Avoid modifying source objects when merging cache results. #4089
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As #4081 demonstrates, if there are key collisions during result merging, it's possible for source objects that were previously merged by reference into the target object (finalResult.result) to be modified destructively by later merges, which in some cases can lead to cycles in the results returned by the cache. This version of the merge function uses a copy-on-first-write strategy to ensure we never modify target objects that might once have been source objects. The code could have been considerably simpler if I didn't try to track pastCopies, but performance and memory usage are very important for this code, which is why I went to the trouble of limiting the number of shallow copies made during a single merge.
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I had to check my GraphQL facts, but overlapping/duplicated fields within the same selection set are allowed, as long as they follow certain rules: https://facebook.github.io/graphql/draft/#sec-Field-Selection-Merging I don't think this code is the place to enforce those rules, but it definitely is important to tolerate field collisions when merging results, rather than doing something easy/drastic like rejecting the query. |
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As #4081 demonstrates, if there are key collisions during result merging, it's possible for source objects that were previously merged by reference into the
targetobject (finalResult.result) to be modified destructively by later merges, which in some cases can lead to cycles in the results returned by the cache.This version of the
mergefunction uses a copy-on-first-write strategy to ensure we never modifytargetobjects that might once have beensourceobjects. The code could have been considerably simpler if I didn't try to trackpastCopies, but performance and memory usage are very important for this code, which is why I went to the trouble of limiting the number of shallow copies made during a singlemerge.