qpdf/TODO

Next
====

Before Release:

* At next release, hide release-qpdf-10.6.3.0cmake* versions at readthedocs
* Stay on top of https://github.com/pikepdf/pikepdf/pull/315
* Release qtest with updates to qtest-driver and copy back into qpdf

Next:
* Support json v2 in the C API. At a minimum, write_json,
  create_from_json, and update_from_json need to be there and should
  take the same kinds of functions as the C API for logger.

Pending changes:

* Consider also exposing a way to set a new logger and to get the
  logger from QPDF and QPDFJob in the C API.
* Check about runpath in the linux-bin distribution. I think the
  appimage build specifically is setting the runpath, which is
  actually desirable in this case. Make sure to understand and
  document this. Maybe add a check for it in the build.
* Decide what to do about #664 (get*Box)
* Add an option --ignore-encryption to ignore encryption information
  and treat encrypted files as if they weren't encrypted. This should
  make it possible to solve #598 (--show-encryption without a
  password). We'll need to make sure we don't try to filter any
  streams in this mode. Ideally we should be able to combine this with
  --json so we can look at the raw encrypted strings and streams if we
  want to, though be sure to document that the resulting JSON won't be
  convertible back to a valid PDF. Since providing the password may
  reveal additional details, --show-encryption could potentially retry
  with this option if the first time doesn't work. Then, with the file
  open, we can read the encryption dictionary normally.
* In libtests, separate executables that need the object library
  from those that strictly use public API. Move as many of the test
  drivers from the qpdf directory into the latter category as long
  as doing so isn't too troublesome from a coverage standpoint.
* Consider adding fuzzer code for JSON
* Consider generating a non-flat pages tree before creating output to
  better handle files with lots of pages. If there are more than 256
  pages, add a second layer with the second layer nodes having no more
  than 256 nodes and being as evenly sizes as possible. Don't worry
  about the case of more than 65,536 pages. If the top node has more
  than 256 children, we'll live with it.

Parent pointer idea:

* Add std::weak_ptr<QPDFObject> parent to QPDFObject. When adding a
  direct object to an array or dictionary, set its parent. When
  removing it, clear the parent pointer.
* When a direct object that already has a parent is added to
  something, it is a warning and will become an error in qpdf 12.
  There needs to be unsafe add methods used by unsafeShallowCopy.
  These will add but not modify the parent pointer.

This allows an object to be moved from one object to another by
removing it, which returns the now orphaned object, and then inserting
it somewhere else. It also doesn't break the pattern of adding a
direct object to something and subsequently mutating it. It just
prevents the same object from being added to more than one thing.

Note that arrays and dictionaries still need to contain
QPDFObjectHandle because of indirect objects. This only pertains to
direct objects, which are always "resolved" in QPDFObjectHandle.

Soon: Break ground on "Document-level work"

Possible future JSON enhancements
=================================

* Add to JSON output the information available from a few additional
  informational options:

  * --check: add but maybe not by default?

  * --show-linearization: add but maybe not by default? Also figure
    out whether warnings reported for some of the PDF specs (1.7) are
    qpdf problems. This may not be worth adding in the first
    increment.

  * --show-xref: add

* Consider having --check, --show-encryption, etc., just select the
  right keys when in json mode. I don't think I want check on by
  default, so that might be different.

* Consider having warnings be included in the json in a "warnings" key
  in json mode.


QPDFJob
=======

Here are some ideas for QPDFJob that didn't make it into 10.6. Not all
of these are necessarily good -- just things to consider.

* How do we chain jobs? The idea would be that the input and/or output
  of a QPDFJob could be a QPDF object rather than a file. For input,
  it's pretty easy. For output, none of the output-specific options
  (encrypt, compress-streams, objects-streams, etc.) would have any
  affect, so we would have to treat this like inspect for error
  checking. The QPDF object in the state where it's ready to be sent
  off to QPDFWriter would be used as the input to the next QPDFJob.
  For the job json, I think we can have the output be an identifier
  that can be used as the input for another QPDFJob. For a json file,
  we could the top level detect if it's an array with the convention
  that exactly one has an output, or we could have a subkey with other
  job definitions or something. Ideally, any input
  (copy-attachments-from, pages, etc.) could use a QPDF object. It
  wouldn't surprise me if this exposes bugs in qpdf around foreign
  streams as this has been a relatively fragile area before.

