Fix for a rare race condition in System.IO.Packaging

[themcoo]

Fix for a rare race condition when initializing System.IO.Packaging.PackageRelationship.s_containerRelationshipPartName._partUriExtension

In rare circumstances, when there are at least two ooxml packages created in parallel for the first time in application's lifetime, it is possible that PackageRelationship.s_containerRelationshipPartName._partUriExtension will be initialized to "els" instead of "rels". Once it's initialized like that, every Package is saved with an invalid extension for package relationships content type in [Content_Types].xml:
<Default ContentType="application/vnd.openxmlformats-package.relationships+xml" Extension="els"/>
When reading such damaged package later, the relationships are not to be found.

As this is extremely non-deterministic, no unit tests were provided. I developed a tiny app to reproduce this issue, launching it in a loop I was able to get the issue 3 times over 8000 runs.

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Issue Details

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Fix for a rare race condition when initializing System.IO.Packaging.PackageRelationship.s_containerRelationshipPartName._partUriExtension

In rare circumstances, when there are at least two ooxml packages created in parallel for the first time in application's lifetime, it is possible that PackageRelationship.s_containerRelationshipPartName._partUriExtension will be initialized to "els" instead of "rels". Once it's initialized like that, every Package is saved with an invalid extension for package relationships content type in [Content_Types].xml:
<Default ContentType="application/vnd.openxmlformats-package.relationships+xml" Extension="els"/>
When reading such damaged package later, the relationships are not to be found.

Author:	themcoo
Assignees:	-
Labels:	area-System.IO, community-contribution
Milestone:	-

[adamsitnik]

LGTM, thank you for your contribution @themcoo ! 👍

Is there any chance you could share the repro app? It would help me to estimate whether we should backport this fix.

[themcoo]

LGTM, thank you for your contribution @themcoo ! 👍

Is there any chance you could share the repro app? It would help me to estimate whether we should backport this fix.

Cool, happy to see it merged so quickly ;)

@adamsitnik I pushed my little messy proofing app to repo https://github.com/themcoo/SystemIOPackaging.Race

Just build, run run.bat and wait for the bug to occur (with message "Race happened!").
My latest 5 launches of run.bat ended with the bug occurring at the following attempts:
2302
97
334
4491
489

Maybe it's worth to mention why I was investigating this. This bug actually occurred on production release of our applications (1000+ ish instances, probably restarting daily). First time circa 10 months ago, second time a week ago (app was released a year ago). The second time made me dig into the dotnet source to find the reason behind the malformed content type xml....