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https://issues.apache.org/jira/browse/PDFBOX-6201?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18077239#comment-18077239
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Michael Klink commented on PDFBOX-6201:
---------------------------------------
So if I correctly understand you, the issue here is that a PDFBox repair
mechanism for broken cross references can be improved, and the "Form field
values lost when loading PDFs with many incremental saves" effect is just an
arbitrary example of what can be prevented by improving that repair mechanism.
If that is the case, this issue IMO should be classified as an improvement
instead of a bug, and its title should be something like "Keep good cross
reference info when encountering bad cross reference data".
As an aside, it looks like the {{COSParser}} class in question has been
seriously overhauled in the current development branch, so you might want to
test against that branch, too.
> PDFBox Bug: Form field values lost when loading PDFs with many incremental
> saves
> --------------------------------------------------------------------------------
>
> Key: PDFBOX-6201
> URL: https://issues.apache.org/jira/browse/PDFBOX-6201
> Project: PDFBox
> Issue Type: Bug
> Components: Parsing
> Affects Versions: 3.0.7 PDFBox
> Reporter: Stefan Ziegler
> Priority: Major
> Attachments: COSParser.java, COSParser.patch, pdfbox-6201-1.pdf,
> pdfbox-6201-2.pdf
>
>
> PDFBox Bug: Form field values lost when loading PDFs with many incremental
> saves
> ================================================================================
> Component: pdfbox - COSParser, BruteForceParser
> Affects: 3.0.7 (confirmed); likely all prior versions
> Severity: Major - visible data loss (form field values silently set to empty)
> SYMPTOM
> -------
> Loading a PDF with many incremental saves (e.g. 1948 startxref/%%EOF sections)
> causes PDFBox to silently lose form field values. The original PDF, when
> viewed in
> Adobe Acrobat, Chrome, or qpdf, correctly shows filled-in values such as
> "xxxx", "xxxx", "xxxx", "xxxx". After loading with
> PDFBox and saving, all fields are empty.
> qpdf and Ghostscript process the same PDF without errors or warnings.
> Running "qpdf" on the PDF beforehand produces a clean file that
> PDFBox handles correctly.
> ROOT CAUSE
> ----------
> The bug is in COSParser.checkXrefOffsets() (called from parseXref(), lenient
> mode only).
> Step-by-step trace:
> 1. parseXref() correctly traverses the full /Prev chain (5 XRef streams):
> Depth 0: XRef@7165114 /Size=721 8 entries /Prev=7148230
> Depth 1: XRef@7148230 /Size=715 10 entries /Prev=7144285
> Depth 2: XRef@7144285 /Size=708 340 entries /Prev=116 <- has
> Obj 185
> Depth 3: XRef@116 /Size=159 131 entries /Prev=128867
> Depth 4: XRef@128867 /Size=28 28 entries /Prev=none
> After setStartxref(), xrefTrailerResolver.getXrefTable() has 384 entries.
> Obj 185 -> offset 2523997, which contains /V (xxxx). CORRECT.
> 2. checkXrefOffsets() is called (lenient mode). It calls
> validateXrefOffsets().
> 3. validateXrefOffsets() iterates over all 384 entries. At the FIRST entry
> whose
> offset cannot be dereferenced (findObjectKey returns null), it immediately
> returns false -- without checking the remaining entries.
> 4. Back in checkXrefOffsets(), because validateXrefOffsets() returned false:
> xrefOffset.clear(); // DESTROYS all 384 correct
> entries
> xrefOffset.putAll(bfCOSObjectKeyOffsets); // replaces with
> brute-force results
> 5. BruteForceParser.getBFCOSObjectOffsets() scans the file linearly using
> map.put()
> (not putIfAbsent). For each "N 0 obj" marker found, it overwrites the
> previous
> entry for that object number. The LAST physical occurrence wins.
> 6. Object 185 appears 44 times physically. The last occurrence (offset
> 7019154) is
> an empty copy written by a later auto-save -- it has no /V entry.
> 7. PDFBox loads the empty object. Text4.getValueAsString() returns "".
> Verification:
> Before checkXrefOffsets: xrefTable.size()=384, obj185=2523997 <- CORRECT
> After checkXrefOffsets: xrefTable.size()=85, obj185=null <- BUG
> THE FIX
> -----------------------
> FIX 1: COSParser.checkXrefOffsets()
> Replace the all-or-nothing logic with selective correction.
> Collect all invalid keys (don't stop at first failure), then only replace
> those
> specific invalid entries with brute-force results. Leave valid entries
> untouched.
> See attached COSParser.java for the full implementation:
> - checkXrefOffsets() now calls collectInvalidXrefKeys() instead of
> validateXrefOffsets()
> - collectInvalidXrefKeys() checks ALL entries and returns only the invalid
> ones
> - Only invalid entries are corrected via brute force; valid ones are preserved
> Note: Fix 1 alone fully resolves the reported issue.
> BruteForceParser is not involved in this bug path at all -- obj 185 has a
> valid
> XRef entry and is therefore never touched by the brute-force scan after the
> fix.
> FULL XRef CHAIN
> ---------------
> Offset Obj Size Entries Prev Contains obj 185?
> 7165114 720 721 8 7148230 no
> 7148230 714 715 10 7144285 no (has obj 184, not 185)
> 7144285 707 708 340 116 YES -> offset 2523997
> 116 67 159 131 128867 no
> 128867 6 28 28 - no
> All 5 XRef streams decompress without error. The chain is valid.
> PDFBox reads all 384 entries correctly before checkXrefOffsets() destroys
> them.
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