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qpdf/TODO
Jay Berkenbilt 5c9c5d537b Update TODO with some small enhancements 
There have been a few enhancements requested that only affect the qpdf
command line tool and that should be relatively quick to implement.
Work out the details and acknowledge these publicly in the TODO file.
2015-05-24 16:26:05 -04:00

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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).
5.2.0
=====
* Before release: remember to bump minor shared library version since
new methods were added (even though private).
* Figure out what about a3576a73593987b26cd3eff346f8f7c11f713cbd
broke binary compatibility.
* Implement automated testing for binary compatibility and add to
release checklist.
* 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
=======
* 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.
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