[oryan-block]

Fixes #886

Checklist

• Pull requests follows the contribution guide
• New or modified functionality is covered by tests

Description

Unions that list each other, or a union that lists itself, failed the build with a StackOverflowError:

union Result = Book | Media

union Media = Movie | Result

SchemaParser#getLeafUnionObjects flattens nested unions recursively and didn't keep track of the unions it was already expanding. It now passes down the path of unions it's in, and when a member is already on that path it throws a SchemaError that names the cycle and points at the member that closes it:

Union 'Result' contains itself through Result -> Media -> Result at line 7, column 23 in schema.graphqls.

I went with the error instead of flattening each union once, since the spec doesn't allow union members at all and nobody asked for cycles. These schemas never built, so nothing that builds today breaks. Only the current path is checked, so two nested unions that share a member without a cycle still build.

mvn clean verify passes, also against graphql-java 25.1. Also built the schema above through the public API: master overflows, with this change it fails with the error above. Through the same harness, the existing nested union case (union Media = Movie) and a shared member (Result = Media | Shelf, both listing Book) still build and return both a Book and a Movie.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[oryan-block]

PASS+NOTES
Commit c70aaed, built into the probe jar and run against master a24067d on graphql-java 26.1. mvn clean verify on the head passes (366 tests, 0 failures).
Loops on master all fail with schema build: FAILED java.lang.StackOverflowError. That covers the issue schema, reversed member order, a field returning the inner union, union Result = Book | Result, union Result = Result, a 3-union loop that doesn't contain the returned union, a 4-union loop, a loop that only exists through extend union Media = Result, and the issue schema with includeUnusedTypes(true).
The same loops on the head all fail with a SchemaError, for example Union 'Result' contains itself through Result -> Media -> Result at line 7, column 23 in scenarios/ul-pair/schema.graphqls., Union 'A' contains itself through A -> B -> C -> A at line 11, column 11 for the loop that excludes Result, A -> B -> C -> D -> A for the 4-loop, and Result -> Result for the self case. Checked again with --graphql-java 25.1 for the issue schema, self-member, 3-loop and extend union loop, with the same messages.
No-loop unions build on master and head with identical printed schemas and query results. Covered: a chain Result = Book | A, A = Movie | B, B = Gadget, extend union without a loop, a shared member (Result = Media | Shelf, both listing Book), and a nested union reached by two paths (Shelf = Media | Gadget). Each returned Book, Movie and Gadget with the right __typename, and 25.1 matched.
Note 1. A loop that no field reaches still builds on both master and head (unused definitions: Media, Movie, Result, schema build: OK). It only fails with includeUnusedTypes(true), where master overflows and the head gives the new error. Same as master, so not a regression.
Note 2. The cycle is reported from whichever union the parser builds first, so when a field returns Media the message reads Union 'Media' contains itself through Media -> Result -> Media at line 5, column 23. It is still a correct cycle.