qpdf/TODO

Fuzz Errors
===========

* https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=<N>

* Ignoring these:
  * Problems inside the jpeg library: 15454, 15471, 15751, 15987
  * Timeout in xref reconstruction (won't fix): 15659
  * Out of memory: 15730

Lexical
=======

 * Make it possible to run the lexer (tokenizer) over a whole file
   such that the following things would be possible:

   * Rewrite fix-qdf in C++ so that there is no longer a runtime perl
     dependency

   * Make it possible to replace all strings in a file lexically even
     on badly broken files. Ideally this should work files that are
     lacking xref, have broken links, etc., and ideally it should work
     with encrypted files if possible. This should go through the
     streams and strings and replace them with fixed or random
     characters, preferably, but not necessarily, in a manner that
     works with fonts. One possibility would be to detect whether a
     string contains characters with normal encoding, and if so, use
     0x41. If the string uses character maps, use 0x01. The output
     should otherwise be unrelated to the input. This could be built
     after the filtering and tokenizer rewrite and should be done in a
     manner that takes advantage of the other lexical features. This
     sanitizer should also clear metadata and replace images.

Page splitting/merging
======================

 * Update page splitting and merging to handle document-level
   constructs with page impact such as interactive forms and article
   threading. Check keys in the document catalog for others, such as
   outlines, page labels, thumbnails, and zones. For threads,
   Subramanyam provided a test file; see ../misc/article-threads.pdf.
   Email Q-Count: 431864 from 2009-11-03.

 * bookmarks (outlines) 12.3.3
   * support bookmarks when merging
   * prune bookmarks that don't point to a surviving page when merging
     or splitting
   * make sure conflicting named destinations work possibly test by
     including the same file by two paths in a merge
   * see also comments in issue 343

   Note: original implementation of bookmark preservation for split
   pages caused a very high performance hit. The problem was
   introduced in 313ba081265f69ac9a0324f9fe87087c72918191 and reverted
   in the commit that adds this paragraph. The revert includes marking
   a few tests cases as $td->EXPECT_FAILURE. When properly coded, the
   test cases will need to be adjusted to only include the parts of
   the outlines that are actually copied. The tests in question are
   "split page with outlines". When implementing properly, ensure that
   the performance is not adversely affected by timing split-pages on
   a large file with complex outlines such as the PDF specification.

   When pruning outlines, keep all outlines in the hierarchy that are
   above an outline for a page we care about. If one of the ancestor
   outlines points to a non-existent page, clear its dest. If an
   outline does not have any children that point to pages in the
   document, just omit it.

   Possible strategy:
   * resolve all named destinations to explicit destinations
   * concatenate top-level outlines
   * prune outlines whose dests don't point to a valid page
   * recompute all /Count fields

   Test files
   * page-labels-and-outlines.pdf: old file with both page labels and
     outlines. All destinations are explicit destinations. Each page
     has Potato and a number. All titles are feline names.
   * outlines-with-actions.pdf: mixture of explicit destinations,
     named destinations, goto actions with explicit destinations, and
     goto actions with named destinations; uses /Dests key in names
     dictionary. Each page has Salad and a number. All titles are
     silly words. One destination is an indirect object.
   * outlines-with-old-root-dests.pdf: like outlines-with-actions
     except it uses the PDF-1.1 /Dests dictionary for named
     destinations, and each page has Soup and a number. Also pages are
     numbered with upper-case Roman numerals starting with 0. All
     titles are silly words preceded by a bullet.

   If outline handling is significantly improved, see
   ../misc/bad-outlines/bad-outlines.pdf and email:
   https://mail.google.com/mail/u/0/#search/rfc822msgid%3A02aa01d3d013%249f766990%24de633cb0%24%40mono.hr)

 * Form fields: should be similar to outlines.

General
=======

NOTE: Some items in this list refer to files in my personal home
directory or that are otherwise not publicly accessible. This includes
things sent to me by email that are specifically not public. Even so,
I find it useful to make reference to them in this list

 * Add support for writing name and number trees

 * Figure out how to render Gajić correctly in the PDF version of the
   qpdf manual.

 * Consider creating a PPA for Ubuntu

 * Investigate whether there is a way to automate the memory checker
   tests for Windows.

 * Support user-pluggable stream filters.  This would enable external
   code to provide interpretation for filters that are missing from
   qpdf.  Make it possible for user-provided filters to override
   built-in filters.  Make sure that the pluggable filters can be
   prioritized so that we can poll all registered filters to see
   whether they are capable of filtering a particular stream.

 * If possible, consider adding CCITT3, CCITT4, or any other easy
   filters. For some reference code that we probably can't use but may
   be handy anyway, see
   http://partners.adobe.com/public/developer/ps/sdk/index_archive.html

 * If possible, support the following types of broken files:

    - Files that have no whitespace token after "endobj" such that
      endobj collides with the start of the next object

    - See ../misc/broken-files

 * Additional form features
   * set value from CLI? Specify title, and provide way to
     disambiguate, probably by giving objgen of field

 * replace mode: --replace-object, --replace-stream-raw,
   --replace-stream-filtered
   * update first paragraph of QPDF JSON in the manual to mention this
   * object numbers are not preserved by write, so object ID lookup
     has to be done separately for each invocation
   * you don't have to specify length for streams
   * you only have to specify filtering for streams if providing raw data

 * Pl_TIFFPredictor is pretty slow.

