Lexical ======= * Make it possible to run the lexer (tokenizer) over a whole file such that the following things would be possible: * Rewrite fix-qdf in C++ so that there is no longer a runtime perl dependency * Make it possible to replace all strings in a file lexically even on badly broken files. Ideally this should work files that are lacking xref, have broken links, etc., and ideally it should work with encrypted files if possible. This should go through the streams and strings and replace them with fixed or random characters, preferably, but not necessarily, in a manner that works with fonts. One possibility would be to detect whether a string contains characters with normal encoding, and if so, use 0x41. If the string uses character maps, use 0x01. The output should otherwise be unrelated to the input. This could be built after the filtering and tokenizer rewrite and should be done in a manner that takes advantage of the other lexical features. This sanitizer should also clear metadata and replace images. Page splitting/merging ====================== * Update page splitting and merging to handle document-level constructs with page impact such as interactive forms and article threading. Check keys in the document catalog for others, such as outlines, page labels, thumbnails, and zones. For threads, Subramanyam provided a test file; see ../misc/article-threads.pdf. Email Q-Count: 431864 from 2009-11-03. * bookmarks (outlines) 12.3.3 * support bookmarks when merging * prune bookmarks that don't point to a surviving page when merging or splitting * make sure conflicting named destinations work possibly test by including the same file by two paths in a merge 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. * Form fields: should be similar to outlines. General ======= NOTE: Some items in this list refer to files in my personal home directory or that are otherwise not publicly accessible. This includes things sent to me by email that are specifically not public. Even so, I find it useful to make reference to them in this list * See if there is a way to provide an option to use higher compression with zlib. See #113. This can be done by adding options to Pl_Flate and having QPDFWriter optionally use those options. Also increasing the buffer size may help. * Add support for writing name and number trees * Figure out how to render Gajić correctly in the PDF version of the qpdf manual. * Decide whether errors thrown by checkLinearization should be converted to warnings. Take a pass through the linearization code to see whether it's correct. Be able to view linearization data even when there are errors. See linearization label in github. * Consider creating a PPA for Ubuntu * Investigate whether there is a way to automate the memory checker tests for Windows. * Support user-pluggable stream filters. This would enable external code to provide interpretation for filters that are missing from qpdf. Make it possible for user-provided filters to override built-in filters. Make sure that the pluggable filters can be prioritized so that we can poll all registered filters to see whether they are capable of filtering a particular stream. * If possible, consider adding CCITT3, CCITT4, or any other easy filters. For some reference code that we probably can't use but may be handy anyway, see http://partners.adobe.com/public/developer/ps/sdk/index_archive.html * If possible, support the following types of broken files: - Files that have no whitespace token after "endobj" such that endobj collides with the start of the next object - See ../misc/broken-files * Additional form features * set value from CLI? Specify title, and provide way to disambiguate, probably by giving objgen of field * replace mode: --replace-object, --replace-stream-raw, --replace-stream-filtered * update first paragraph of QPDF JSON in the manual to mention this * object numbers are not preserved by write, so object ID lookup has to be done separately for each invocation * you don't have to specify length for streams * you only have to specify filtering for streams if providing raw data * Pl_TIFFPredictor is pretty slow. * Support for handling file names with Unicode characters in Windows is incomplete. qpdf seems to support them okay from a functionality standpoint, and the right thing happens if you pass in UTF-8 encoded filenames to QPDF library routines in Windows (they are converted internally to wchar_t*), but file names are encoded in UTF-8 on output, which doesn't produce nice error messages or output on Windows in some cases. * If we ever wanted to do anything more with character encoding, see ../misc/character-encoding/, which includes machine-readable dump of table D.2 in the ISO-32000 PDF spec. This shows the mapping between Unicode, StandardEncoding, WinAnsiEncoding, MacRomanEncoding, and PDFDocEncoding. * Some test cases on bad files fail because qpdf is unable to find the root dictionary when it fails to read the trailer. Recovery could find the root dictionary and even the info dictionary in other ways. In particular, issue-202.pdf can be opened by evince, and there's no real reason that qpdf couldn't be made to be able to recover that file as well. * Audit every place where qpdf allocates memory to see whether there are cases where malicious inputs could cause qpdf to attempt to grab very large amounts of memory. Certainly there are cases like this, such as if a very highly compressed, very large image stream is requested in a buffer. Hopefully normal input to output