Make fileIDs deterministic

for reproducible builds

Author: bmwiedemann

fop-core/src/main/java/org/apache/fop/pdf/FileIDGenerator.java

@@ -86,7 +86,9 @@ byte[] getUpdatedFileID() {
 
         private void generateFileID() {
             DateFormat df = new SimpleDateFormat("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'SSS");
-            digest.update(PDFDocument.encode(df.format(new Date())));
+            String sde = System.getenv("SOURCE_DATE_EPOCH");
+            Date d = (sde == null) ? new Date() : new Date(1000 * Long.parseLong(sde));
+            digest.update(PDFDocument.encode(df.format(d)));
             // Ignoring the filename here for simplicity even though it's recommended
             // by the PDF spec
             digest.update(PDFDocument.encode(String.valueOf(document.getCurrentFileSize())));