Give a repeatable PDF the same document id however the project was made (#1178)

With SOURCE_DATE_EPOCH set, the document id came from the project's uuid
and a hash of the .qet file. A project generated again from the same
data has new uuids, so its PDF differed in the id alone (XMP DocumentID
and trailer /ID, 120 bytes) although every page was the same.

Qt now writes a fixed placeholder id, and once the file is written the
placeholder is replaced with an id derived from the bytes of the PDF:
the same pages give the same id. Same length, so no offset changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/cli_export.cpp

@@ -42,7 +42,6 @@
 #include <QDir>
 #include <QDirIterator>
 #include <QDomDocument>
-#include <QCryptographicHash>
 #include <QDate>
 #include <QDateTime>
 #include <QFile>
@@ -63,7 +62,6 @@
 #include <QTextStream>
 #include <QTimeZone>
 #include <QTransform>
-#include <QUuid>
 
 namespace {
 
@@ -189,18 +187,16 @@ int exportPdf(QETProject &project, const QString &output,
 
 	// SOURCE_DATE_EPOCH (reproducible-builds.org) asks for the same file
 	// from the same input: the time it names instead of now, and a document
-	// id from the project file instead of a random one. Qt has no setter for
-	// the dates, so they are rewritten once the file is written. Before
-	// Qt 6.8 there is no document id to set: it is only written for PDF/A.
+	// id from what the PDF shows instead of a random one. Qt has no setter
+	// for the dates, and the content is not known yet, so both are
+	// rewritten once the file is written; Qt writes a fixed id until then.
+	// The id does not come from the project file, whose uuids are new each
+	// time a project is generated again from the same data. Before Qt 6.8
+	// there is no document id to set: it is only written for PDF/A.
 	const QDateTime sourceDate = sourceDateEpoch();
 #if QT_VERSION >= QT_VERSION_CHECK(6, 8, 0)
-	if (sourceDate.isValid()) {
-		QCryptographicHash hash(QCryptographicHash::Sha256);
-		QFile file(project.filePath());
-		if (file.open(QIODevice::ReadOnly))
-			hash.addData(&file);
-		writer.setDocumentId(QUuid::createUuidV5(project.uuid(), hash.result()));
-	}
+	if (sourceDate.isValid())
+		writer.setDocumentId(PdfLinks::placeholderDocumentId());
 #endif
 
 	QPainter painter;
@@ -277,8 +273,10 @@ int exportPdf(QETProject &project, const QString &output,
 	// the cross-references jump inside the document in any PDF viewer.
 	PdfLinks::convertUriToGoTo(output);
 	PdfLinks::removeUnusedPdfxNamespace(output);
-	if (sourceDate.isValid())
+	if (sourceDate.isValid()) {
 		PdfLinks::setDocumentDate(output, sourceDate);
+		PdfLinks::setDocumentIdFromContent(output);
+	}
 
 	out << "Exported " << diagrams.size() << " page(s) -> " << output << "\n";
 	return 0;

sources/pdf_links.cpp

@@ -28,6 +28,7 @@
 #include <private/qpdf_p.h>
 
 #include <QByteArray>
+#include <QCryptographicHash>
 #include <QFile>
 #include <QGraphicsTextItem>
 #include <QList>
@@ -437,6 +438,35 @@ void removeUnusedPdfxNamespace(const QString &pdfPath)
 	f.close();
 }
 
+QUuid placeholderDocumentId()
+{
+	// Any fixed uuid: it is also the namespace of the ids made from it.
+	return QUuid(QStringLiteral("{6f1c2d4e-9a3b-4c5d-8e7f-0a1b2c3d4e5f}"));
+}
+
+void setDocumentIdFromContent(const QString &pdfPath)
+{
+	QFile f(pdfPath);
+	if (!f.open(QIODevice::ReadOnly)) return;
+	QByteArray data = f.readAll();
+	f.close();
+
+	// Qt writes the id as text in the XMP ("uuid:6f1c...") and as the hex
+	// of that same text in the trailer ("/ID [ <3666...> <3666...> ]").
+	const QUuid placeholder = placeholderDocumentId();
+	const QByteArray text = placeholder.toByteArray(QUuid::WithoutBraces);
+	if (!data.contains(text) && !data.contains(text.toHex())) return;
+
+	const QUuid id = QUuid::createUuidV5(
+		placeholder, QCryptographicHash::hash(data, QCryptographicHash::Sha256));
+	const QByteArray idText = id.toByteArray(QUuid::WithoutBraces);
+	data.replace(text, idText);
+	data.replace(text.toHex(), idText.toHex());
+
+	if (!f.open(QIODevice::WriteOnly | QIODevice::Truncate)) return;
+	f.write(data);
+}
+
 void setDocumentDate(const QString &pdfPath, const QDateTime &when)
 {
 	QFile f(pdfPath);

sources/pdf_links.h

@@ -24,6 +24,7 @@
 #include <QRectF>
 #include <QString>
 #include <QTransform>
+#include <QUuid>
 #include <functional>
 
 class QPdfEngine;
@@ -94,6 +95,19 @@ namespace PdfLinks {
 	*/
 	void setDocumentDate(const QString &pdfPath, const QDateTime &when);
 
+	/// The document id to give QPdfWriter before a repeatable export, for
+	/// setDocumentIdFromContent() to find and replace afterwards.
+	QUuid placeholderDocumentId();
+
+	/**
+		Post-process a PDF written with placeholderDocumentId(): replace that
+		id, in the XMP metadata and in the trailer's /ID, with one derived
+		from the bytes of the file.  The same pages give the same id, however
+		the project was made.  Same length, so no offset changes.  No-op when
+		the placeholder is absent.
+	*/
+	void setDocumentIdFromContent(const QString &pdfPath);
+
 	struct ComponentInfo {
 		QString contents;
 	};