qpdf/TODO

Small, command-line tool only enhancements to do soon
=====================================================

 * Handle input file = output file as a special case. See issue 29.
   Behavior: detect if output file is the same as one of the input
   files. If so, refuse to operate unless --allow-overwrite is
   specified. In that case, write to a temporary file and, if there
   are no errors or warnings, rename the temporary output file over
   the input file. If rename fails, delete the temporary file.

 * Consider providing alternative methods for specifying passwords.
   The methods should be general enough to use for both encryption and
   decryption passwords. Example methods could be reading the password
   from a file, a file descriptor, or prompting. Prompting should
   never be done with being specifically requested though; we don't
   want to create a situation where running qpdf might block waiting
   for input where it previously did not. Test case: encrypt an
   encrypted file with the output file having different user/owner
   passwords. Make sure we have a predictable way to read all three
   passwords (input, output user, output owner). Maybe we have
   something like --password-source=<method>:<which>,... where method could
   be file=/path, fd=n, or prompt and which could be one of input,
   user, owner. If a password source is provided for input, it takes
   precedence over --password if specified later on the command line.
   If a password source is specified for output passwords, the
   corresponding passwords must be '-'. If more than one password is
   read from the same source, passwords are newline separated.
   Trailing newlines are ignored. Example:

   qpdf --password-source=fd=3:input,owner a.pdf b.pdf

   would read two lines from file descriptor 3. The first would the
   password for reading a.pdf, and the second would be the owner
   password for b.pdf. The encryption arguments would specify the
   actual user password for b.pdf and - as the owner password.

   qpdf --password-source=file=/tmp/a:input --password=source=prompt:user,owner

   would read the input file from /tmp/a and would prompt twice: one
   for the user password and once for the owner password.

 * 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).


6.1.0
=====

 * Add method to push inheritable resources to a single page by
   walking up and copying without overwrite.  Above logic will also be
   sufficient to fix the limitation in
   QPDFObjectHandle::getPageImages().  Maybe add a method to get the
   effective resources for a page without modifying the page and then
   implement both changes in terms of that method.

 * Provide an option for QPDFWriter to preserve unreferenced objects
   when writing out a file.

 * Look at all the exceptions and error conditions in QPDF_stream and
   figure out which ones should be converted to warnings and treating
   the stream as not filterable.

 * 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 fitlers 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 RLE, 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 lack %%EOF at the end but otherwise have a valid
      startxref near the end

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

    - Files with individual corrupted streams.  Just leave the streams
      unfiltered after giving a warning, or maybe do something else
      like applying as many of the filters as possible, etc.
      QPDFWriter can have some kind of retry mechanism on streams
      where filtering fails after filterable returns true.

    - Files whose PDF header is malformed, perhaps with no version
      number (as literally %PDF-a.b).  Maybe keep track of features to
      try to infer a version based on encryption formats and object
      streams.

    - For really hard errors like corrupted streams where there is
      virtually guaranteed to be loss, maybe require an additional
      option to tell qpdf that it's okay to continue and treat those
      as warnings.  Probably need separate options for each type of
      error plus a generic tryReallyHard kind of method that enables
      them all.  Then the qpdf command-line tool can have a single
      flag that enables all supported aggressive recovery techniques.

    - See ../misc/broken-files



General
=======

 * Implement automated testing for binary compatibility and add to
   release checklist.

 * Figure out how to find Visual Studio in Windows registry and see if
   I can get it to work with make so I can simplify creation of
   Windows releases.

 * 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 providing a Windows installer for qpdf using NSIS.

 * 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.

 * Consider impact of article threads on page splitting/merging.
   Subramanyam provided a test file; see ../misc/article-threads.pdf.
   Email Q-Count: 431864 from 2009-11-03.  Other things to consider:
   outlines, page labels, thumbnails, zones.  There are probably
   others.

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

 * For debugging linearization bugs, consider adding an option to save
   pass 1 of linearization.  This code is sufficient.  Change the
   interface to allow specification of a pass1 file, which would
   change the behavior as in this patch.

------------------------------
Index: QPDFWriter.cc
===================================================================
--- QPDFWriter.cc	(revision 932)
+++ QPDFWriter.cc	(working copy)
@@ -1965,11 +1965,15 @@
 
     // Write file in two passes.  Part numbers refer to PDF spec 1.4.
 
+    FILE* XXX = 0;
     for (int pass = 1; pass <= 2; ++pass)
     {
 	if (pass == 1)
 	{
-	    pushDiscardFilter();
+//	    pushDiscardFilter();
+	    XXX = QUtil::safe_fopen("/tmp/pass1.pdf", "w");
+	    pushPipeline(new Pl_StdioFile("pass1", XXX));
+	    activatePipelineStack();
 	}
 
 	// Part 1: header
@@ -2204,6 +2208,8 @@
 
 	    // Restore hint offset
 	    this->xref[hint_id] = QPDFXRefEntry(1, hint_offset, 0);
+	    fclose(XXX);
+	    XXX = 0;
 	}
     }
 }
------------------------------

 * 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.

 * Pl_PNGFilter is only partially implemented.  If we ever decoded
   images, we'd have to finish implementing it along with the other
   filter decode parameters and types.  For just handling xref
   streams, there's really no need as it wouldn't make sense to use
   any kind of predictor other than 12 (PNG UP filter).

 * If we ever want to have check mode check the integrity of the free
   list, this can be done by looking at the code from prior to the
   object stream support of 4/5/2008.  It's in an if (0) block and
   there's a comment about it.  There's also something about it in
   qpdf.test -- search for "free table".  On the other hand, the value
   of doing this seems very low since no viewer seems to care, so it's
   probably not worth it.

 * QPDFObjectHandle::getPageImages() doesn't notice images in
   inherited resource dictionaries.  See comments in that function.

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

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

 * From github issue 2, provide more in-depth output for examining
   hint stream contents.

 * An attempt was made to replace pcre with std::regex, but it failed
   because std::regex lacks some features of PCRE that we use and
   because the result was many times slower. The work was saved in
   refs/attic/std-regex on github.