    }
+
     public List<Transformer> getTransformers() {
         return this.transformers;
     }

src/test/groovy/com/github/jengelman/gradle/plugins/shadow/ShadowPluginSpec.groovy

@@ -1,5 +1,6 @@
 package com.github.jengelman.gradle.plugins.shadow
 
+import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowCopyAction
 import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
 import com.github.jengelman.gradle.plugins.shadow.util.AppendableMavenFileRepository
 import com.github.jengelman.gradle.plugins.shadow.util.PluginSpecification
@@ -61,7 +62,7 @@ class ShadowPluginSpec extends PluginSpecification {
                 .withArguments('--stacktrace')
                 .withProjectDir(dir.root)
                 .forwardOutput()
-                .withDebug(true)
+//                .withDebug(true)
                 .withTestKitDir(getTestKitDir())
 
 
@@ -484,6 +485,36 @@ class ShadowPluginSpec extends PluginSpecification {
 
     }
 
+    def 'zeroes zip entry timestamps if requested'() {
+        given:
+        file('src/main/java/shadow/Passed.java') << '''
+            package shadow;
+            public class Passed {}
+        '''.stripIndent()
+
+        buildFile << """
+            dependencies { compile 'junit:junit:3.8.2' }
+
+            // tag::rename[]
+            shadowJar {
+               baseName = 'shadow'
+               classifier = null
+               version = null
+
+               zeroEntryTimestamps()
+            }
+            // end::rename[]
+        """.stripIndent()
+
+        when:
+        runner.withArguments('-S', 'shadowJar').build()
+
+        then:
+        getZipEntries(output("shadow.jar")).each {
+            assert it.getTime() == ShadowCopyAction.DOS_TIMESTAMP_ZERO
+        }
+    }
+
     private String escapedPath(File file) {
         file.path.replaceAll('\\\\', '\\\\\\\\')
     }

src/test/groovy/com/github/jengelman/gradle/plugins/shadow/util/PluginSpecification.groovy

@@ -2,6 +2,8 @@ package com.github.jengelman.gradle.plugins.shadow.util
 
 import com.github.jengelman.gradle.plugins.shadow.util.file.TestFile
 import com.google.common.base.StandardSystemProperty
+import org.apache.tools.zip.ZipEntry
+import org.apache.tools.zip.ZipFile
 import org.codehaus.plexus.util.IOUtil
 import org.gradle.testkit.runner.GradleRunner
 import org.junit.Rule
@@ -100,6 +102,10 @@ class PluginSpecification extends Specification {
         return sw.toString()
     }
 
+    List<ZipEntry> getZipEntries(File f) {
+        new ZipFile(f).entries.toList()
+    }
+
     void contains(File f, List<String> paths) {
         JarFile jar = new JarFile(f)
         paths.each { path ->