Fix issues related to JsonSerializerOptions mutation and caching.

[eiriktsarpalis]

Supersedes #65799. After conversation with @layomia in #65799 (comment) I realized that the caching implementation introduced in #64646 contains a bug. More specifically, a locked/immutable JsonSerializerOptions instance can still be mutated by the serialization infrastructure in couple of important ways:

• Setting the JsonSerializerContext field.
• Materializing the _typeInfoCreationFunc field if the options instance has been passed to a reflection serialization API.

Both of these changes alter the JsonTypeInfo resolution semantics in important ways, and as such they can invalidate the current contents of the metadata cache. This PR makes the following changes:

• JsonSerializerContext values can now only be attached to mutable JsonSerializerOptions instances. This is a potential breaking change, however the new throwing behavior is arguably better than the current one (nondeterministic use of serialization metadata).
• Make IsInitializedForReflectionSerializer an instance method instead of a static, and ensure it gets propagated correctly to shared caching contexts.
• Fix Repeatedly initialize the static variables s_defaultSimpleConverters and s_defaultFactoryConverters when calling the method GetConverter of JsonSerializerOptions #65770.

I ran the benchmark suite and no performance regressions were recorded.

[ghost]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

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Tagging @dotnet/compat for awareness of the breaking change.

[ghost]

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Issue Details

---

Supersedes #65799. After conversation with @layomia in #65799 (comment) I realized that the caching implementation introduced in #64646 contains a bug. More specifically, a locked/immutable JsonSerializerOptions instance can still be mutated by the serialization infrastructure in couple of important ways:

• Setting the JsonSerializerContext field.
• Materializing the _typeInfoCreationFunc field if the options instance has been passed to a reflection serialization API.

Both of these changes alter the JsonTypeInfo resolution semantics in important ways, and as such they can invalidate the current contents of the metadata cache. This PR makes the following changes:

• JsonSerializerContext values can now only be attached to mutable JsonSerializerOptions instances. This is a potential breaking change, however the new throwing behavior is arguably better than the current one (nondeterministic use of serialization metadata).
• Make IsInitializedForReflectionSerializer an instance method instead of a static, and ensure it gets propagated correctly to shared caching contexts.
• Fix Repeatedly initialize the static variables s_defaultSimpleConverters and s_defaultFactoryConverters when calling the method GetConverter of JsonSerializerOptions #65770.

Author:	eiriktsarpalis
Assignees:	eiriktsarpalis
Labels:	area-System.Text.Json, breaking-change
Milestone:	7.0.0

[layomia]

LGTM.

[jkotas]

This PR was merged with failures in the tests introduced by this PR. I am reverting it in #66235. Please fix the test failures and resubmit.
cc @agocke

[eiriktsarpalis]

Apologies for that, I've reposted the changes in #66248 with relevant tests disabled in netfx.

[eiriktsarpalis]

I'm removing the breaking-change label since in hindsight, the removed behavior (i.e. ability to add a JsonSerializerContext to a locked JsonSerializerOptions instance) is fairly pathological (generally resulting in nondeterministic contract resolution in .NET 6) so it probably doesn't warrant mention in the documentation.