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Comments about incremental update support
Also remove some trivial, non-functional code.
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34
TODO
34
TODO
@ -64,6 +64,40 @@
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General
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=======
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* Provide support in QPDFWriter for writing incremental updates.
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Provide support in qpdf for preserving incremental updates. The
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goal should be that QDF mode should be fully functional for files
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with incremental updates including fix_qdf.
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Note that there's nothing that says an indirect object in one
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update can't refer to an object that doesn't appear until a later
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update. This means that QPDF has to treat indirect null objects
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differently from how it does now. QPDF drops indirect null objects
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that appear as members of arrays or dictionaries. For arrays, it's
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handled in QPDFWriter where we make indirect nulls direct. This is
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in a single if block, and nothing else in the code cares about it.
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We could just remove that if block and not break anything except a
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few test cases that exercise the current behavior. For
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dictionaries, it's more complicated. In this case,
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QPDF_Dictionary::getKeys() ignores all keys with null values, and
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hasKey() returns false for keys that have null values. We would
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probably want to make QPDF_Dictionary able to handle the special
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case of keys that are indirect nulls and basically never have it
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drop any keys that are indirect objects.
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If we make a change to have qpdf preserve indirect references to
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null objects, we have to note this in ChangeLog and in the release
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notes since this will change output files. We did this before when
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we stopped flattening scalar references, so this is probably not a
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big deal. We also have to make sure that the testing for this
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handles non-trivial cases of the targets of indirect nulls being
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replaced by real objects in an update. I'm not sure how this plays
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with linearization, if at all. For cases where incremental updates
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are not being preserved as incremental updates and where the data
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is being folded in (as is always the case with qpdf now), none of
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this should make any difference in the actual semantics of the
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files.
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* When decrypting files with /R=6, hash_V5 is called more than once
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with the same inputs. Caching the results or refactoring to reduce
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the number of identical calls could improve performance for
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@ -980,10 +980,6 @@ QPDFWriter::enqueueObject(QPDFObjectHandle object)
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" another file.");
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}
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if (object.isNull())
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{
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// This is a place-holder object for an object stream
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}
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QPDFObjGen og = object.getObjGen();
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if (obj_renumber.count(og) == 0)
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@ -2014,10 +2010,7 @@ QPDFWriter::prepareFileForWrite()
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// Do a traversal of the entire PDF file structure replacing all
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// indirect objects that QPDFWriter wants to be direct. This
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// includes stream lengths, stream filtering parameters, and
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// document extension level information. Also replace all
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// indirect null references with direct nulls. This way, the only
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// indirect nulls queued for output will be object stream place
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// holders.
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// document extension level information.
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std::list<QPDFObjectHandle> queue;
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queue.push_back(getTrimmedTrailer());
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