[jira] [Comment Edited] (PDFBOX-5263) Suggestion: Signing actual document changes - Enhancing incremental saving

["Christian Appl (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/PDFBOX-5263?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17407952#comment-17407952 ] 

Christian Appl edited comment on PDFBOX-5263 at 9/1/21, 8:22 AM:
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*Edit:*
 (This will make more sense, once you had a look at the COSBase#setDocument() #copyContextTo() methods.)
 Currently "copyContextTo()" will not overwrite a preexisting context. If a COSBase already has a root document, it will keep it. (Stay with me - this really is one issue with this patch!)

*Reasons:*
 There is one simple reason for that and that can be found in the usage of COSDictionary. Some items of COSDictionaries contain references to their parent Dictionary (e.g. via COSName.P) and in some PD objects that entry is set before adding the child to the parent dictionary.
 If I - for example - copied an item from one document to such an PD object of another document, the child´s root document would overwrite the parent´s root document (which must never happen!).
 I could have changed the order of such calls (in *all* PDObjects, that do such things :)) and that would also solve the issue. But I can not enforce future implementations of PDObjects to also obey that - therefore this would not only be a tedious task, but would also dig a trap for anyone following.

One way to prevent this, would be to determine the pathes of the objects and determining which object is the parent, which is the child and which of them must be updated. Also resulting (again) in traversing the COS structure repeatedly. (which the current solution does avoid - following the idea: objects shall _know and tell_ not _search_.)

*Current state of things:*
 If this remained as it is: if nodes are adopted from another document, it is up to the user, to call "setDocument()" for the adopted nodes. (Possibly documents could have an "adopt" method or something)
 This answer is dissatisfying for the same reason I gave above: "PDFBox should do that for me. Don't make me think!"
 Which still is a valid point... A real solution would be preferable, but I did not have the time to come up with a better idea.

*Why this should not be an issue concerning incremental saving:*
 A root document currently serves the purpose to prevent changes to COS objects. Aslong as the document is parsed or dereferenced objects are being loaded - this will prevent objects from being marked as "updated", that were originally contained in the document. Aslong as a root document is present (no matter which root document) such load states can be determined.
 A node adopted from another document is most likely a new node in the target document and the document they are adopted from will be "ready" at that point (otherwise the object could not be touched).
 For those reasons this does not undermine the intended functionality concerning incremental updating.

*The reason this still is a problem otherwise:*
 If some solution in the future relied on "getDocument()" to return the correct root document - that being: the object does actually reside in - this would be an ugly thing to discover and would cause issues, that would be infuriating to resolve.
 Currently the priority is to make incremental saving work...
 But #getDocument() and #setDocument() introduce new features, that might be used later for other purposes - therefore they should work reliably and should automatically make the correct decision.
 Therefore this still is a TODO. (which I should have marked as such....)


was (Author: capsvd):
*Edit:*
(This will make more sense, once you had a look at the COSBase#setDocument() #copyContextTo() methods.)
Currently "copyContextTo()" will not overwrite a preexisting context. If a COSBase already has a root document, it will keep it. (Stay with me - this really is one issue with this patch!)

*Reasons:*
There is one simple reason for that and that can be found in the usage of COSDictionary. Some items of COSDictionaries contain references to their parent Dictionary (e.g. via COSName.P) and in some PD objects that entry is set before adding the child to the parent dictionary.
If I - for example - copied an item from one document to such an PD object of another document, the child´s root document would overwrite the parent´s root document (which must never happen!).
I could have changed the order of such calls (in *all* PDObjects, that do such things :)) and that would also solve the issue. But I can not enforce future implementations of PDObjects to also obey that - therefore this would not only be a tedious task, but would also dig a trap for anyone following.

One way to prevent this, would be to determine the pathes of the objects and determining which object is the parent, which is the child and which of them must be updated. Also resulting (again) in traversing the COS structure repeatedly.

*Current state of things:*
If this remained as it is: if nodes are adopted from another document, it is up to the user, to call "setDocument()" for the adopted nodes. (Possibly documents could have an "adopt" method or something)
This answer is dissatisfying for the same reason I gave above: "PDFBox should do that for me. Don't make me think!"
Which still is a valid point... A real solution would be preferable, but I did not have the time to come up with a better idea.

*Why this should not be an issue concerning incremental saving:*
A root document currently serves the purpose to prevent changes to COS objects. Aslong as the document is parsed or dereferenced objects are being loaded - this will prevent objects from being marked as "updated", that were originally contained in the document. Aslong as a root document is present (no matter which root document) such load states can be determined.
A node adopted from another document is most likely a new node in the target document and the document they are adopted from will be "ready" at that point (otherwise the object could not be touched).
For those reasons this does not undermine the intended functionality concerning incremental updating.

*The reason this still is a problem otherwise:*
If some solution in the future relied on "getDocument()" to return the correct root document - that being: the object does actually reside in - this would be an ugly thing to discover and would cause issues, that would be infuriating to resolve.
Currently the priority is to make incremental saving work...
But #getDocument() and #setDocument() introduce new features, that might be used later for other purposes - therefore they should work reliably and should automatically make the correct decision.
Therefore this still is a TODO. (which I should have marked as such....)