Documentation
=============

* Do a full pass through the documentation.

  * Make sure `qpdf` is consistent. Use QPDF when just referring to
    the package.
  * Make sure markup is consistent
  * Autogenerate where possible
  * Consider which parts might be good candidates for moving to the
    wiki.

* Commit 'Manual - enable line wrapping in table cells' from
  Mon Jan 17 12:22:35 2022 +0000 enables table cell wrapping. See if
  this can be incorporated directly into sphinx_rtd_theme and the
  workaround can be removed.

* When possible, update the debian package to include docs again. See
  https://bugs.debian.org/1004159 for details.

Document-level work
===================

* Ideas here may by superseded by #593.

* QPDFPageCopier -- object for moving pages around within files or
  between files and performing various transformations. Reread/rewrite
  _page-selection in the manual if needed.

  * Handle all the stuff of pages and split-pages
  * Do n-up, booklet, collation
  * Look through cli and see what else...flatten-*?
  * See comments in QPDFPageDocumentHelper.hh for addPage -- search
    for "a future version".
  * Make it efficient for bulk operations
  * Make certain doc-level features selectable
  * qpdf.cc should do all its page operations, including
    overlay/underlay, splitting, and merging, using this
  * There should also be example code

* After doc-level checks are in, call --check on the output files in
  the "Copy Annotations" tests.

* Document-level checks. For example, for forms, make sure all form
  fields point to an annotation on exactly one page as well as that
  all widget annotations are associated with a form field. Hook this
  into QPDFPageCopier as well as the doc helpers. Make sure it is
  called from --check.

* See also issues tagged with "pages"

* Add flags to CLI to select which document-level options to
  preserve or not preserve. We will probably need a pair of mutually
  exclusive, repeatable options with a way to specify all, none, only
  {x,y}, or all but {x,y}.

* If a page contains a reference a file attachment annotation, when
  that page is copied, if the file attachment appears in the top-level
  EmbeddedFiles tree, that entry should be preserved in the
  destination file. Otherwise, we probably will require the use of
  --copy-attachments-from to preserve these. What will the strategy be
  for deduplicating in the automatic case?

Text Appearance Streams
=======================

This is a list of known issues with text appearance streams and things
we might do about it.

* For variable text, the spec says to pull any resources from /DR that
  are referenced in /DA but if the resource dictionary already has
  that resource, just use the one that's there. The current code looks
  only for /Tf and adds it if needed. We might want to instead merge
  /DR with resources and then remove anything that's unreferenced. We
  have all the code required for that in ResourceFinder except
  TfFinder also gets the font size, which ResourceFinder doesn't do.

* There are things we are missing because we don't look at font
  metrics. The code from TextBuilder (work) has almost everything in
  it that is required. Once we have knowledge of character widths, we
  can support quadding and multiline text fields (/Ff 4096), and we
  can potentially squeeze text to fit into a field. For multiline,
  first squeeze vertically down to the font height, then squeeze
  horizontally with Tz. For single line, squeeze horizontally with Tz.
  If we use Tz, issue a warning.

* When mapping characters to widths, we will need to care about
  character encoding. For built-in fonts, we can create a map from
  Unicode code point to width and then go from the font's encoding to
  unicode to the width. See misc/character-encoding/ (not on github)
  and font metric information for the 14 standard fonts in my local
  pdf-spec directory.

* Once we know about character widths, we can correctly support
  auto-sized variable text fields (0 Tf). If this is fixed, search for
  "auto-sized" in cli.rst.