 * Support for handling file names with Unicode characters in Windows
   is incomplete. qpdf seems to support them okay from a functionality
   standpoint, and the right thing happens if you pass in UTF-8
   encoded filenames to QPDF library routines in Windows (they are
   converted internally to wchar_t*), but file names are encoded in
   UTF-8 on output, which doesn't produce nice error messages or
   output on Windows in some cases.

 * If we ever wanted to do anything more with character encoding, see
   ../misc/character-encoding/, which includes machine-readable dump
   of table D.2 in the ISO-32000 PDF spec. This shows the mapping
   between Unicode, StandardEncoding, WinAnsiEncoding,
   MacRomanEncoding, and PDFDocEncoding.

 * Some test cases on bad files fail because qpdf is unable to find
   the root dictionary when it fails to read the trailer. Recovery
   could find the root dictionary and even the info dictionary in
   other ways. In particular, issue-202.pdf can be opened by evince,
   and there's no real reason that qpdf couldn't be made to be able to
   recover that file as well.

 * Audit every place where qpdf allocates memory to see whether there
   are cases where malicious inputs could cause qpdf to attempt to
   grab very large amounts of memory. Certainly there are cases like
   this, such as if a very highly compressed, very large image stream
   is requested in a buffer. Hopefully normal input to output
   filtering doesn't ever try to do this. QPDFWriter should be checked
   carefully too. See also bugs/private/from-email-663916/

 * Interactive form modification:
   https://github.com/qpdf/qpdf/issues/213 contains a good discussion
   of some ideas for adding methods to modify annotations and form
   fields if we want to make it easier to support modifications to
   interactive forms. Some of the ideas have been implemented, and
   some of the probably never will be implemented, but it's worth a
   read if there is an intention to work on this. In the issue, search
   for "Regarding write functionality", and read that comment and the
   responses to it.

 * Look at ~/Q/pdf-collection/forms-from-appian/

 * Consider adding "uninstall" target to makefile. It should only
   uninstall what it installed, which means that you must run
   uninstall from the version you ran install with. It would only be
   supported for the toolchains that support the install target
   (libtool).

 * Provide support in QPDFWriter for writing incremental updates.
   Provide support in qpdf for preserving incremental updates.  The
   goal should be that QDF mode should be fully functional for files
   with incremental updates including fix_qdf.

   Note that there's nothing that says an indirect object in one
   update can't refer to an object that doesn't appear until a later
   update.  This means that QPDF has to treat indirect null objects
   differently from how it does now.  QPDF drops indirect null objects
   that appear as members of arrays or dictionaries.  For arrays, it's
   handled in QPDFWriter where we make indirect nulls direct.  This is
   in a single if block, and nothing else in the code cares about it.
   We could just remove that if block and not break anything except a
   few test cases that exercise the current behavior.  For
   dictionaries, it's more complicated.  In this case,
   QPDF_Dictionary::getKeys() ignores all keys with null values, and
   hasKey() returns false for keys that have null values.  We would
   probably want to make QPDF_Dictionary able to handle the special
   case of keys that are indirect nulls and basically never have it
   drop any keys that are indirect objects.

   If we make a change to have qpdf preserve indirect references to
   null objects, we have to note this in ChangeLog and in the release
   notes since this will change output files.  We did this before when
   we stopped flattening scalar references, so this is probably not a
   big deal.  We also have to make sure that the testing for this
   handles non-trivial cases of the targets of indirect nulls being
   replaced by real objects in an update.  I'm not sure how this plays
   with linearization, if at all.  For cases where incremental updates
   are not being preserved as incremental updates and where the data
   is being folded in (as is always the case with qpdf now), none of
   this should make any difference in the actual semantics of the
   files.

 * When decrypting files with /R=6, hash_V5 is called more than once
   with the same inputs.  Caching the results or refactoring to reduce
   the number of identical calls could improve performance for
   workloads that involve processing large numbers of small files.

 * Consider adding a method to balance the pages tree.  It would call
   pushInheritedAttributesToPage, construct a pages tree from scratch,
   and replace the /Pages key of the root dictionary with the new
   tree.

 * Secure random number generation could be made more efficient by
   using a local static to ensure a single random device or crypt
   provider as long as this can be done in a thread-safe fashion.  In
   the initial implementation, this is being skipped to avoid having
   to add any dependencies on threading libraries.

 * Study what's required to support savable forms that can be saved by
   Adobe Reader.  Does this require actually signing the document with
   an Adobe private key?  Search for "Digital signatures" in the PDF
   spec, and look at ~/Q/pdf-collection/form-with-full-save.pdf, which
   came from Adobe's example site.

 * See if we can avoid preserving unreferenced objects in object
   streams even when preserving the object streams.