filtering doesn't ever try to do this. QPDFWriter should be checked carefully too. See also bugs/private/from-email-663916/ * Interactive form modification: https://github.com/qpdf/qpdf/issues/213 contains a good discussion of some ideas for adding methods to modify annotations and form fields if we want to make it easier to support modifications to interactive forms. Some of the ideas have been implemented, and some of the probably never will be implemented, but it's worth a read if there is an intention to work on this. In the issue, search for "Regarding write functionality", and read that comment and the responses to it. * Look at ~/Q/pdf-collection/forms-from-appian/ * Consider adding "uninstall" target to makefile. It should only uninstall what it installed, which means that you must run uninstall from the version you ran install with. It would only be supported for the toolchains that support the install target (libtool). * Provide support in QPDFWriter for writing incremental updates. Provide support in qpdf for preserving incremental updates. The goal should be that QDF mode should be fully functional for files with incremental updates including fix_qdf. Note that there's nothing that says an indirect object in one update can't refer to an object that doesn't appear until a later update. This means that QPDF has to treat indirect null objects differently from how it does now. QPDF drops indirect null objects that appear as members of arrays or dictionaries. For arrays, it's handled in QPDFWriter where we make indirect nulls direct. This is in a single if block, and nothing else in the code cares about it. We could just remove that if block and not break anything except a few test cases that exercise the current behavior. For dictionaries, it's more complicated. In this case, QPDF_Dictionary::getKeys() ignores all keys with null values, and hasKey() returns false for keys that have null values. We would probably want to make QPDF_Dictionary able to handle the special case of keys that are indirect nulls and basically never have it drop any keys that are indirect objects. If we make a change to have qpdf preserve indirect references to null objects, we have to note this in ChangeLog and in the release notes since this will change output files. We did this before when we stopped flattening scalar references, so this is probably not a big deal. We also have to make sure that the testing for this handles non-trivial cases of the targets of indirect nulls being replaced by real objects in an update. I'm not sure how this plays with linearization, if at all. For cases where incremental updates are not being preserved as incremental updates and where the data is being folded in (as is always the case with qpdf now), none of this should make any difference in the actual semantics of the files. * When decrypting files with /R=6, hash_V5 is called more than once with the same inputs. Caching the results or refactoring to reduce the number of identical calls could improve performance for workloads that involve processing large numbers of small files. * Consider adding a method to balance the pages tree. It would call pushInheritedAttributesToPage, construct a pages tree from scratch, and replace the /Pages key of the root dictionary with the new tree. * Secure random number generation could be made more efficient by using a local static to ensure a single random device or crypt provider as long as this can be done in a thread-safe fashion. In the initial implementation, this is being skipped to avoid having to add any dependencies on threading libraries. * Study what's required to support savable forms that can be saved by Adobe Reader. Does this require actually signing the document with an Adobe private key? Search for "Digital signatures" in the PDF spec, and look at ~/Q/pdf-collection/form-with-full-save.pdf, which came from Adobe's example site. * Consider the possibility of doing something locale-aware to support non-ASCII passwords. Update documentation if this is done. Consider implementing full Unicode password algorithms from newer encryption formats. See ../misc/unicode-password*. If code is added to properly encode Unicode passwords, figure out how to deal with backward compatibility. Either require some additional flag to decode the password or provide a `--raw-password` flag to suppress decoding. While automatically encoding breaks backward compatibility, it's probably the right behavior because the current behavior is arguably a bug. Alternatively, if the password doesn't work as a raw password and contains characters outside US-ASCII, try various encoding methods to see if any work. See section 7.6.3.3, algorithms 2 and 2A, in the ISO spec for details. (This is tracked in https://github.com/qpdf/qpdf/issues/215.) * See if we can avoid preserving unreferenced objects in object streams even when preserving the object streams. * Provide APIs for embedded files. See *attachments*.pdf in test suite. The private method findAttachmentStreams finds at least cases for modern versions of Adobe Reader (>= 1.7, maybe earlier). PDF Reference 1.7 section 3.10, "File Specifications", discusses this. A sourceforge user asks if qpdf can handle extracting and embedded resources and references these tools, which may be useful as a reference. http://multivalent.sourceforge.net/Tools/pdf/Extract.html http://multivalent.sourceforge.net/Tools/pdf/Embed.html * The description of Crypt filters is unclear with respect to how to use them to override /StmF for specific