> Suggestion: Signing actual document changes - Enhancing incremental saving
> --------------------------------------------------------------------------
>
>                 Key: PDFBOX-5263
>                 URL: https://issues.apache.org/jira/browse/PDFBOX-5263
>             Project: PDFBox
>          Issue Type: Improvement
>          Components: Parsing, PDModel, Writing
>    Affects Versions: 3.0.0 PDFBox
>            Reporter: Christian Appl
>            Priority: Major
>             Fix For: 3.0.0 PDFBox
>
>         Attachments: Enhanced_incremental_saving_.patch, Enhanced_incremental_saving_PDFBox3.patch, image-2021-08-23-14-55-24-077.png, image-2021-08-26-09-52-33-567.png, image-2021-08-26-09-54-24-897.png, image-2021-08-26-10-00-07-383.png, image-2021-08-26-10-02-08-003.png, image-2021-08-26-10-03-47-940.png, image-2021-08-26-10-06-42-925.png, image-2021-08-26-10-09-12-698.png, image-2021-08-26-10-12-19-265.png
>
>
> *TL;DR:*
> Currently it is rather tedious to create incremental changes in between signatures via PDFBox. I attempted to simplify that and wrote a patch.
> This is rather a POC, than an actual suggestion for direct inclusion. (For reasons explained later.)
> *Signatures and incremental PDF documents:*
> A typical reason for wanting to sign a document multiple times (creating an incremental PDF) is , that in between signatures the document changed and the additional signature shall sign the new state of the document.
> If one wanted to implement such incremental changes using PDFBox, he would find, that most of the time made changes are completly ignored, when calling "saveIncremental".
> As documented for the "saveIncremental" methods and especially the matching constructors in "COSWriter", this would require, to identify the "path" of all made changes and one would need to set the "needToBeUpdated" flag of all elements of that path.
> *But:*
> As documented one would have to have exact understanding of what he did and how the PDF standard does implement this, he would have to identify said structures and the more complex the changes were, the more tedious this would become.
> *Also:*
> Because of the implementation of incremental saving in COSWriter, the whole path must be informed that it required an update.
> Resulting in unnecessary large increments, as not all ancestors might actually have changed.
> e.g. If one added an image to a preexisting page of the document - the contentstream, the resources of the page and the page dictionary would have changed. But the "pages" array and all it's ancestors would not have changed a bit, but still would have to be informed and included.
> *Assumptions that lead to this patch:*
> - COSWriter should not stop iterating a COSTree just because a parent element did not change. It's descendants still could have changed!
> - Externally managing an object´s update state is tedious and error-prone.
> Objects that implement "COSUpdateInfo" should know and manage by themselves whether they were freshly created or altered
> (e.g.: A COSDictionary should be able to remember, that a setter had been called).
> - If "COSUpdateInfo" objects were self aware and would solve this by themselves, it would not be necessary anymore to set update states manually.
> *Problems:*
> The first and obvious problem is, that the initial loading of a document is creating and altering new COS structures and we obviously don't want objects to observe and remember those changes. An object that is created during document initialization must be treated as preexisting.
> However: COSBase is not context aware - it does know it's descendants, but neither does it know it's parent, nor does it know it's root.
> If it was, that actually would present the optimal solution, as in that case the Object could ask it's root for the current load state and therefore would be able to ignore said changes caused by the initial loading of a document.
> But it is not. (My opinion is - it should be! But more on that later.)
> Therefore a a helper named COSUpdateInfoList was implemented, which was capable of finding COSUpdateInfo objects in a COS structure, and that allowed resetting their update state after loading was completed.
> *Description of the patch:*
> The patch implements selfaware COSUpdateInfo objects, which the COSWriter has been adapted to process. PDFBox therefore is capable of monitoring changes in realtime and to automatically include altered structures in an incremental save of the document, therefore creating increments (or an increment), that a signature would sign.
> *Result:*
> Using this patch documents could be created:
> incrementally adding pages, adding contents to pages, adding annotations, altering structures, removing structures.
> As far as has been initially tested the resulting documents were valid, viewable in a reader and the objects overwritten in increments seemed correct.
> *But -* *Caveat:*
> This patch does introduce atleast one ugly class (most likely you will be able to point out more, that could be optimized :)) and that is "COSUpdateInfoList" - as already explained: In my opinion such a class should not exist, the COSUpdateInfo objects should be context aware and should be capable of regulating their own behaviour.
> Whenever the alternatives are to either manage an object externally, or to "teach" an object to solve problems autonomously, I will tend to prefer the latter... but I did not dare to do that.
> This would require, that either further constructors or setters would have to be introduced for COSBase objects, that allowed setting parent/root/context for the object.
> Which would result in further massive changes for using applications and PDFBox itself - as all instantiations of COSBase objects (PDObjects) would have to be adapted.
> However: I would prefer if COSBase objects actually were context aware.
> But as stated... I did not dare to touch it and instead chose the ugly workarround, that would introduce yet another iteration over the whole COSDocument structure.
> Eliminating COSUpdateInfoList would be preferable!
> *Suggestion:*
> As PDFBox 3 is already changing how documents and objects are handled, I would suggest, that also COSBase objects should be made context and selfaware in PDFBox 3.
> This would allow simplifying handling COS objects using PDFBox and it would allow for an easier and automized handling of incremental saving.
> *Usage example:*
> The following "pseudo code" (actually using simplified Helper classes) demonstrates the intended usage:
> !image-2021-08-23-14-55-24-077.png!
> *As always:* Thank you very much for your work and support! I hope this suggestion is to your liking.



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