 * Provide APIs for embedded files.  See *attachments*.pdf in test
   suite.  The private method findAttachmentStreams finds at least
   cases for modern versions of Adobe Reader (>= 1.7, maybe earlier).
   PDF Reference 1.7 section 3.10, "File Specifications", discusses
   this.

   A sourceforge user asks if qpdf can handle extracting and embedded
   resources and references these tools, which may be useful as a
   reference.

   http://multivalent.sourceforge.net/Tools/pdf/Extract.html
   http://multivalent.sourceforge.net/Tools/pdf/Embed.html

 * The description of Crypt filters is unclear with respect to how to
   use them to override /StmF for specific streams.  I'm not sure
   whether qpdf will do the right thing for any specific individual
   streams that might have crypt filters, but I believe it does based
   on my testing of a limited subset.  The specification seems to imply
   that only embedded file streams and metadata streams can have crypt
   filters, and there are already special cases in the code to handle
   those.  Most likely, it won't be a problem, but someday someone may
   find a file that qpdf doesn't work on because of crypt filters.
   There is an example in the spec of using a crypt filter on a
   metadata stream.

   For now, we notice /Crypt filters and decode parameters consistent
   with the example in the PDF specification, and the right thing
   happens for metadata filters that happen to be uncompressed or
   otherwise compressed in a way we can filter.  This should handle
   all normal cases, but it's more or less just a guess since I don't
   have any test files that actually use stream-specific crypt filters
   in them.

 * The second xref stream for linearized files has to be padded only
   because we need file_size as computed in pass 1 to be accurate.  If
   we were not allowing writing to a pipe, we could seek back to the
   beginning and fill in the value of /L in the linearization
   dictionary as an optimization to alleviate the need for this
   padding.  Doing so would require us to pad the /L value
   individually and also to save the file descriptor and determine
   whether it's seekable.  This is probably not worth bothering with.

 * The whole xref handling code in the QPDF object allows the same
   object with more than one generation to coexist, but a lot of logic
   assumes this isn't the case.  Anything that creates mappings only
   with the object number and not the generation is this way,
   including most of the interaction between QPDFWriter and QPDF.  If
   we wanted to allow the same object with more than one generation to
   coexist, which I'm not sure is allowed, we could fix this by
   changing xref_table.  Alternatively, we could detect and disallow
   that case.  In fact, it appears that Adobe reader and other PDF
   viewing software silently ignores objects of this type, so this is
   probably not a big deal.

 * Based on an idea suggested by user "Atom Smasher", consider
   providing some mechanism to recover earlier versions of a file
   embedded prior to appended sections.

 * From a suggestion in bug 3152169, consider having an option to
   re-encode inline images with an ASCII encoding.

 * From github issue 2, provide more in-depth output for examining
   hint stream contents. Consider adding on option to provide a
   human-readable dump of linearization hint tables. This should
   include improving the 'overflow reading bit stream' message as
   reported in issue #2. There are multiple calls to stopOnError in
   the linearization checking code. Ideally, these should not
   terminate checking. It would require re-acquiring an understanding
   of all that code to make the checks more robust. In particular,
   it's hard to look at the code and quickly determine what is a true
   logic error and what could happen because of malformed user input.
   See also ../misc/linearization-errors.

 * There are a few known limitations of copying foreign streams. These
   are somewhat discussed in github issue 219. They are most likely
   not worth fixing.

   * The original pipeStreamData in QPDF_Stream has various logic for
     reporting warnings and letting the caller retry. This logic is
     not implemented for stream data providers. When copying foreign
     streams, qpdf uses a stream data provider
     (QPDF::CopiedStreamDataProvider) to read the stream data from the
     original file. While a warning is issued for that case, there is
     no way to actually propagate failure information back through
     because StreamDataProvider::provideStreamData doesn't take the
     suppress_warnings or will_retry options, and adding them would
     break source compatibility.

   * When copying streams that use StreamDataProvider provider, the
     original QPDF object must still be kept around. This is the case
     of where QPDF q1 has a stream for which replaceStreamData was
     called to provide a StreamDataProvider, and then that stream from
     q1 was subsequently copied to q2. In that case, q1 must be kept
     around until q2 is written. That is a pretty unusual case as,
     most of the time, people could just attach the stream data
     provider directly to the q2. That said, it actually appears in
     qpdf itself. qpdf allows you to combine the --pages and --split
     options. In that case, a merged file, which uses
     StreamDataProvider to access the original stream, is subsequently
     copied to the final split output files. Solving this restriction
     is difficult and would probably require breaking source
     compatibility because QPDF_Stream doesn't have enough information
     to know whether the original QPDF object is needed by the
     stream's data provider. Also, the interface is designed to pass
     the object ID and generation of the object whose data is being
     provided so the provider can use it for lookup or any other
     purpose. As such, copying a stream data provider to a new stream
     potentially changes the meaning of the parameters passed to the
     provider. This happens with CopiedStreamDataProvider, which uses
     the object ID and generation of the new stream to locate the
     parameters of the old stream.

 * If I ever decide to make appearance stream-generation aware of
   fonts or font metrics, see email from Tobias with Message-ID
   <5C3C9C6C.8000102@thax.hardliners.org> dated 2019-01-14.