Introduce CleanProperties

PropertiesFileTransformer uses `java.utils.Properties` internally as a storage.

`java.utils.Properties` `store0()` contains `bw.write("#" + new Date().toString());` that prepends current timestamp before any content (after comments).

 This effectively breaks reproducible builds that use PropertiesFileTransformer because every new build has different timestamp in transformed files.

 CleanProperties implementation is introduced in order to remove prepended timestamp when creating output stream.

src/main/groovy/com/github/jengelman/gradle/plugins/shadow/internal/CleanProperties.groovy

@@ -0,0 +1,32 @@
+/*
+ * Source https://stackoverflow.com/a/39043903/519333
+ */
+package com.github.jengelman.gradle.plugins.shadow.internal
+
+class CleanProperties extends Properties {
+    private static class StripFirstLineStream extends FilterOutputStream {
+
+        private boolean firstLineSeen = false
+
+        StripFirstLineStream(final OutputStream out) {
+            super(out)
+        }
+
+        @Override
+        void write(final int b) throws IOException {
+            if (firstLineSeen) {
+                super.write(b);
+            } else if (b == '\n') {
+                super.write(b);
+
+                firstLineSeen = true;
+            }
+        }
+
+    }
+
+    @Override
+    void store(final OutputStream out, final String comments) throws IOException {
+        super.store(new StripFirstLineStream(out), null)
+    }
+}

src/main/groovy/com/github/jengelman/gradle/plugins/shadow/transformers/PropertiesFileTransformer.groovy

@@ -19,6 +19,7 @@
 
 package com.github.jengelman.gradle.plugins.shadow.transformers
 
+import com.github.jengelman.gradle.plugins.shadow.internal.CleanProperties
 import org.apache.tools.zip.ZipEntry
 import org.apache.tools.zip.ZipOutputStream
 import org.codehaus.plexus.util.IOUtil
@@ -117,7 +118,7 @@ import static groovy.lang.Closure.IDENTITY
 class PropertiesFileTransformer implements Transformer {
     private static final String PROPERTIES_SUFFIX = '.properties'
 
-    private Map<String, Properties> propertiesEntries = [:]
+    private Map<String, CleanProperties> propertiesEntries = [:]
 
     @Input
     List<String> paths = []
@@ -179,15 +180,17 @@ class PropertiesFileTransformer implements Transformer {
     }
 
     private Properties loadAndTransformKeys(InputStream is) {
-        Properties props = new Properties()
-        props.load(is)
+        Properties props = new CleanProperties()
+        if (is != null) {
+            props.load(is)
+        }
         return transformKeys(props)
     }
 
     private Properties transformKeys(Properties properties) {
         if (keyTransformer == IDENTITY)
             return properties
-        def result = new Properties()
+        def result = new CleanProperties()
         properties.each { key, value ->
             result.put(keyTransformer.call(key), value)
         }

src/test/groovy/com/github/jengelman/gradle/plugins/shadow/transformers/PropertiesFileTransformerTest.groovy

@@ -0,0 +1,76 @@
+package com.github.jengelman.gradle.plugins.shadow.transformers
+
+import com.github.jengelman.gradle.plugins.shadow.ShadowStats
+import com.github.jengelman.gradle.plugins.shadow.relocation.Relocator
+import org.apache.tools.zip.ZipOutputStream
+import org.junit.Before
+import org.junit.Test
+
+import java.util.zip.ZipFile
+
+import static java.util.Arrays.asList
+import static org.junit.Assert.*
+
+/**
+ * Test for {@link PropertiesFileTransformer}.
+ */
+final class PropertiesFileTransformerTest extends TransformerTestSupport {
+    static final String MANIFEST_NAME = "META-INF/MANIFEST.MF"
+
+    private PropertiesFileTransformer transformer
+
+    @Before
+    void setUp() {
+        transformer = new PropertiesFileTransformer()
+    }
+
+    @Test
+    void testHasTransformedResource() {
+        transformer.transform(new TransformerContext(MANIFEST_NAME))
+
+        assertTrue(transformer.hasTransformedResource())
+    }
+
+    @Test
+    void testHasNotTransformedResource() {
+        assertFalse(transformer.hasTransformedResource())
+    }
+
+    @Test
+    void testTransformation() {
+        transformer.transform(new TransformerContext(MANIFEST_NAME, getResourceStream(MANIFEST_NAME), Collections.<Relocator>emptyList(), new ShadowStats()))
+
+        def testableZipFile = File.createTempFile("testable-zip-file-", ".jar")
+        def fileOutputStream = new FileOutputStream(testableZipFile)
+        def bufferedOutputStream = new BufferedOutputStream(fileOutputStream)
+        def zipOutputStream = new ZipOutputStream(bufferedOutputStream)
+
+        try {
+            transformer.modifyOutputStream(zipOutputStream, false)
+        } finally {
+            zipOutputStream.close()
+        }
+        def targetLines = readFrom(testableZipFile, MANIFEST_NAME)
+
+        assertFalse(targetLines.isEmpty())
+
+        assertTrue(targetLines.contains("Manifest-Version=1.0"))
+    }
+
+    static List<String> readFrom(File jarFile, String resourceName) {
+        def zip = new ZipFile(jarFile)
+        try {
+            def entry = zip.getEntry(resourceName)
+            if (!entry) {
+                return Collections.emptyList()
+            }
+            return zip.getInputStream(entry).readLines()
+        } finally {
+            zip.close()
+        }
+    }
+
+    InputStream getResourceStream(String resource) {
+        this.class.classLoader.getResourceAsStream(resource)
+    }
+}