Add JSON annotation in internal/gqlparser/ast to Position fields

[robmyersrobmyers]

Annotate internal/gqlparser/ast structs not to include Position when marshaled to JSON.
This makes the JSON roundtrip succeed without allocating a ton of memory for JSON fields that will be subsequently pruned.

# Without this change:
$ time ./reproducer-pre-fix
Now passing 1262568 bytes to builtinGraphQLParseSchema() to reproduce the issue in ast.InterfaceToValue()
Alloc = 4159 MiB   TotalAlloc = 5625 MiB   Sys = 5584 MiB  NumGC = 18
Alloc = 8312 MiB   TotalAlloc = 11169 MiB  Sys = 11125 MiB NumGC = 19
Alloc = 8312 MiB   TotalAlloc = 11169 MiB  Sys = 11125 MiB NumGC = 19
Alloc = 16615 MiB  TotalAlloc = 22247 MiB  Sys = 22209 MiB NumGC = 20
Alloc = 16615 MiB  TotalAlloc = 22247 MiB  Sys = 22209 MiB NumGC = 20
Alloc = 16615 MiB  TotalAlloc = 22247 MiB  Sys = 22209 MiB NumGC = 20
Alloc = 38765 MiB  TotalAlloc = 44397 MiB  Sys = 44377 MiB NumGC = 20
Alloc = 33223 MiB  TotalAlloc = 44397 MiB  Sys = 44377 MiB NumGC = 21

# With this change:
$ time ./reproducer-post-fix
Now passing 1262568 bytes to builtinGraphQLParseSchema() to reproduce the issue in ast.InterfaceToValue()

real       0m0.545s
user       0m0.201s
sys        0m0.028s

• This doesn't change any internal or external APIs or add any functionality, so new tests are not needed.
• A reproducer is attached
• Resolves issue discussed in Possible to cache graphql.parse() results? #5377 (comment) with ast.InterfaceToValue never returning since it relies on a RoundTrip to JSON.

I have also submitted this change for inclusion in the upstream gqlparser project.

[anderseknert]

Amazing find! Excellent work 👏 I hope it's accepted upstream too.

As for testing, I guess adding a test just to verify that parsing some schemas don't cause infinite recursion (as would happen before the fix) might make some sense?

[robmyersrobmyers]

It didn't turn out to be a cycle as I previously speculated. It's just using a ton of memory because each JSON node includes a copy of the entire original source file. In the AST this is just a pointer to a single copy, but when JSON marshaled each Position includes the whole source file.

I'll add a self contained test to the graphql caching PR based off this updated reproducer.

EDIT: The commit with the test.

[johanfylling]

Thank you! 😃

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