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

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

* Ignoring these:
  * Out of memory in dct: 35001, 32516

External Libraries
==================

Current state (10.0.2):

* qpdf/external-libs repository builds external-libs on a schedule.
  It detects and downloads the latest versions of zlib, jpeg, and
  openssl and creates source and binary distribution zip files in an
  artifact called "distribution".

* Releases in qpdf/external-libs are made manually. They contain
  qpdf-external-libs-{bin,src}.zip.

* The qpdf build finds the latest non-prerelease release and downloads
  the qpdf-external-libs-*.zip files from the releases in the setup
  stage.

* To upgrade to a new version of external-libs, create a new release
  of qpdf/external-libs (see README-maintainer in external-libs) from
  the distribution artifact of the most recent successful build after
  ensuring that it works.

Desired state:

* The qpdf/external-libs repository should create release candidates.
  Ideally, every scheduled run would make its zip files available. A
  personal access token with actions:read scope for the
  qpdf/external-libs repository is required to download the artifact
  from an action run, and qpdf/qpdf's secrets.GITHUB_TOKEN doesn't
  have this access. We could create a service account for this
  purpose. As an alternative, we could have a draft release in
  qpdf/external-libs that the qpdf/external-libs build could update
  with each candidate. It may also be possible to solve this by
  developing a simple GitHub app.

* Scheduled runs of the qpdf build in the qpdf/qpdf repository (not a
  fork or pull request) could download external-libs from the release
  candidate area instead of the latest stable release. Pushes to the
  build branch should still use the latest release so it always
  matches the main branch.

* Periodically, we would create a release of external-libs from the
  release candidate zip files. This could be done safely because we
  know the latest qpdf works with it. This could be done at least
  before every release of qpdf, but potentially it could be done at
  other times, such as when a new dependency version is available or
  after some period of time.

Other notes:

* The external-libs branch in qpdf/qpdf was never documented. We might
  be able to get away with deleting it.

* See README-maintainer in qpdf/external-libs for information on
  creating a release. This could be at least partially scripted in a
  way that works for the qpdf/qpdf repository as well since they are
  very similar.


ABI Changes
===========

This is a list of changes to make next time there is an ABI change.
Comments appear in the code prefixed by "ABI". Always Search for ABI
in source and header files to find items not listed here. Also search
for "[[deprecated" to find deprecated APIs that can be removed.


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.

Analytics
=========

Consider features that make it easier to detect certain patterns in
PDF files. The information below could be computed using an external
program that reads the existing json, but if it's useful enough, we
could add it directly to the json output.

 * Add to "pages" in the json:
   * "inheritsresources": bool; whether there are any inherited
     attributes from ancestor page tree nodes
   * "sharedresources": a list of indirect objects that are
     "/Resources" dictionaries or "XObject" resource dictionary subkeys
     of either the page itself or of any form XObject referenced by the
     page.

 * Add to "objectinfo" in json: "directpagerefcount": the number of
   pages that directly reference this object (i.e., you can find an
   indirect reference to the object in the page dictionary without
   traversing over any indirect objects)

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.

 * Look at https://bestpractices.coreinfrastructure.org/en

 * Rework tests so that nothing is written into the source directory.
   Ideally then the entire build could be done with a read-only
   source tree.

 * Large file tests fail with linux32 before and after cmake. This was
   first noticed after 10.6.3. I don't think it's worth fixing.

 * Consider updating the fuzzer with code that exercises
   copyAnnotations, file attachments, and name and number trees. Check
   fuzzer coverage.

 * Add code for creation of a file attachment annotation. It should
   also be possible to create a widget annotation and a form field.
   Update the pdf-attach-file.cc example with new APIs when ready.

 * Flattening of form XObjects seems like something that would be
   useful in the library. We are seeing more cases of completely valid
   PDF files with form XObjects that cause problems in other software.
   Flattening of form XObjects could be a useful way to work around
   those issues or to prepare files for additional processing, making
   it possible for users of the qpdf library to not be concerned about
   form XObjects. This could be done recursively; i.e., we could have a
   method to embed a form XObject into whatever contains it, whether
   that is a form XObject or a page. This would require more
   significant interpretation of the content stream. We would need a
   test file in which the placement of the form XObject has to be in
   the right place, e.g., the form XObject partially obscures earlier
   code and is partially obscured by later code. Keys in the resource
   dictionary may need to be changed -- create test cases with lots of
   duplicated/overlapping keys.

