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

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@parag2507 is already working on this.

Since Oct 18, 2026.

  • #1639 by @parag2507 — open

Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
java
Domain
backend, performance

Research direction

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.

Written by the indexing model from the issue text.

Description

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
Dominant language
Java
Stars
4.4k
Forks
714
No merged PRs in 30d

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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