From 0d1793375fb6bbe87a40902bcac63169482ef35c Mon Sep 17 00:00:00 2001 From: m-holger Date: Sun, 25 Jun 2023 11:30:21 +0100 Subject: Rename TODO file to TODO.md --- TODO.md | 844 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 844 insertions(+) create mode 100644 TODO.md (limited to 'TODO.md') diff --git a/TODO.md b/TODO.md new file mode 100644 index 00000000..4eba6f6b --- /dev/null +++ b/TODO.md @@ -0,0 +1,844 @@ +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= + +* 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 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. + 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 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_POINTER_TRANSITION = 2 -- the QPDF constructor is private; all calls to std::make_shared 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 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. -- cgit v1.2.3-54-g00ecf