 * Part of closed_file_input_source.cc is disabled on Windows because
   of odd failures. It might be worth investigating so we can fully
   exercise this in the test suite. That said, ClosedFileInputSource
   is exercised elsewhere in qpdf's test suite, so this is not that
   pressing.

 * 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

    - See ../misc/bad-files-issue-476. This directory contains a
      snapshot of the google doc and linked PDF files from issue #476.
      Please see the issue for details.

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

 * 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/

 * 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.

 * 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 also
   ../misc/digital-sign-from-trueroad/. If digital signatures are
   implemented, update the docs on crypto providers, which mention
   that this may happen in the future.

 * Qpdf does not honor /EFF when adding new file attachments. When it
   encrypts, it never generates streams with explicit crypt filters.
   Prior to 10.2, there was an incorrect attempt to treat /EFF as a
   default value for decrypting file attachment streams, but it is not
   supposed to mean that. Instead, it is intended for conforming
   writers to obey this when adding new attachments. Qpdf is not a
   conforming writer in that respect.

 * 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.

 * 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.

 * 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.

 * Look at places in the code where object traversal is being done and,
   where possible, try to avoid it entirely or at least avoid ever
   traversing the same objects multiple times.

----------------------------------------------------------------------

HISTORICAL NOTES

Performance
===========

As described in https://github.com/qpdf/qpdf/issues/401, there was
great performance degradation between qpdf 7.1.1 and 9.1.1. Doing a
bisect between dac65a21fb4fa5f871e31c314280b75adde89a6c and
release-qpdf-7.1.1, I found several commits that damaged performance.
I fixed some of them to improve performance by about 70% (as measured
by saying that old times were 170% of new times). The remaining
commits that broke performance either can't be correct because they
would re-introduce an old bug or aren't worth correcting because of
the high value they offer relative to a relatively low penalty. For
historical reference, here are the commits. The numbers are the time
in seconds on the machine I happened to be using of splitting the
first 100 pages of PDF32000_2008.pdf 20 times and taking an average
duration.

Commits that broke performance:

* d0e99f195a987c483bbb6c5449cf39bee34e08a1 -- object description and
  context: 0.39 -> 0.45
* a01359189b32c60c2d55b039f7aefd6c3ce0ebde (minus 313ba08) -- fix
  dangling references: 0.55 -> 0.6
* e5f504b6c5dc34337cc0b316b4a7b1fca7e614b1 -- sparse array: 0.6 -> 0.62

Other intermediate steps that were previously fixed:

* 313ba081265f69ac9a0324f9fe87087c72918191 -- copy outlines into
  split: 0.55 -> 4.0
* a01359189b32c60c2d55b039f7aefd6c3ce0ebde -- fix dangling references:
  4.0 -> 9.0

This commit fixed the awful problem introduced in 313ba081:

* a5a016cdd26a8e5c99e5f019bc30d1bdf6c050a2 -- revert outline
  preservation: 9.0 -> 0.6

Note that the fix dangling references commit had a much worse impact
prior to removing the outline preservation, so I also measured its
impact in isolation.

A few important lessons (in README-maintainer)

* Indirection through PointerHolder<Members> is expensive, and should
  not be used for things that are created and destroyed frequently
  such as QPDFObjectHandle and QPDFObject.
* Traversal of objects is expensive and should be avoided where
  possible.

Also, it turns out that PointerHolder is more performant than
std::shared_ptr. (This was true at the time but subsequent
implementations of std::shared_ptr became much more efficient.)

QPDFPagesTree
=============

On a few occasions, I have considered implementing a QPDFPagesTree
object that would allow the document's original page tree structure to
be preserved. See comments at the top QPDF_pages.cc for why this was
abandoned.