streams. I'm not sure whether qpdf will do the right thing for any specific individual streams that might have crypt filters, but I believe it does based on my testing of a limited subset. The specification seems to imply that only embedded file streams and metadata streams can have crypt filters, and there are already special cases in the code to handle those. Most likely, it won't be a problem, but someday someone may find a file that qpdf doesn't work on because of crypt filters. There is an example in the spec of using a crypt filter on a metadata stream. For now, we notice /Crypt filters and decode parameters consistent with the example in the PDF specification, and the right thing happens for metadata filters that happen to be uncompressed or otherwise compressed in a way we can filter. This should handle all normal cases, but it's more or less just a guess since I don't have any test files that actually use stream-specific crypt filters in them. * The second xref stream for linearized files has to be padded only because we need file_size as computed in pass 1 to be accurate. If we were not allowing writing to a pipe, we could seek back to the beginning and fill in the value of /L in the linearization dictionary as an optimization to alleviate the need for this padding. Doing so would require us to pad the /L value individually and also to save the file descriptor and determine whether it's seekable. This is probably not worth bothering with. * The whole xref handling code in the QPDF object allows the same object with more than one generation to coexist, but a lot of logic assumes this isn't the case. Anything that creates mappings only with the object number and not the generation is this way, including most of the interaction between QPDFWriter and QPDF. If we wanted to allow the same object with more than one generation to coexist, which I'm not sure is allowed, we could fix this by changing xref_table. Alternatively, we could detect and disallow that case. In fact, it appears that Adobe reader and other PDF viewing software silently ignores objects of this type, so this is probably not a big deal. * Based on an idea suggested by user "Atom Smasher", consider providing some mechanism to recover earlier versions of a file embedded prior to appended sections. * From a suggestion in bug 3152169, consider having an option to re-encode inline images with an ASCII encoding. This is also https://github.com/qpdf/qpdf/issues/278. It would be interesting to make optimize-images work with inline images. * From github issue 2, provide more in-depth output for examining hint stream contents. Consider adding on option to provide a human-readable dump of linearization hint tables. This should include improving the 'overflow reading bit stream' message as reported in issue #2. There are multiple calls to stopOnError in the linearization checking code. Ideally, these should not terminate checking. It would require re-acquiring an understanding of all that code to make the checks more robust. In particular, it's hard to look at the code and quickly determine what is a true logic error and what could happen because of malformed user input. See also ../misc/linearization-errors. * There are a few known limitations of copying foreign streams. These are somewhat discussed in github issue 219. They are most likely not worth fixing. * The original pipeStreamData in QPDF_Stream has various logic for reporting warnings and letting the caller retry. This logic is not implemented for stream data providers. When copying foreign streams, qpdf uses a stream data provider (QPDF::CopiedStreamDataProvider) to read the stream data from the original file. While a warning is issued for that case, there is no way to actually propagate failure information back through because StreamDataProvider::provideStreamData doesn't take the suppress_warnings or will_retry options, and adding them would break source compatibility. * When copying streams that use StreamDataProvider provider, the original QPDF object must still be kept around. This is the case of where QPDF q1 has a stream for which replaceStreamData was called to provide a StreamDataProvider, and then that stream from q1 was subsequently copied to q2. In that case, q1 must be kept around until q2 is written. That is a pretty unusual case as, most of the time, people could just attach the stream data provider directly to the q2. That said, it actually appears in qpdf itself. qpdf allows you to combine the --pages and --split options. In that case, a merged file, which uses StreamDataProvider to access the original stream, is subsequently copied to the final split output files. Solving this restriction is difficult and would probably require breaking source compatibility because QPDF_Stream doesn't have enough information to know whether the original QPDF object is needed by the stream's data provider. Also, the interface is designed to pass the object ID and generation of the object whose data is being provided so the provider can use it for lookup or any other purpose. As such, copying a stream data provider to a new stream potentially changes the meaning of the parameters passed to the provider. This happens with CopiedStreamDataProvider, which uses the object ID and generation of the new stream to locate the parameters of the old stream. * If I ever decide to make appearance stream-generation aware of fonts or font metrics, see email from Tobias with Message-ID <5C3C9C6C.8000102@thax.hardliners.org> dated 2019-01-14.