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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
These are some ideas I had before m-holger's changes to split
QPDFValue from QPDFObject. These notes were written prior to the
split of QPDFObject into QPDFObject and QPDFValue and don't work
directly with the new implementation. I think they are still basic
valid after adjusting to the new structure, but I think they would
come at too high a performance cost to be worth doing.
This idea was rejected because it would be complicated to implement
and would likely have a high performance cost to fix what is not
really that big of a problem in practice.
It is possible for a QPDFObjectHandle for a direct object to be
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
direct object to an array or dictionary, set its parent. When
removing it, clear the parent pointer.
* When a direct object that already has a parent is added to
something, it is a warning and will become an error in qpdf 12.
There needs to be unsafe add methods used by unsafeShallowCopy.
These will add but not modify the parent pointer.
removing it, clear the parent pointer. The parent pointer would
always be null for indirect objects, so the parent pointer, which
would reside in QPDFObject, would have to be managed by
QPDFObjectHandle. This is because QPDFObject can't tell the
difference between a resolved indirect object and a direct object.
This allows 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.
* Phase 1: When a direct object that already has a parent is added
to a dictionary or array, issue a warning. There would need to be
unsafe add methods used by unsafeShallowCopy. These would add but
not modify the parent pointer.
Note that arrays and dictionaries still need to contain
QPDFObjectHandle because of indirect objects. This only pertains to
direct objects, which are always "resolved" in QPDFObjectHandle.
* Phase 2: In the next major release, make the multiple parent case
an error. Require people to create a copy. The unsafe operations
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.