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Fix TODO notes on multiple direct object parent issue

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Jay Berkenbilt 2022-09-08 11:47:48 -04:00
parent 18a583e8d9
commit 5911a348a5
1 changed files with 32 additions and 19 deletions
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@ -787,27 +787,40 @@ Rejected Ideas
* Fix Multiple Direct Object Parent Issue * Fix Multiple Direct Object Parent Issue
These are some ideas I had before m-holger's changes to split This idea was rejected because it would be complicated to implement
QPDFValue from QPDFObject. These notes were written prior to the and would likely have a high performance cost to fix what is not
split of QPDFObject into QPDFObject and QPDFValue and don't work really that big of a problem in practice.
directly with the new implementation. I think they are still basic
valid after adjusting to the new structure, but I think they would It is possible for a QPDFObjectHandle for a direct object to be
come at too high a performance cost to be worth doing. 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<QPDFObject> parent to QPDFObject. When adding a * Add std::weak_ptr<QPDFObject> parent to QPDFObject. When adding a
direct object to an array or dictionary, set its parent. When direct object to an array or dictionary, set its parent. When
removing it, clear the parent pointer. removing it, clear the parent pointer. The parent pointer would
* When a direct object that already has a parent is added to always be null for indirect objects, so the parent pointer, which
something, it is a warning and will become an error in qpdf 12. would reside in QPDFObject, would have to be managed by
There needs to be unsafe add methods used by unsafeShallowCopy. QPDFObjectHandle. This is because QPDFObject can't tell the
These will add but not modify the parent pointer. difference between a resolved indirect object and a direct object.
This allows an object to be moved from one object to another by * Phase 1: When a direct object that already has a parent is added
removing it, which returns the now orphaned object, and then inserting to a dictionary or array, issue a warning. There would need to be
it somewhere else. It also doesn't break the pattern of adding a unsafe add methods used by unsafeShallowCopy. These would add but
direct object to something and subsequently mutating it. It just not modify the parent pointer.
prevents the same object from being added to more than one thing.
Note that arrays and dictionaries still need to contain * Phase 2: In the next major release, make the multiple parent case
QPDFObjectHandle because of indirect objects. This only pertains to an error. Require people to create a copy. The unsafe operations
direct objects, which are always "resolved" in QPDFObjectHandle. 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.