OverlappingFieldsCanBeMergedRule stack-overflows on cyclic fragment spreads

Validating a document that contains a cyclic fragment spread crashes the process with a stack overflow.

Reproducer

fragment A on Dog { ...B }
fragment B on Dog { ...A }
{ ...A }

Running this through graphql.ValidateDocument against any schema produces:

runtime: goroutine stack exceeds 1000000000-byte limit
fatal error: stack overflow

The stack points at collectConflictsBetweenFieldsAndFragment in rules_overlapping_fields_can_be_merged.go recursing on itself.

Impact

Any server using this library can be crashed by a small malformed query. NoFragmentCyclesRule correctly reports the cycle as a validation error, but OverlappingFieldsCanBeMergedRule runs in parallel via the visitor and crashes the process before that result is returned.

Fix

PR #741 proposes a fix.