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@@ -1,844 +0,0 @@ -Always -====== - -* Evaluate issues tagged with `next` and `bug`. Remember to check - discussions and pull requests in addition to regular issues. -* When close to release, make sure external-libs is building and - follow instructions in ../external-libs/README - -Next -==== - -* Fix #874 -- make args in --encrypt to match the json and make - positional fill in the gaps -* Maybe fix #553 -- use file times for attachments -* std::string_view transition -- work being done by m-holger -* Break ground on "Document-level work" -- TODO-pages.md lives on a - separate branch. -* Standard for CLI and Job JSON support for JSON-based command-line - arguments. Come up with a standard way of supporting command-line - arguments that take JSON specifications of things so that - * there is a predictable way to indicate whether an argument is a - file or a JSON blob - * with QPDFJob JSON, make sure it is possible to directly include - the JSON rather than having to stringify a JSON blob - * One option might be to prepend file:// to a filename or otherwise - to take a JSON blob. We could have that as a particular type of - argument that would behave properly for both job JSON and CLI. - - -Possible future JSON enhancements -================================= - -* Consider not including unreferenced objects and trimming the trailer - in the same way that QPDFWriter does (except don't remove `/ID`). - This means excluding the linearization dictionary and hint stream, - the encryption dictionary, all keys from trailer that are removed by - QPDFWriter::getTrimmedTrailer except `/ID`, any object streams, and - the xref stream as long as all those objects are unreferenced. (They - always should be, but there could be some bizarre case of someone - creating a PDF file that has an indirect reference to one of those, - in which case we need to preserve it.) If this is done, make - `--preserve-unreferenced` preserve unreference objects and also - those extra keys. Search for "linear" and "trailer" in json.rst to - update the various places in the documentation that discuss this. - Also update the help for --json and --preserve-unreferenced. - -* 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". Include closed issues. - -* 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 -* Search for "[[deprecated" to find deprecated APIs that can be removed -* Search for issues, pull requests, and discussions with the "abi" label -* Check discussion "qpdf X planning" where X is the next major - version. This should be tagged `abi` - -For qpdf 12, see https://github.com/qpdf/qpdf/discussions/785 - -C++ Version Changes -=================== - -Use -// C++NN: ... -to mark places in the code that should be updated when we require at -least that version of C++. - -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. - -* Consider enabling code scanning on GitHub. - -* 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. If this is - done, search for "raw, encrypted" in json.rst. - -* 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 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. This is only safe if all - intermediate page nodes have only /Kids, /Parent, /Type, and /Count. - -* Look at https://bestpractices.coreinfrastructure.org/en - -* Consider adding fuzzer code for JSON - -* 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/ and - ../misc/digital-signatures/digitally-signed-pdf-xfa.pdf. 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. - -* Removal of raw QPDF* from the API. Discussions in #747 and #754. - This is a summary of the arguments I put forth in #754. The idea was - to make QPDF::QPDF() private and require all QPDF objects to be - shared pointers created with QPDF::create(). This would enable us to - have QPDFObjectHandle::getOwningQPDF() return a std::weak_ptr<QPDF>. - Prior to #726 (QPDFObject/QPDFValue split, released in qpdf 11.0.0), - getOwningQPDF() could return an invalid pointer if the owning QPDF - disappeared, but this is no longer the case, which removes the main - motivation. QPDF 11 added QPDF::create() anyway though. - - Removing raw QPDF* would look something like this. Note that you - can't use std::make_shared<T> unless T has a public constructor. - - QPDF_POINTER_TRANSITION = 0 -- no warnings around calling the QPDF constructor - QPDF_POINTER_TRANSITION = 1 -- calls to QPDF() are deprecated, but QPDF is still available so code can be backward compatible and use std::make_shared<QPDF> - QPDF_POINTER_TRANSITION = 2 -- the QPDF constructor is private; all calls to std::make_shared<QPDF> have to be replaced with QPDF::create - - If we were to do this, we'd have to look at each use of QPDF* in the - interface and decide whether to use a std::shared_ptr or a - std::weak_ptr. The answer would almost always be to use a - std::weak_ptr, which means we'd have to take the extra step of - calling lock(), and it means there would be lots of code changes - cause people would have to pass weak pointers instead of raw - pointers around, and those have to be constructed and locked. - Passing std::shared_ptr around leaves the possibility of creating - circular references. It seems to be too much trouble in the library - and too much toil for library users to be worth the small benefit of - not having to call resetObjGen in QPDF's destructor. - -* Fix Multiple Direct Object Parent Issue - - This idea was rejected because it would be complicated to implement - and would likely have a high performance cost to fix what is not - really that big of a problem in practice. - - It is possible for a QPDFObjectHandle for a direct object to be - contained inside of multiple QPDFObjectHandle objects or even - replicated across multiple QPDF objects. This creates a potentially - confusing and unintentional aliasing of direct objects. There are - known cases in the qpdf library where this happens including page - splitting and merging (particularly with page labels, and possibly - with other cases), and also with unsafeShallowCopy. Disallowing this - would incur a significant performance penalty and is probably not - worth doing. If we were to do it, here are some ideas. - - * 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. The parent pointer would - always be null for indirect objects, so the parent pointer, which - would reside in QPDFObject, would have to be managed by - QPDFObjectHandle. This is because QPDFObject can't tell the - difference between a resolved indirect object and a direct object. - - * Phase 1: When a direct object that already has a parent is added - to a dictionary or array, issue a warning. There would need to be - unsafe add methods used by unsafeShallowCopy. These would add but - not modify the parent pointer. - - * Phase 2: In the next major release, make the multiple parent case - an error. Require people to create a copy. The unsafe operations - would still have to be permitted. - - This approach would allow 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. |