Partial work is in refs/attic/QPDFPagesTree. QPDFPageTree is mostly
implemented and mostly tested. There are not enough cases of different
kinds of operations (pclm, linearize, json, etc.) with non-flat pages
trees. Insertion is not implemented. Insertion is potentially complex
because of the issue of inherited objects. We will have to call
pushInheritedAttributesToPage before adding any pages to the pages
tree. The test suite is failing on that branch.

Some parts of page tree repair are silent (no warnings). All page tree
repair should warn. The reason is that page tree repair will change
object numbers, and knowing that is important when working with JSON
output.

If we were to do this, we would still need keep a pages cache for
efficient insertion. There's no reason we can't keep a vector of page
objects up to date and just do a traversal the first time we do
getAllPages just like we do now. The difference is that we would not
flatten the pages tree. It would be useful to go through QPDF_pages
and reimplement everything without calling flattenPagesTree. Then we
can remove flattenPagesTree, which is private. That said, with the
addition of creating non-flat pages trees, there is really no reason
not to flatten the pages tree for internal use.

In its current state, QPDFPagesTree does not proactively fix /Type or
correct page objects that are used multiple times. You have to
traverse the pages tree to trigger this operation. It would be nice if
we would do that somewhere but not do it more often than necessary so
isPagesObject and isPageObject are reliable and can be made more
reliable. Maybe add a validate or repair function? It should also make
sure /Count and /Parent are correct.

Rejected Ideas
==============

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

* 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.

* 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.

* 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.

* Consider creating a sanitizer to make it easier for people to send
  broken files. Now that we have json mode, this is probably no
  longer worth doing. Here is the previous idea, possibly implemented
  by making it possible to run the lexer (tokenizer) over a whole
  file. 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, duplicated dictionary keys, syntax
  errors, 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. If I ever do this, the file from issue #494 would
  be a great one to look at.

* Here are some notes about having stream data providers modify
  stream dictionaries. I had wanted to add this functionality to make
  it more efficient to create stream data providers that may
  dynamically decide what kind of filters to use and that may end up
  modifying the dictionary conditionally depending on the original
  stream data. Ultimately I decided not to implement this feature.
  This paragraph describes why.

  * When writing, the way objects are placed into the queue for
    writing strongly precludes creation of any new indirect objects,
    or even changing which indirect objects are referenced from which
    other objects, because we sometimes write as we are traversing
    and enqueuing objects. For non-linearized files, there is a risk
    that an indirect object that used to be referenced would no
    longer be referenced, and whether it was already written to the
    output file would be based on an accident of where it was
    encountered when traversing the object structure. For linearized
    files, the situation is considerably worse. We decide which
    section of the file to write an object to based on a mapping of
    which objects are used by which other objects. Changing this
    mapping could cause an object to appear in the wrong section, to
    be written even though it is unreferenced, or to be entirely
    omitted since, during linearization, we don't enqueue new objects
    as we traverse for writing.

  * There are several places in QPDFWriter that query a stream's
    dictionary in order to prepare for writing or to make decisions
    about certain aspects of the writing process. If the stream data
    provider has the chance to modify the dictionary, every piece of
    code that gets stream data would have to be aware of this. This
    would potentially include end user code. For example, any code
    that called getDict() on a stream before installing a stream data
    provider and expected that dictionary to be valid would
    potentially be broken. As implemented right now, you must perform
    any modifications on the dictionary in advance and provided
    /Filter and /DecodeParms at the time you installed the stream
    data provider. This means that some computations would have to be
    done more than once, but for linearized files, stream data
    providers are already called more than once. If the work done by
    a stream data provider is especially expensive, it can implement
    its own cache.

  The example examples/pdf-custom-filter.cc demonstrates the use of
  custom stream filters. This includes a custom pipeline, a custom
  stream filter, as well as modification of a stream's dictionary to
  include creation of a new stream that is referenced from
  /DecodeParms.