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`PdfSmartCopy` can take effectively unbounded time on nested shared Form XObjects

Aperta
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Dal 18/10/2026.

  • #1639 di @parag2507 — aperta

Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
48/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Attiva
Stack tecnologico
java
Ambito
backend, performance

Direzione di ricerca

Start in PdfSmartCopy.ByteStore, which the issue identifies as recursively traversing shared indirect objects during deduplication. Use the provided createPdfWithNestedFormXObjects example to reproduce the slowdown, then compare processing with PdfCopy. Done when PdfSmartCopy handles the shared nested Form XObjects without the reported excessive time and memory use.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

bug
Describe the bug

Using PdfSmartCopy to process a valid PDF with Form XObjects that reference nested resource dictionaries that repeatedly contain many Form XObjects from the next level can take a long time and use a lot of memory even for relatively small files. The problem seems to be that, in order to perform deduplication, PdfSmartCopy.ByteStore recursively traverses shared indirect objects once for every path through the resource graph, without keeping track of already-serialized indirect objects.

Important to note that using PdfCopy to process the same kind of file works fine, without taking too long or using too much memory.

To Reproduce

The document that triggered the issue cannot be shared because it contains private information. It is a 11 page document of roughly 300 KB, generated by the library TCPDF 6.7.5. The following code generates a 1 page pdf document that demonstrates the issue:

      private void test() {
        var pdfBytes = createPdfWithNestedFormXObjects(8, 8);
        try (var doc = new Document(); var out = new ByteArrayOutputStream(); var copy = new PdfSmartCopy(doc, out)) {
            doc.open();
            copy(pdfBytes, copy);
        }
      }

      void copy(byte[] bytes, PdfCopy copy) throws Exception {
        try (var reader = new PdfReader(bytes)) {
            for (var page = 1; page <= reader.getNumberOfPages(); page++) {
                copy.addPage(copy.getImportedPage(reader, page));
            }
        }
    }

      private byte[] createPdfWithNestedFormXObjects(int nestedLeves, int fanout) throws Exception {
        var output = new ByteArrayOutputStream();
        var document = new Document(PageSize.A4);
        var writer = PdfWriter.getInstance(document, output);
        document.open();
        document.add(new Paragraph("Test PDF, nested Form XObjects"));

        List<PdfTemplate> layer = new ArrayList<>();
        for (var index = 0; index < fanout; index++) {
            var leaf = PdfTemplate.createTemplate(writer, 20, 20);
            leaf.rectangle(0, 0, 10, 10);
            leaf.stroke();
            layer.add(leaf);
        }

        for (var level = 1; level < nestedLeves; level++) {
            var parents = new ArrayList<PdfTemplate>();
            for (var index = 0; index < fanout; index++) {
                var parent = PdfTemplate.createTemplate(writer, 20, 20);
                for (var nested : layer) {
                    parent.addTemplate(nested, 0, 0);
                }
                parents.add(parent);
            }
            layer = parents;
        }

        for (var form : layer) {
            writer.getDirectContent().addTemplate(form, 0, 0);
        }
        document.close();
        return output.toByteArray();
    }

Varying the parameters of createPdfWithNestedFormXObjects allows to see the cases that trigger the problem.

Expected behavior

Using PdfSmartCopy should take more or less the same amount of time and use more or less the same amount of memory of using PdfCopy to process a document with multple levels of Form XObjects that reference shared Form XObjects.

System
  • OpenPDF version: 3.0.3, 3.0.4, 3.0.5
Lingua principale
Java
Stelle
4.4k
Fork
714
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